 × ## Introduction

Light is an extraordinary entity of nature that makes things visible. It behaves both as a wave as well as a particle, which has perplexed scientists since centuries. However, there have been recent developments in the concepts of quantum mechanics. This has allowed us to develop a better understanding of light.

Reflection and refraction are phenomena associated with the wave nature of light and have no analogue for particles. Reflection refers to the bouncing of light upon hitting certain surfaces, while refraction is a change in path that occurs when light waves traverse across different media.

## What Is Reflection?

Light can behave in three ways upon hitting a surface. It can pass through unaltered, be absorbed, or be reflected towards a different path. The last of these processes is known as reflection. During reflection, light rays change direction and bounce back. Note that it is not necessary for the wave to retrace the path it took. It can take a different path as well but there are certain rules which must always be satisfied. These laws are known as the laws of reflection and are universally valid.

## Types Of Reflection

1. Specular reflection

Imagine looking into a plain mirror, the kind you see in washrooms. The image you see is sharply focussed, intense, and not distorted. Even with curved mirrors, the intensity and focus of the image aren’t modified. Such a type of reflection wherein all light is reflected uniformly is known as specular reflection. Here, the image isn’t broken or jagged.

2. Diffused reflection

When the reflecting surface isn’t smooth and instead, non-uniform, the image appears broken and jagged. Some light is reflected in one direction while other goes in a different direction or isn’t reflected at all. This leads to diffused reflection and the image isn’t sharp and clear.

## Refractive Index

The refractive index is a physical quantity that measures the amount by which, light rays slow down while traversing through a medium. It is a dimensionless quantity whose value can never be less than one.

Light in free space travels with a constant velocity of 3E8 m/s. When propagating through a medium, it slows down ever so slightly and the amount of this “slowing down” is measured by the refractive index. Mathematically, the refractive index is given by: Here, v is the velocity with which light travels in the given medium.

## Laws Of Reflection

1. During reflection, we have an incident ray and a reflected ray. We also have a surface normal and all these three lines lie in the same plane.

This can be understood via the diagram below: Law of reflection

Suppose that in the diagram, an imaginary mirror is kept along the X-Y plane. A light ray incident on it gets reflected. Since the mirror is perpendicular to the Z-axis, the Z-axis forms the surface normal here.

If you look closely, the incident and reflected rays as well as the surface normal all lie in the same red, translucent plane represented in the image. In this particular case, it happens to be the X-Z plane. This is the first law of reflection.

2. The angle of incidence and the angle of reflection are equal

When you look into the mirror from an angle, you can see objects that are on the side opposite to the one you are standing in. This occurs due to the second law of reflection. When measured from the surface normal, the angle of incidence and the angle of reflection are equal. The following diagram illustrates it. Whenever light is reflected, the relation is always satisfied. Normal plane

## Examples

1. Rear-view mirrors of cars.

We adjust the rear-view mirrors of our cars so that we can see the vehicles coming from behind. This is where the second law of reflection comes in useful since we don’t point the mirrors toward the cars. Instead, we adjust them so that the cars become visible.

2. Dentists’ instruments

Dentists use mirrors to change the path of light and see inside our mouths, which won’t be possible directly.

## Basic Terminology Used in Laws of Reflection

The following terminologies are commonly used while discussing the laws of reflection:

1. The incident ray

The ray of light coming from the object and hitting the mirror before getting reflected is known as the incident ray. For instance, light from your face hits the mirror and is reflected toward your eyes, allowing you to see an image of yourself in the mirror.

2. The reflected ray

A light ray reflected after an incident is known as the reflected ray and we can take the same example as before. When you see an image of yourself in the mirror, you are seeing the reflected ray.

3. The Surface Normal

The term “surface normal” refers to an imaginary line drawn at the point where the incident ray falls onto the mirror. This line is always at an angle of 900 with respect to the surface of the mirror, even if the mirror is curved. In such a case, the surface normal is perpendicular to the tangent drawn at the point of incidence. All angles are measured with respect to this line only.

4. Angle of incidence

The angle between the incident ray and surface normal is known as the angle of incidence and it can vary from zero to ninety degrees.

5. Angle of reflection

The angle between the reflected ray and the surface normal is referred to as the angle of reflection. For the case of reflection, it must always equal the angle of incidence.

## Summary

Reflection refers to the phenomenon of bouncing back light after hitting a surface and the intensity of the reflected light depends upon the smoothness and nature of the reflecting surface. Sometimes, it can also depend on the angle of incidence.

It is classified into specular and diffused reflections. Specular reflection forms clear and sharp images and is caused by smooth surfaces which reflect light uniformly. On the other hand, diffused reflection forms broken or jagged images and occurs when the reflecting surface is non-uniform.

1. How are the laws of reflection derived?

The laws of reflection can be derived by equating the electric and magnetic fields on either side of the boundary. This can be done since light is composed of electromagnetic fields.

Q2. What is meant by the dual nature of light?

In different situations, light behaves differently. Sometimes, it exhibits wave phenomena while other times, it shows particle nature. This is known as the dual nature of light.

Q3. What happens to the phase of light upon reflection?

The phase of the light wave changes by 1800 if the refractive index of the material behind the reflector is higher than the one in front. Otherwise, it remains unchanged.

Q4. What is total internal reflection?

Beyond a certain incidence angle known as the critical angle, when light falls on the interface of two media, it doesn’t penetrate the second one. Instead, it is reflected “internally” in the medium. This is known as total internal reflection. The value of this critical angle is given as:

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