Plane Mirrors

Plane Mirrors - Types of Images formed, Laws of Reflection and Uses of Mirror

Plane mirrors can be found very easily in our daily life. Though an average person is not familiar with this technical term, they can easily relate that to the normal mirrors which are polished on one side with mercury so that they can reflect light falling on them. A plane mirror can be easily defined as the mirrors those are flat in surface and are without any inward or outward curve. They can easily reflect light in various directions, undergoing certain phenomenon namely reflection, refraction or absorption. In the plane mirrors, we require at least two rays through which the image of the object can be formed by observing the endpoints of the 2 light rays taken. The plane mirror can be polished with various materials that can be used for various purposes but all the mirrors function the same way regardless of their constructing material. According to their reflection of light, they can be classified into 3 types as concave, convex and plane mirror. In the case of a plane mirror, the angle at which the ray is reflected is equal to the angle at which the ray of light is incident, regardless of whether the image is real or virtual, where the angle of incidence is the angle formed at an imaginary surface which is normal to the mirror (which is perpendicular to the surface) at the point of incidence. While the angle of reflection can be defined as the angle formed at the point of contact of the reflected ray and the surface normal of the plane mirror. 

Image formation on plane mirrors

To see any image in the mirror a person should be in the line of sight of the mirror, as when the person is in the line of sight of the mirror then the reflected ray reaches that person’s eye through which anyone can see the image of the object if they are in the line of sight of the mirror, this is because light has the property of moving in a straight line. The important phenomenon of the plane mirror is that the reflections of the objects form the virtual image with the same magnification, size, and distance of the same as the object really are. The image of the object depends on the geometrical line through which a person sees in the mirror, though all the line of sight will provide the image of the object, as long as the person‘s eye is in line with the mirror. The image produced is on the opposite side of the mirror with the distance of the image from the surface is equal to the distance of the object from the surface, that is the reason why a person can see what’s exactly behind him.

There can be 2 types of an image formed by mirrors, which are:

1. Real Image
2. Virtual Image

The main difference between the real and virtual image is that in a real image the rays of light passes through the mirror while in a virtual image the rays of light strike the surface of the mirror and bounces back to the eye of the user.

Real Image

The real images of the objects are generally formed by the curved mirrors as in the curved mirrors the rays of light get reflected and pass through the mirror to form the real image. The real images formed by the curved mirrors are always inverted. Some examples of a real image are the image formed on the retina of the eye or the image formed on the film of the camera.

Virtual Image

The virtual image of the objects is generally formed by the plane mirrors as the plane mirrors are polished on one side so the reflection of the object strikes the mirror and get reflected towards the direction of the observer's eye. So due to this, the observer sees the image at the same distance as the object is from the mirror. The image formed by the plane mirror has the same magnification, size, and distance of the object. These virtual images of plane mirrors are not formed on the screen as that of a real image. Some examples of Virtual image are the image formed by a magnifying glass when used to look at the small objects.

Laws of reflection:

There are generally two laws of reflection which can be stated as follows:-

1. The angle of incidence (i) is always equal to the angle of reflection (r).
2. The reflected ray, the normal at the point of incidence and the incident ray lie on the same plane.

Properties of reflection in plane mirrors:

The magnification of the plane mirror is always 1, which is calculated by combining these properties of reflection. There are 3 things that can happen when the light ray falls on the surface of the mirror, which are as follows:

1. A fraction ‘r’ can be reflected.
2. A fraction ‘a’ can be absorbed.
3. A fraction ‘t’ can be transmitted.
For any given surface of a plane mirror, these above terms should add up to form 1.
I.e. r + a + t = 1.

Characteristics of plane mirrors:


1. Images formed by the plane mirrors are always virtual in nature.
2. Images formed by the plane mirrors are erect/ upright and is of the same size as of the object.
3. The image formed by the plane mirror is of the same size as the object.
4. An image formed by the plan mirror is of the same magnification as that of the object.
5. One of the main characteristics of the plane mirror is that the image formed by the plan mirror is inverted, that means if you raise your left hand then the image of the plan mirror will show the right hand going upwards.

Types of reflection:

There are two types of reflection in plane mirrors, which are:
1. Specular/ Regular Reflection
2. Diffused/ Irregular Reflection

Specular / Regular Reflection:

The Specular / Regular reflection, as the name says, provides the perfect image of the object without any distortion. We can simply say when the light strikes the smooth surfaces the ray of light gets reflected in the same direction, as each incident ray reflected along the reflected ray having the same angle as that of the incident ray. For e.g. the mountains covered with lakes, the image of the mountains shown in the lake is perfect as the lake is the smooth surface.

Diffused / Irregular reflection:

The Diffused / Irregular reflection is the type of reflection in which the light after striking the rough surface gets reflected in all directions, it includes any light which we can see through our eyes. In this type of reflection, the incident ray reflected along the reflected ray does not have the same angle as that of the incident ray. For e.g. when the light hits a bird which is a rough surface, the reflected light scatters in all direction, when reaches our eye and hits the retina it gets processed in the brain from an electrical signal to form an image of a bird.

Uses of plane mirrors:


1. Seeing around corners
2. Dental mirrors
3. Periscope
4. Flat Telescope
5. Illuminating Egyptian tombs
6. Overhead projectors
7. Kaleidoscope
8. Optical lever