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:-