Mirrors

When a light source illuminates a perfectly diffused surface, it usually appears bright in all directions. Meanwhile, a polished surface will tend to appear illuminated only in one direction for a given position of an observer. 

So, the phenomenon due to which illumination takes place on a diffused or polished surface is known as a regular reflection of light. Moreover, the surface on which the reflection of light takes place is known as a mirror. 

Mirror Definition in Physics

In physics, a mirror is defined as a surface that reflects nearly all kinds of light incidents on it. Moreover, a mirror will keep on reflecting light in a regular fashion until or unless an opaque object is introduced between the reflecting surface and the source of light. 

Working of Mirrors 

When light hits a mirror surface, it reflects every colour of the spectrum. As a matter of fact, mirrors work on the principle of reflection. 

We know that the light either gets reflected or absorbed. In the case of a mirror, light is always reflected. The reason why the light gets reflected when it falls on a mirror is that at the microscopic level, it is a smooth surface.

In the case of a rough surface, it is reflected back in all directions and this is known as diffuse reflection but for smooth surfaces, it gets reflected in one single direction and is known as specular reflection. So, in the case of a mirror, it is a specular reflection. 

Terms Related to Mirrors 

• Incident Ray: The ray of light that hits the surface. 

• Reflected Ray: The ray of light that gets reflected after getting hit on the surface. 

• Normal Ray: The ray of light that is perpendicular to the reflecting surface.

• Refracted Ray: When the incident ray passes through a second medium, it is known as a refracted ray.

Types of Mirrors

There are different types of mirrors in physics. Mirrors can be broadly classified as plane mirrors, rotating mirrors, inclined mirrors, and spherical mirrors. Moreover, spherical mirrors can be further classified into two types, i.e. a concave spherical mirror and a convex spherical mirror.

Furthermore, this classification is explained with an illustration given below.

Before we carry on with our topic on Mirrors, brush up your memory on these essential pointers in the Q&A section discussed below.

Test Your Knowledge

1. What do you mean by a mirror image?

Ans: When light is incident on any type of mirror from a particular source, it is not only reflected, but it also creates a virtual image inside the reflecting surface. This virtual image, thus formed, is known as a mirror image. The features of a mirror image depend primarily on the type of mirror used. 

2. What is an incident ray and a reflected ray? 

Ans: A light ray, which travels from a source to a reflecting surface, is known as an incident ray. 

On the other hand, a ray of light which travels from a mirror to the image, formed due to reflection, is known as a reflected ray.

3. How do mirrors reflect light?

Ans. In the case of an ordinary house mirror, when a ray of light falls on its surface, the silver molecules behind the glass get excited by absorbing the photons. This makes the atoms unstable, so they tend to become stable again by reflecting those photons in the opposite direction. 

As you get to know about how many types of mirrors are there in optics, you should now also understand the detailed explanation on the important types individually. 

Plane Mirror

For example, let us consider a ray of light is incident on a plane mirror at a point. Regular reflection takes place, and an image is formed at point B. 

Now, the angle between this incident ray and the normal is the angle of incidence. Consequently, the angle between the normal and the incident ray is known as the angle of reflection. 

As per the Laws of Reflection of light, we can conclude that –

• The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. 

• The angle of incidence is the same as that of the angle of reflection

Example:

Any regular reflecting material which has a polished surface and is not curved in nature is an example of a plane mirror, like household mirrors, looking glass, etc.

Spherical Mirror

A spherical mirror can be defined as a portion of a hollow sphere whose one side comprises a reflecting surface whereas the other side has been silvered. 

As discussed earlier, there are two types of such mirrors which are –

• Concave Mirrors

A concave mirror is a type of spherical mirror which has a reflecting surface that is located inwards at the centre of the sphere. A concave mirror is that whose outer walls have been silvered and the inner surface is used for reflection.

• Convex Mirrors

In the case of a convex mirror, this reflecting surface is situated away from the center of the sphere. A convex mirror is that whose inner walls have been silvered while the outer surface is used for reflection.