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 reflect light– some more & 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 show image 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).