As an extension of the THINKING MAPS I referred to in my previous blog, I introduce here another type of Thinking Map – The BRACE MAP.
The BRACE MAP is an effective tool for learning (or teaching) Whole- to– Part relationship for different topics/ concepts of Science.
A BRACE MAP is a great way of remembering different parts of any structure (say Parts of a Plant/ Organism/ Organ), remembering spellings of difficult Scientific Terms with their Meanings with the greatest ease…..the uses of BRACE MAPS are multiple & have been discussed in detail in the following ‘video clip’ prepared by me…
One of the important Takeaways from this video clip is that– BRACEMAPS can be veryuseful in arousing interest towards Science in students & at the same time giving them a cue abouthow to proceedtowardslearning a difficult‘Structure’ with ease…
Learning science is fun. A good teacher makes the learning a lifetime experience. Science becomes interesting so far it is imparted with practical, every day connect.
In the last few decades, we have witnessed a great breakthrough in the way science concepts are taught and learnt. Science learning is no more a bookish affair. A whole new methods and techniques have evolved to reach out to even the weakest child.
Learning science has been made workable through various Interactive methods like low-cost classroom experimentation, field trips, storytelling, role-play, text cards, word walls, projects, video clips, PowerPoints, Science fair, Science clubs, Science- at- home, Quiz, model making etc.
Here, in this blog, I have tried to introduce to my readers, a teaching- learning technique which can act as a powerful tool in remembering various science concepts. The tool I’m talking about is called a ‘Thinking Map’.
Thinking maps are visual tools for learning. There are eight such types of thinking maps, each linked to a specific cognitive process.
Thinking maps are illustrations which can effectively communicate information using precise and brief language.
Here, I am considering one of the thinking maps which is called a BRIDGE MAP.
A ‘bridge map’ is a thinking map that is used for Analogies or comparisons to understand similarities and relationships between the pairs that are being considered.
In each pair top item relates to the bottom item. Such relating items are connected by ‘bridges’ (triangles) to show analogies.
Let’s understand the BRIDGE MAP with the help of the following ‘video clip’ prepared by me.
Here’s a video composed by me to help ease out the learning technique called BRIDGE MAP, for the viewers….
I find these Bridge Maps useful asthey-
Help teacher to illustrate concepts in a brief & precise way, so makes it interesting for students.
Help students to take down notes briefly with a good and precise understanding of the topic.
A ‘bridge map’ can extend upto any length, only it should include related items.
There is no hard and fast rule for the content. The teacher- taught can use them extensively, creating their own Bridge maps.
Readers do give a feedback, telling how useful you find these Bridge maps.
Just as the basic unit of all non-living things is ‘Atom’, the basic unit of all living organisms is ‘Cell’.
PS- The atom is still very smaller than the cell.
Now, the standard definition of the Cell says that “cell is the basic structural and functional unit of living organisms”.
By the word “structural” we mean that cells give shape or form to organs and consequently to the body parts.
By “functional” we mean that a cell contains the whole machinery (organelles) for performing various life processes required for sustaining life. A Cell processes nutrients to give energy and undergoes replication/ division to give rise to more cells. There are specialised cells to perform specific functions in Eukaryotes.
A typical Eukaryotic animal cell contains membrane- bound organelles like nucleus, mitochondria, ribosomes, endoplasmic reticulum(ER), Golgi apparatus which have specific functions to carry on various life processes.
There can be variations in number or presence/absence of a few organelles in different Specialized Cells, depending on the job they do.
1. Mature RBCs get rid of nuclei and all other cell organelles like mitochondria, Golgi apparatus and ER to free up space for more haemoglobin, which is an oxygen carrier.
2. Mature neurons lack centrioles which are responsible for mitotic cell division.
3. Muscle cells and liver cells have more number of mitochondria.
Some more interesting facts about the Cell:
– Human body is composed of about 200 different types of specialized cells.
– All cells are capable of carrying out certain basic functions like nutrition, respiration, growth and replication which are essential for the survival of cells.
– Since the cell carries out so many functions, so there is “division of labour” within the cell which means that different organelles of the cell perform different specific functions. For example- making of new material like protein synthesis by the organelle ribosomes, cellular metabolism/ respiration to release energy by mitochondria, DNA replication in nucleus of Eukaryotes, clearing up the waste substances from the cells by lysosomes etc.
Let us know about the Cell size, Cell shape, Cell volume, Cell number and Cell structure:
– The cells are microscopic. In human, the cell size varies from 3 to 4 micron (leucocytes) to over 90 cm (nerve cells).
– The cell size is correlated with its function and not to the size of organism.
– In a large cell, the cytoplasm requires more proteins and consequently more RNA.
– The DNA content of the cell in a given organism remains constant.
– Small cells have more surface area per unit volume and with an increase in size this ratio decreases.
– Small cells are metabolically more active because they have greater surface area, so more exchange of materials occur with outside environment.
– The shapes of the cells are also related to their functions.
– The shapes also depend on the surface tension and viscosity of protoplasm, mutual pressure of the adjoining cells and rigidity of the cell membrane.
– The Cell volume is almost constant for a particular cell type and is independent of the size of the organism.
– The total mass of an organism depends on the number of cells present in its body and not on the volume of the cells. So, the cells of an elephant are not larger than any other tiny animal.
– The number of cells is correlated with the size of organisms. So, small organisms have less number of cells than large organisms.
– The entire body of an adult animal or plant consists of a fixed number of cells and that remains the same in all members of the species. This phenomenon of cell or nuclear constancy is called “Eutely”.
– In human beings, the number of cells is around 100 trillion.
All cells have three major functional regions:
1. Cell membrane or Plasma membrane (cell wall also, in case of plants only)
What is the physical significance of Plasma Membrane?
– Plasma membrane is the outer boundary of the cell. It is present in all types of cells- both eukaryotic and prokaryotic cells, cells of plants, animals and microorganisms.
– It physically separates the cytoplasm from the surrounding cellular environment.
– Most cellular organelles like mitochondria, lysosomes, Golgi apparatus, nucleus, endoplasmic reticulum, and chloroplast (in plant cells only) are enclosed by the plasma membrane.
Is plasma membrane living or dead?
The plasma membrane is thin, elastic, living and selectively permeable membrane. It ranges from 6 to 10 nm. Chemically, membrane is 75% phospholipids and also contains proteins, cholesterol and polysaccharides.
Let’s know the ultrastructure of plasma membrane to know more about the living nature of plasmamembrane.
Various models, to know the structure of plasma membrane, have been proposed by different scientists.
“Fluid Mosaic Model” of plasma membrane is well accepted which we take up as follows:
– The Fluid Mosaic Model was propounded by Singer and Nicholson.
– According to this, the plasma membrane is made up of a bilayer of phospholipids.
– Two types of proteins- Intrinsic or Integral and Extrinsic or Peripheral float about in the fluid phospholipid bilayer.
– Intrinsic proteins penetrate lipid bilayer partially or wholly.
– Extrinsic proteins are present either on the outer or inner surface of the lipid bilayer.
– The Lipids and Intrinsic proteins are amphipathic in nature ie. these molecules have both hydrophobic (non-polar) and hydrophilic (polar) groups.
– The proteins are present to serve as (a)enzymes (b)transport proteins or permeases (c)pumps (d)receptor proteins
– Lipids and proteins provide flexibility to the plasma membrane which helps in processes like Endocytosis.
– Plasma membrane is selectively permeable ie. it permits the entry and exit of some materials only in the cell.
– Substances allowed inside the cell include food, water, salts, oxygen, vitamins and hormones.
– Substances thrown out of the cell include nitrogenous waste and carbon dioxide, secretions like proteins, proenzymes, hormones, milk, tear, mucus, immunoglobulins (antibodies) etc.
– Since plasma membrane regulates the transport of various substances in and out of the cell, so it is living in nature. This is done to maintain the concentration of various substances and ions inside the cell.
Transport can be Passive or Active.
– Here, the particles or molecules move from a region of higher concentration to lower concentration through plasma membrane by DIFFUSION. So, this is also called “downhill transport”. Here, the movement occurs only due to the concentration gradient without consuming energy. The hydrophobic substances are readily transported by this method because these are soluble in lipids.
– Sometimes a carrier molecule called “Carrier Protein or Permeases” assist the transport (without the use of energy) and this is called “Facilitated transport”. It is helpful in the transport of hydrophilic nutrients like glucose and amino acids.
– When water molecules pass through the plasma membrane along the concentration gradient without the use of energy, the process is called OSMOSIS.
– This is the movement of molecules or ions against the concentration gradient (Uphill movement) using energy (ATP) to counteract against gradient.
– The most important active transport in all animals is the sodium-potassium transport between cells and the surrounding extra cellular fluid. This transport is called “Sodium pump”.
– The animal cell requires a high concentration of potassium ions inside the cells for protein synthesis by ribosomes and for certain enzymatic functions.
– The desirable potassium ion concentration is 20 to 50 times greater inside the cell than outside and sodium ion concentration maybe 10 times more outside the cell than inside.
How Sodium- Potassium ionic gradients are maintained or how the sodium pump works?
– There is a higher concentration of sodium ions outside the plasma membrane of the animal cell. The sodium ions are transported outside with the use of a carrier molecule- A Carrier Transport Complex is formed which utilises ATP and transports sodium ion outside the cell. Simultaneously, potassium ions are transported inside the cell by similar way.
– This unbalanced charge transfer leads to separation of charges across the plasma membrane. This difference helps in the Action Potential produced by nerve cells.
How are the macromolecules, solid food particles, etc transported inside the cell?
These are transported by the mechanism called ENDOCYTOSIS.
Depending on the nature of the substance, Endocytosis can be of the following types:
1. Pinocytosis (cell drinking)
– Pinocytosis is the process of “ingestion of fluid droplets & small solute particles” by the cell. The substances like protein, amino acids, which cannot enter by simple osmosis are ingested by pinocytosis.
– Here, the plasma membrane invaginates to form small vesicles or “Pinosomes”. The vesicles pinch off from the plasma membrane, move through the cytoplasm and fuse with the plasma membrane of the other side, there by discharging the contents.
– Pinocytosis is seen in microvilli of small intestine and in kidney cells.
– Phagocytosis is the process of “engulfing solid food particles” by cells through plasma membrane (as seen in protozoa also). Vesicles formed here are called “Phagosomes” (1 to 2 µm).
– Phagosomes move through cytoplasm and are dissolved and ingested by enzymes of lysosomes. The residues are ejected out of the plasma membrane by process called EPHAGY.
– In Phagocytosis, bacteria etc are engulfed.
This is the transfer of small quantities of cytoplasm, together with their inclusions, from one cell to the other. This was demonstrated in bone marrow tissue.
PS- All types of Endocytosis processes occur by “Active Transport”.
Q1. Can we call the plasma membrane as unit membrane?
Ans. Unit membrane means the limiting membrane of the cell and the organelles, viewed formerly as a three layered membrane, composed of inner lipid layer and two outer protein layers. This concept has been rejected as Fluid Mosaic model is the current accepted one.
Q2. Do proteins present in the plasma membrane give strength to the cell?
We have seen in nature that solids, liquids and gases, and empty spaces become hot when they are heated. But, to our surprise, they all get heated by different ways. The simple reason behind this difference is the fact that arrangement of particles in all these differ. In solids, particles are very tightly packed. In liquids and gases, the particles are free to move about, and in vacuum, there are no particles.
I have already discussed in my previous blog that solids get heated by the phenomenon called Conduction.
The transfer of heat ( ie. the way things become hotter) in liquids and gases occurs by the phenomenon called CONVECTION. It is the transfer of heat from hotter parts of liquids/ gases to its colder parts due to the movement of freely moving particles in liquids and gases.
The process of Convection in liquids and gases is just the same. Let’s see with examples.
Convection in Liquids
Let’s take water as the liquid. When we heat water in a saucepan from below, first the water at the bottom becomes hot, so it expands (ie. particles gain K.E. due to heat energy, vibrate vigourously and move apart, increasing the space they occupy) and becomes lighter and rises up. The colder denser water above in the saucepan sinks down, taking the space occupied by hot water. In this way, circulation of water is set in the saucepan where hot water rises up, colder water sinks down- then it gets heated and rises up and again the colder water from top sinks down creating a circular motion of water in the saucepan called CONVECTION CURRENTS. So, these convection currents transfer heat from water at the bottom to the water at the top, or in the other words, water is heated by Convection Currents.
A Small Experiment For Readers
Try heating water from top ie. bring the burner on the top of the saucepan containing water.
Does the entire water get heated or is the heat transferred from top to bottom?
You will find that the answer is ‘NO’.
REASON is- the water at the top gets heated- it becomes lighter, so, does not sink down and hence, the water at the bottom remains cold as no Convection Currents are formed.
So, heat is transferred in water by Convection only in the upward direction ie. from bottom to top.
Now you can answer the following Queries:-
Q1. Why we always place the burner below the utensil while boiling, cooking and likewise?
Q2. Why the heating element of an electric kettle placed at the bottom?
Convection in Gases (air)
The heat is transferred from hotter parts of air to colder parts by Convection, the process being exactly the same as in water because particles of air/ gases are also not fixed and can move about freely. So, air gets heated by CONVECTION CURRENTS in the same way as in water.
Now, you will be able to explain how a Room Heater kept at the floor heats the entire room.
It is to be noted that heat can be transferred in air only in the upward direction (because the hot air becomes lighter and rises up) as in liquids.
Where do you find Convection Currents in air in nature?
Ans. We find Convection Currents in the blowing of Sea breeze during the day and Land breeze during the night.
Now, a question comes to our mind- Will the heat be transferred when there are no particles ie. when there is no medium (ie. in vacuum)?
The best example to explain this, is the way by which heat is transferred from the sun to the objects on the earth.
The vast distance between the sun and the earth is mainly empty space or vacuum. Still, sun’s heat rays fall on objects on the earth and heat them. This heating of objects on the earth is not possible by Conduction or Convection as both these processes require a medium. So, this means that the transfer of heat from the sun to the earth takes place through a different process. The invisibleheat rays of the sun are Infrared rays. These heat rays from the hot sun transfer heat energy to the colder objects on the earth, without any medium by the process called RADIATION.
So, it is evident, that in Radiation, the heat is transferred from top to bottom (just reverse of Conduction or Convection).
Can you think of examples where heat is transferred by Radiation in the presence of medium?
Ans. YES, see the following situations:- When we keep our hands below or around a glowing bulb or by the sides of a burning candle or around a campfire, we feel the heat on our hands. This heating is not due to Convection because in Convection heat is always transferred upwards, whereas here, it is being transferred in all directions.
So, we reach to a big conclusion :- In Radiation, heat is transferred from top to bottom and also in all directions and also, Radiation can take place both in the presence and absence of the medium.
Be it cooking, use of insulators, temperature measurement by thermometer, protection of body from cold, hot fomentation or any other such phenomenon, it all involves transfer of heat.
Heat is also transferred in boiling water or by a room heater to the surrounding air or by sun’s rays to our body. But, in these cases, heat is transferred by different ways which we will discuss in next blog.
Now, what is heat?
Heat is a form of energy. It makes the substance hotter. When heat is removed from the substance, the substance cools down in sometime, or we can say that its temperature becomes lower or it decreases.
Now, how we can define temperature?
Can we make out correctly always, by touching a substance, whether the substance is hot or cold? Most of the times we can. But sometimes, the sense of touch can be misinforming us, like when we dip our hand in very cold water and then dip it in lukewarm water, we will feel the lukewarm water to be quite hot and vice versa.
So, temperature can be defined as being an authentic or dependable measure of hotness or coldness of a substance.
Man has devised an instrument for measuring temperature called THERMOMETER. Thermometer gives an exact degree of hotness or coldness ie. the temperature of a substance.
Mercury, which is a liquid metal, is used in thermometer because heat is transferred quickly from the substance to the mercury by touching the mercury bulb in thermometer, where the level of mercury shows the temperature of the substance on the graduated scale (Celsius/Fahrenheit scale) encompassing the mercury.
So, by now, we know that when heat is transferred (given to a substance or taken away from the substance) a substance becomes hot or cold.
So, how does this transfer of heat occur?
Let’s take an example
We dip a room temperature stainless steel (metal) spoon in a cup of hot coffee. After sometime, we find that the steel spoon also becomes hot ie. its temperature also rises. So, it’s obvious that heat contained in hot coffee has been transferred to cold spoon.
Similarly, when we keep a steel saucepan (metal) on flame, the pan becomes hot. So, heat from hot flame has been transferred to the cold pan.
Now, if we remove the hot pan from flame and keep it aside, the pan cools down slowly. This shows that heat has been transferred from hot pan to the colder surroundings.
So, we can say that heat flows from hot substance to cold substance or from a substance at high temperature to a substance at lower temperature.
Here, it is important to know that heat may be transferred from one substance to another or within the same substance.
Example:- When we boil an egg in water, the heat is transferred in the following manner:-
Hot flame ➡️ saucepan ➡️ water ➡️ outer portion of egg ➡️ inner and innermost portion of egg
The transfer of heat within the inner portions of egg is the heat transfer within the same substance.
Is the heat transferred in all substances – solid, liquid and gases, by the same phenomenon?
The transfer of heat from a hot solid substance to a cold solid substance or within the same solid substance, without the movement of substance, is called Conduction. So, we can say, that the heat is transferred in solids by the phenomenon called CONDUCTION.
In liquids and gases, this transfer of heat takes place by the phenomenon called Convection and in the absence of medium (vacuum), this occurs by the phenomenon called Radiation (these two phenomena will be discussed in my next blog).
Now, a very tickling question- how the heat is transferred in solids without the movement of substance?
We know, from our previous knowledge that in solids, particles (atoms/ molecules) are very tightly packed and they remain fixed at their position ie. they do not move.
When these particles get heat energy, they vibrate or wiggle at their position (there is no actual movement of particles from hotter end to colder end during heat transfer by conduction).
Due to this vibration, the particles possess kinetic energy (KE). The more the heat energy given, the more is the vibration of particles and more is the kinetic energy that they possess and so, consequently more is the temperature that the particles attain.
The vibrating particle causes the neighbouring particle to vibrate and the chain continues causing heat energy to flow from particle to particle. The heat is transferred to another substance (solid) till a thermal equilibrium is attained ie. both substances (solids) acquire same temperature.
Good conductors and bad conductors
The substances that conduct heat easily and quickly are called good conductors or simply CONDUCTORS. All metals and their alloys are good conductors of heat. Metals like copper, silver, aluminium, and metal alloys like brass, steel, stainless steel are very good conductors.
The substances that do not conduct heat easily are called bad or poor conductors or INSULATORS. Plastic, cloth, wool, wood, paper, clay, cork etc are poor conductors of heat.
Also, in general, liquids are poor conductors and air is a very poor conductor of heat.
Uses of good and poor conductors of heat
1. The cooking metal utensils, being good conductors, transfer heat very quickly to the food being cooked.
2. Plastic or wooden handles of utensils like saucepan, frying pan, electric iron are poor conductors, so help in easy handling of these hot objects.
3. Woollen clothes keep us warm because the fibres of wool trap air, so stop flow of heat from our body warm body to cold surroundings. Since our body doesn’t lose its heat, so we feel warm.
4. Fur of animals and feathers of birds trap air and keep them warm.
5. Use of hollow bricks in construction of houses traps air in the walls and so protects from extreme cold or heat.
Some Questions For Readers
Q1. Which will keep us warmer- wearing a single thick layer of clothing or wearing more thin layers of clothing?
Q2. Which will feel colder in winters- a metal utensil or wooden utensil and why?
Q3. Why copper base below stainless steel utensil is better?
Q4. Why roasting a chicken becomes difficult if we take care that outside doesn’t get overcooked?
At a glance, we may feel that lenses (example- magnifying glass) and plane mirrors (example- looking mirror) are more or less the same, as both are made of glass. But, the two are very different from each other, when we consider their properties as regards their behaviour towards light rays.
Outwardly, the basic difference between the two is that Plane mirrors are flat, shiny glasses, do not let any ray of light to pass through them, instead Reflect (send back) all the light rays falling on them.
Lenses, on the other hand, are spherical, transparent glasses through which the light rays can pass and while passing through the lens, they get Refracted (bent) to finally converge or diverge, at or from the Focus.
What is a lens?
A lens is made up of two spherical glasses joined together.
Lenses are of 2 types depending on their shape and properties:-
It is thickened (bulging out) in the middle and thinner towards the periphery. It converges all the light rays passing through it at the Focus. It is said to have a Positive Focus or Positive Focal length because the Incident ray and the refracted ray, both travel in the same direction.
It is thinner (bulging inwards) in the middle and thickened towards the periphery. It appears to diverge all the light rays passing through it from the Focus. It is said to have a Negative Focus or Negative Focal length because the refracted ray appear to diverge from the opposite direction of the Incident ray.
What is Focus, Focal length, Centre of curvature and Pole of the lens?
It is the point where all the light rays coming from Infinity converge (as in convex lens) or appear to diverge (as in concave lens) after passing through the lenses. The Focus lies on the right side of the lens as well as on the left side of the lens because the light rays can pass through the lens from any side. If light rays pass through the lens from the right side, image is formed on the left side and vice versa.
It is the distance between the Focus and the Centre/ Pole of the lens. Focal length is half of the distance of Centre of curvature.
Centre of curvature
It is the centre of the Sphere of which the lens is part of. It is ‘Twice the distance of Focal length. It is also written as 2F. Any distance beyond 2F is called Infinity. A lens has 2 Centres of curvature because the lens is part of 2 Spheres. Centre of curvature for any spherical lens can be measured by measuring the Radius of the sphere of which the lens is a part (remember lens is made up of parts of 2 spheres joined together).
Pole of the lens
It is the point where the Principal axis meets the surface of the lens. The central vertical dotted line (shown in fig.) is taken as the Centre/ Pole of the Lens for ray diagram purposes.
Are the images formed by a lenses, real or virtual?
Generally speaking, the images formed by Convex lenses are REAL ie. they can be obtained on the screen while the images formed by Concave lenses are VIRTUAL.
How and where the images are formed in a Convex lens?
There can be 5 conditions (as shown in the above figure) of the different positions of the OBJECT. They are:-
1. Beyond 2F
2. At 2F
3. Between 2F and F
4. At F
5. Between F and Centre of the lens
By keeping the object at above positions, one by one, we can obtain images on the other side of the lens on the screen, by moving the screen closer or farther so as to obtain sharp images.
Let’s see what will be the positions of Images for the above 5 locations of the Object
When the object is beyond 2F, the image is formed between 2F1 and F1 on the other side of the lens. The image so formed is real, inverted and diminished.
When the object is at 2F, the image is formed at 2F1 on the other side of the lens. The image so formed is real, inverted and equal in size to Object.
When the object is between 2F and F, the image is formed beyond 2F1 on the other side of the lens. The image so formed is real, inverted and enlarged.
When the object is at F, No image is formed. After refracting, the light Rays are travelling parallel to each other and cannot produce an image.
When the object is between F and O, the image is formed somewhere behind the Object on the object side of the lens. The image so formed is virtual, erect and enlarged or magnified. Example- magnifying glass.
How and where the images are formed in a Concave lens?
By keeping the OBJECT at any of the 5 different positions (as described in conditions above) in front of the Concave lens, we always get Virtual (image that cannot be obtained on screen), Erect and Diminished images.
This can be verified in the same way as in Convex lens, by holding the Object (torch or candle) at different positions, one by one, on one side of the Concave lens and moving the screen on the other side of the lens at different positions till we get sharp images.
Uses of Convex lens
1. Used as a magnifying glass
2. Used in making spectacles, microscopes, telescopes, binoculars, cameras, projectors etc.
Uses of Concave lens
1. Used in making spectacles ie. convex lens to correct hypermetropia or farsightedness and concave lens to correct myopia or shortsightedness
2. Used in ‘Peep holes’ in the entrance doors
3. Used in TV dish antenna
Q1.Why are Ray diagrams important?
Ans. Ray diagrams are valuable tools for determining the path of light from the object to our eyes.
Q2. Why do the light rays ‘bend’ when they pass through the lens?
Ans. Because while the light rays pass through the lens, the medium changes. So, when the light rays pass from rarer (air) medium to denser (glass) medium, they bend and vice versa.
Q3. How many incident rays do we take into consideration while making Ray diagrams for a convex lens?
Q4. Where do these three incident rays are shown to pass from while making Ray diagrams to show the location of the image formed?
Ans. First incident ray travelling parallel to the Principal axis, passes through the focal point after refraction. Second incident ray travelling through the focus runs parallel to the principal axis after refraction. Third incident ray travelling through the centre of the lens continues to travel in the same direction after refraction.
Q5. How do we determine the location of the image?
Ans. The point of intersection of the three incident rays (explained above) determines the image location.
Home Assignment For Readers
Draw Ray diagrams to show the location of Images, individually, for all the 5 conditions for the positions of the Object in front of the convex lens.
Take help of the following fig. to draw the Ray diagrams:-
We, practically every day, see our reflection in the mirror and have always wondered how we get our reflection or image in the mirror. It almost looks like a magic, when we see our image doing exactly the same actions what we actually do in front of the mirror. In this blog, we will decipher, how we get an image in a plane mirror; Is that image real or virtual; How is it different from the image on a photograph and so on.
To know all the answers, we must first know that we can see reflection of an object only when there is a source of light illuminating the object.
We see our image or reflection in the mirror because our body is being illuminated, say by a bulb or a tube-light (our body itself has no light of its own and so are the many objects that we see around us).
Now, when we stand in front of the plane mirror, several parallel rays of light (called Incident Rays) strike the plane mirror surface and all these incident rays are sent back or reflected (called Reflected Rays) at various angles in accordance with their Angles Of Incidence.In a plane mirror, the ‘angle of incidence’ is always equal to the ‘angle of reflection’. This is the basic Law Of Reflection due to which images are formed. When the reflected ray reaches our eyes, we are able to see the image in the plane mirror.
Now, the question arises that if we are able to see the image due to the reflected ray of light, then why can’t we see our image in the wall or a paper or any other such surface, as all surfaces reflectlight– somemore & some less?
The surface of the plane mirror is smooth and there is a silver coating at the back to make the plane mirror shiny and this is protected by red paint to not allow the light to pass through the mirror. Very shiny surfaces like mirror reflect all the light. Here, the Incident Ray is sent back as Reflected Ray in a definite direction ( called Regular Reflection) & the angle of incidence is exactly equal to the angle of reflection, so image is formed on such a surface.
On the other hand, the surface of the wall or paper is rough, composed of several tiny particles. Each tiny particle disperses the Incident Ray in different directions (called Irregular or Diffuse Reflection), so no images are formed on such a surface.
Examples of smooth surfaces are- plane mirror, still water, polished metal surface etc.
Examples of rough surfaces are- paper, wall, stone, wood etc.
Is the image formed in a plane mirror- Real or Virtual (unreal)?
The image formed in a plane mirror is virtual (unreal) because it cannot be obtained on the screen. It appears to be formed behind the mirror. So, it’s just an illusion. If we extend the reflected rays backwards (shown by dotted lines) behind the mirror, they will meet at a point, that will mark the position/ location of the image so formed of the object.
On the other hand a real image can be obtained on the screen. For example- the images on cinema screen shown by a projector are Real Images.
How is a Virtual image different from a Real image?
• A virtual image is unreal or an illusion and can’t be formed on the screen.
• A virtual image is ‘erect’ ie. top of the object is the top in the image and bottom of the object is the bottom in the image.
• A virtual image is at the same distance behind the mirror as the object is in front of the plane mirror.
• A virtual image shows ‘lateral inversion’ ie. the left side of the object becomes the right side of the image and vice-versa.
• A virtual image is exactly the same size as the object.
If several Incident Rays come out of an object, why do we take into consideration only 2 Incident Rays while making a ray diagram to showimage formation?
This is done purely for the sake of convenience and clarity. We consider/take 2 Incident Rays from the two extremities of the object (top & bottom) and correspondingly show their 2 Reflected Rays.
What is ‘angle of incidence’ and ‘angle of reflection’?
If you draw a perpendicular (called Normal) to the surface of plane mirror, the angle formed between incident ray and the Normal is called the ‘angle of incidence’ and that formed between the reflected ray and the Normal is called the ‘angle of reflection’.
According to the law of reflection, these 2 angles should be equal ie. angle of incidence should be equal to angle of reflection.
Q1. How are we able to see the image of another object in the plane mirror even when we are not standing in front of the mirror?
Ans. If the reflected rays from the mirror (originating from the object as incident ray) reach our eyes, we are able to see the image of that object in the mirror, even if we are not standing in front of the mirror or we are away or at an angle from the mirror.
Q2. How do we see objects in the universe?
• The object should either be luminous or be illuminated by a source of light (like sun, bulb, tube-light, candle etc.).
• The reflected ray from the object reaches our eyes and pass through the eye- lens and an image is formed on the retina (screen of the eye). This image is ‘inverted’ which is perceived and corrected by the brain by sending signals through the optic nerve.
Q3. Is the photograph a real or a virtual image?
Ans. The photograph is a real image as it is obtained on the screen- the photographic paper, in this case.
Q4. Does the photograph image show ‘lateral inversion’?
Q5. Will the movie (images) taken by a movie camera (Handy cam) be called real image or virtual image?
Ans. Real image
Q6. Why is an incident ray or a reflected ray shown as a straight line with an arrow?
Ans. Because light travels in a straight line and the arrows indicate the direction of the incident ray and the reflected ray respectively.
Applications Of Reflection In Plane Mirror
• We can see our image (same size and erect) in the mirror and so it helps us in viewing ourselves, doing make- up, combing hair etc.
• Plane mirror (made slightly convex) is used as a rear view mirror in vehicles to view the vehicles coming from behind.
• You can view a person, who is in another adjoining room, if you have a mirror in your room and the connecting door is just open enough to let an incident ray from the person (object) come in.
• Plane mirror is used in jewellery shops and other showrooms to make the shop look bigger and glittering / loaded with stuff.
• A parallel arrangement of two plane mirrors can be used to make a Periscope. (Periscope is a device that gives a higher view than normal; used in submarines etc).
Home Assignment For Readers
Find the anomalies in the following ray diagrams:-
As a student, I found it very difficult to memorise the Periodic Table Of Elements in Chemistry course.
Periodic table has been very cautiously designed, considering or arranging all the elements, that exist on the earth, in the increasing order of their Atomic Numbers.
Elements have been recognised ROW-WISE which are called PERIODS (1 to 7). GROUPS 1 to18 have been marked COLUMN- WISE also. Among these, a few Groups show many common properties like the Boron family, Carbon family, Nitrogen family, Oxygen family, Fluorine family, Inert gases etc.
I have tried to figure out the easiest way of sequential remembering of the “names of elements” in the Periodic Table, which I found to be the “Mnemonic” technique.
The beauty of the Mnemonics that I have created is as follows :-
• They make minimal use of helping verbs, prepositions, articles, etc.
• They mostly include words (in Mnemonic) which follow the sound of the element (word) in consideration.
• Common names (wherever approved) of elements have been considered like iron, copper, gold, silver, etc.
• As far as possible, at least the first two letters of the words used in Mnemonics are the beginning letters of the elements ; somewhere even three letters match with the names of the elements.
• Use of names of ‘animals’ in Mnemonics make them interesting to read. At the same time, it aims at Multi- disciplinaryapproach by connectingChemistrywith Biology.
Conventionally, we feel that a child should look exactly like his parents since he inherits 50% of his genetic material from father and 50% from his mother. The siblings, too, should look alike. But the reality is that, you the child may look quite like his parents, somewhat similar, or totally different. The same holds true of siblings (except for monozygotic/ identical twins). This all happens, or depends on the mixing of genes, that is passed on to the child in case of sexual reproduction.
Now, before we move further we should know what genes are?
Genes are small parts of DNA that encode for a specific protein. (These Proteins carry out specific function as was described in the previous blogs). Genes are present on structures called Chromosomes which are present in the nucleus of the cell. There maybe 25,000 to 35,000 genes in a single human cell. These Genes make up the hereditary material where each gene is responsible for the expression of a particular trait. Genes are present in pairs, one each on the corresponding homologous chromosome pair. From the pair, one gene comes from the mother and the other from the father.
From this knowledge, an obvious question comes to our mind- what are chromosomes?
A chromosome is a thread like structure or a strand which bears the genes. It is composed of a single DNA double helix molecule (the double helix is held together by complimentary base pairs as was described in one of the previous blogs). Instead of having one long piece of DNA, our DNA is broken into 23 pairs of shorter pieces called chromosomes. Of the 23 pairs, 22 pairs of chromosomes look the same in both males and females and they are known as AUTOSOMES . The 23rd pair differs in male and female and so is known as the Sex Chromosome pair. The Sex Chromosome pair is XX in a female and XY in a male. In females, one X chromosome is inherited from the mother and the other X chromosome is inherited from the father. In males, the X chromosome is inherited from the mother and the Y chromosome is inherited from the father.
The Chromosomes including the sex chromosomes are present in all the cells, be it somatic or reproductive cell.
How the sex cells are different from the other cells of the body in the type of cell division methods?
The somatic cells other than the sex cells undergo a cell division called MITOSIS (equational cell division) by which new cells are formed (each daughter cell receives a full set of chromosomes from the parent cell ie. diploid condition) from the old cells which have a fixed lifespan. Due to Mitosis, damaged body cells are constantly repaired and replaced. So, in Mitosis, the chromosome number is maintained at 46.
The sex sells undergo another type of cell division called MEIOSIS (reductional cell division) apart from Mitosis. In males, MEIOSIS I followed by MEIOSIS II, occurs at the time of Puberty and as a result of this, 4 haploid Sperms (having half the number of chromosomes from parents cell) are produced from each germ cell.
In females, MEIOSIS I occurs during the embryonic stage, during which a fixed number of germs cells are produced. MEIOSIS II occurs on maturation, when 1 haploid Egg (and 3 Polar bodies which are discarded) is released during each Menstrual cycle.
What are homologous chromosomes?
Homologous chromosomes occur in pairs.
A pair of homologous chromosome contains one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization (ie. zygote formation).
A pair of homologous chromosomes carry the same genes, one from each parent.
The two X chromosomes are considered homologous, whereas the X and Y chromosomes are not considered homologous.
The following diagram explains well about the homologous chromosomes and the sister chromatids :-
What are non- sister chromatids?
Following the DNA replication, now each chromosome is composed of two DNA molecules. The two identical copies of chromosomes are called chromatids. So, now each homologous chromosomes pair contains four chromatids.
Non-sister chromatids are the chromatids of the homologous chromosomes pair.
Where does mixing of genes occur?
Mixing of Genes occurs in Prophase I Stage of MEIOSIS I in both males and females through a process of Crossing Over and Recombination between non-sister chromatids of homologous chromosomes.
How Crossing Over occurs?
Crossing over is basically a phenomenon of breaking, transposition and fusion of chromosomal segments.
The non-sister chromatids first break at certain points due to the activity of enzymes called endonucleases.
Then the broken segments get exchanged and finally get fused with the other chromatid due to the action of the enzyme called ligase.
A little amount (3%) of DNA synthesis occurs during the time of crossing over to repair the broken chromosomes.
The resultant cross after crossing over is called Chiasma or Chiasmata.
Result of Mixing of Genes/ Random chance splitting of genetic material :-
• The spermatozoa (sperms) so formed (after Meiosis II), contain haploid number of chromosomes ie. only one set of chromosomes, and are comprised of Recombinant Chromosomes (due to mixing of genes or crossing over). So,this set of chromosomes/ genes is unique to that particular sperm.
• Similarly, the ova (eggs) so formed (after Meiosis II), contain haploid number of chromosomes ie. single set of chromosomes, which are comprised of Recombinant Chromosomes (due to crossing over). So, this set of chromosomes/ genes is unique to this particular egg.
• Here, it’s worth mentioning that the different sperms and different ova produced by individuals, have different genes due to random chance separation of genetic material. So, siblings who share the same parents, may have little similarities or a lot of dissimilarities. If the siblings look alike, the mix of genes they inherited is similar.
When this ‘unique sperm’ (haploid) fertilizes the ‘unique egg’ (also haploid), a ‘zygote’ is formed which is diploid with two sets of chromosomes- one from sperm and the other from egg. So, this zygote gives rise to a ‘unique individual’.
Significance of Crossing over and Recombination
Meiosis cell division, involving Crossing over and Recombination of chromosomes, lead to segregation of chromosomes and accounts for Genetic Variations, which is the most important evolutionary benefit of sexual reproduction.
Search, explore and find out, the many interesting mysteries, heredity and genetics unfold…
The most tangible way to comprehend science concepts is ‘learning by doing’.
It has been very well said by Confucius –
I hear and I forget. I see and I remember. I do and I understand.
The pragmatic approach for learning science rudiments is by conducting experiments, starting from simpler Hands-on to Lab. experiments.
So, the Motto of science learning becomes –
“Experimentation, Experimentation and Experimentation“.
Experiments are a part and parcel of science learning. Many so-called difficult concepts can be subdivided into simpler rules, derived from previous knowledge, observations and logic. Then, based on previous knowledge/ observations, a chain of experiments can be developed to arrive at the BIG, FINAL Concept/ Rule.
Experiments are hands-on activities every child enjoys and performs with great delight and interest.Even the weakest child feels enthused and motivated when he experiments with his own hands. Here, he uses his own intellect and creativity and sees things happening in the way written in the text books. Such a hands-on activity has an indelible mark on his cognition.
Science concepts which are dependent on a number of factors or conditions can be easily taught and explained by CONTROLLED EXPERIMENTS technique.
In ControlledExperiments, all conditions are kept CONSTANTexceptfor the one which you want to test/ verify.
The ADVANTAGEof a ControlledExperiment is that you can eliminate much oftheUncertainty about yourResults.If youcouldn’tcontroleachfactor, you might end up with a confusingoutcome.
Learning Controlled Experiment Method With An Example
We want to find out which conditions are required for seed germination.
We know from our previous knowledge/ observations that when we sow a seed in the soil, we give it the following things or conditions:-
Now we need to devise an experiment in which we keep controlling one condition at a time which we want to verify, and keep all the other conditions Constant.
Let’s verify each condition one by one–
BOWL 1: We control the factor ‘soil’ by eliminating it and germinating seeds on moist cotton(to provide water). Other conditions ie. air, water and warmth are kept constant.
BOWL 2 : We control the factor ‘water’ by germinating seeds on dry cotton (to eliminate water). Other conditions ie. air and warmth are kept constant.
BOWL 3 : We control the factor ‘air’ by keeping the cotton containing seeds immersed in water (to cut off the air supply). Other conditions ie. water and warmth are kept constant.
BOWL 4 : We control the factor ‘warmth’ by putting the seeds on moist cotton in a refrigerator.So, other conditions ie.air and water are keptconstant.
After a few days we observe the following results :-
BOWL 1 : Seedlings develop; so soil is not necessary for seed germination.
BOWL 2 : No seedlings develop; so ‘water’ is an essential requirement for seed germination.
BOWL 3 : No seedlings develop; so ‘air’ is an essential requirement for seed germination.
BOWL 4 : No seedlings develop; so ‘warmth’ is an essential requirement for seed germination.
Conclusion- Since no seedings develop in Bowl -2, 3 & 4, so we infer that water, air and warmth are the three essential factors/conditions for Seed Germination.
Extrapolation – Since seedlings develop in Bowl 1, we infer that soil is not required for seed germination. This means that seeds can germinate without soil also.This is because, in the initial stages of growth/germination, the embryo utilises the food/ nourishment stored in seed cotyledons/ endosperms to giverise to the baby root (radicle) and the baby shoot (plumule).By the time, the first few leaves appear, the food stored in the cotyledons/endosperms gets exhausted and now the leaves make food for the plant. Also, by this time the seedlings should have been planted in the soil forfurthergrowth.
Some examples of science concepts which can be better understood by Controlled Experiment technique
1) ‘Buoyancy’ which depends on factors like volume of the object and density of the liquid
2) ‘Evaporation’ which depends on factors like surface area and temperature
3) ‘Photosynthesis’ which depends on factors like presence or absence of chlorophyll, carbon dioxide, and sunlight
4) ‘Rate of a chemical reaction ‘ which depends on factors like amount of reactants, temperature, presence or absence of catalyst
Ponder a while and get going …it’s simple & fun!
STAGE 4 SEED GERMINATION (HOME GROWN) IMAGE
Home grown Pinto bean seeds (on moist cotton in a bowl ) showing Stage 4 of Seed Germination. It shows Developing Seedling with coming out of First leaves and Epicotyl leaving Dried Cotyledons below........(.images for Stages 1, 2 & 3 also available immediately )