Written By Anum
Last Modified 27-01-2023

Plane Mirror: Properties, Definition, Focal Length & Diagram

Plane Mirror: Every individual makes use of mirrors on a regular basis. Plane mirror can be described as a flat surface that produces a visually erect image of any real object that has its back and front reversed. What exactly do we see in the mirror? As we stand in front of it, we see ourselves in the mirror, along with other things in our background. Is there any change in our height as seen through a mirror? Clearly no. We appear as tall as we are in the mirror. Is there any change in our width when seen through a mirror?

Obviously no. We appear as wide as we are in the mirror. So can we say that through a mirror, we see ourselves exactly as we are? Well, the answer again is no. To notice a difference, raise your left hand in front of the mirror. Does the image in the mirror repeat your action? It does, but we see the right hand being raised instead of the left in the mirror. What causes this reversal? How are these images formed in a mirror? To learn about this in detail, read further.

What is a Plane Mirror?

A plane mirror is a piece of a flat reflective surface. A plane mirror is formed by adding a layer of silver nitrate or aluminium behind a piece of flat glass. It reflects the light falling on its surface based on the two laws of reflection, i.e. angle of incidence is equal to the angle of reflection, and the reflected ray, the incident ray and the normal to the surface all lie in the same plane.

What is an Image?

Rays that come from a point object may interact with an optical medium and is referred to as diverse or converge. The image is referred to as a real image if the ray converges from a point of convergence. Similarly, the image is referred to as a virtual image if the rays are diverging and they appear to diverge from any specific point. The image forms at infinity when the rays go parallel after interacting.
Real Image: A real image is formed by the actual intersection of light rays after interacting with an optical medium. Real images can be taken on a screen.
Virtual Image: A virtual image is formed when light rays do not actually meet but appear to meet at a given point. Virtual images can not be taken on-screen.

Image Formed by a Plane Mirror

Plane mirrors surround us. From our homes, offices and schools, they are used for multiple purposes, and they come in varying shapes and sizes. The light falling on a plane mirror may be absorbed, reflected or transmitted. Although the mirrors are polished with mercury, most of the light falling on its surface is reflected. The light rays coming off from an object strike at the surface of the mirror and get reflected. At the point where the reflected rays meet (or appear to meet), the image of the object is formed. For the formation of an image, we need at least two light rays coming from an object.

How is an Image Formed?

To understand how the image of an object is formed, consider the diagram shown below:

Consider the mirror kept at point \(C\), where \(BF\) represents the principal axis. An object \(AB\) is kept in front of the mirror at a distance \(d\) from the mirror. Let the height of \(AB\) be \(h\). As we can see from the diagram, three rays start from \(A\) and travel along different paths to form an image behind the mirror at \(E\). Consider the first ray, represented by \(1\), travelling horizontally, and it strikes the mirror normally and hence retraces its path after reflection.

Ray \(2\) strikes the mirror at point \(C\) at an angle as shown and gets reflected at the same angle. Ray \(3\) strikes at mirror point \(D\) at an angle and similarly gets reflected at the same angle. On producing these three rays behind the mirror, they meet at point \(E\). Thus, we get an image \(E\) of the point object \(A\).

Similarly, we can get images of all points of the object to form the image \(EF\). Here, \(CF\) is the same as the distance \(BC\). Therefore the image is formed as far behind the mirror as the object is kept in front of it. Also, Since the rays do not actually meet, they appear to meet at a point behind the mirror. Thus, the image formed by a plane mirror is virtual.

What is the Focal Length of a Plane Mirror?

Focal length is the distance between the mirror and its focal point. It is equal to half of the radius of curvature. Parallel rays hitting a mirror will form an image at the focal point. For a plane mirror, an image of parallel rays is formed at an infinite distance inside the mirror. The focal length of a plane mirror is infinity. A plane mirror can be considered a spherical mirror with an infinite radius of curvature. Thus, the focal length of the plane mirror will also be infinite.

Characteristics of the Image Formed by a Plane Mirror

It is a virtual image.
It is erect.
It is of the same size as the object.
The distance between image and mirror is equal to the distance between object and mirror.
It travels as fast towards or away from the mirror as the object.
It is laterally inverted.

What is a Lateral Inversion?

The image formed by a plane mirror seems to be inverted left to right and not top to bottom. Thus, lateral inversion is the phenomenon of reversal of mirror image where the right side of the object appears on the left side behind the mirror, and that is why when we raise our right hand in front of a plane mirror, our image appears to raise its left hand.

Multiple Reflections

What will happen if two mirrors are kept in front of each other, and an object is kept in the middle of the two mirrors? Both the mirrors will form an image of that object. If these mirrors are kept so that they are in each other’s field of view, then the image formed by the second mirror will act as an object for the first mirror and form an image of the same, and the same will happen for the second mirror. If the two mirrors are kept parallel, we will get infinite images of the object. Barbers use this phenomenon to help us see the hair at the back of our head after a haircut or in trial rooms at malls and children’s amusement parks.

Applications of Plane Mirrors

Mirrors are immensely useful to us. These are used in periscopes and kaleidoscopes, solar cookers, dressing mirrors, etc. Let us discuss some of the uses of mirrors in detail:

a. Periscopes and Kaleidoscopes

Plane mirrors are used in periscopes and kaleidoscopes. In kaleidoscopes, mirrors are used along with coloured patterns, which reflect colourful patterns that are beautiful to look at. Periscopes installed in submarines reflect the images of ships present on the sea surface and thus, help the submarine drivers navigate underwater.

b. Dressing Mirrors

All our day to day activities like combing our hair, putting on our makeup, setting our clothes involve using mirrors.

c. Dental Uses

Plane mirrors are used by the dentist while performing any examination to see images of teeth. In microscopes, these mirrors are used for reflecting the image of the object which is being monitored.

d. Solar Cooker

A plane mirror reflects most of the light falling on its surface, and that is why these are widely used in solar cookers. The light after reflection from the mirror helps in cooking the food. A solar cooker is a really efficient and renewable way to cook food.

e. For Security and Safety Purposes

Plane mirrors help in the detection of explosives while looking for them underneath a vehicle. These mirrors are installed in shops to keep an eye on customers. Such mirrors are also installed at blind turns of busy roads to see the incoming traffic from the other side to avoid any accidents.

Summary

The light rays coming off from an object strike at the surface of the mirror and get reflected. At the point where the reflected rays meet (or appear to meet), the image of the object is formed. For the formation of an image, we need at least two light rays coming from an object. The image formed by a plane mirror is virtual, erect, of the same size as the object and laterally inverted. The distance between image and mirror is equal to the distance between object and mirror. If two or more mirrors are kept close to each other, we can get multiple images of a single object, placed in between the two mirrors.

FAQs on Images Formed by a Plane Mirror

Let’s look at some of the commonly asked questions about image formed by a plane mirror:

Q.1. What are the characteristics of an image formed by a plane mirror?
Ans: The image formed by a plane mirror is virtual, erect, of the same size as the object and laterally inverted.

Q.2. What is the focal length of a plane mirror?
Ans: The focal length of a plane mirror is infinite.

Q.3. Write a few uses of the image formed by a plane mirror.
Ans: A plane mirror is used in periscope, kaleidoscopes and in solar cookers etc.

Q.4. If the distance between object and mirror is \(d\) what will be the distance between object and image?
Ans: The distance between object and image will be \(d + d = 2d.\)

Q.5. What is the magnification of a plane mirror?
Ans: Since the height of the image is the same as the height of the object and magnification, \(m = \frac {\rm{height}\;\rm{of}\;\rm{object}}{\rm{height}\;\rm{of}\;\rm{image}}\)
Thus, the magnification of the plane mirror \(+1\).

Q.6. If the height of the image formed by a plane mirror is \(s\). Can you determine the size of the object?
Ans: Since the image is of the same size as the object thus, the size of the object will be \(s\).

Now that you have a detailed article on Image Formed by a Plane Mirror, we hope you study well. If you get stuck somewhere do let us know in the comments sections. We will get back to you at the earliest.