# 0.1 Geometrical optics - grade 10  (Page 3/14)

 Page 3 / 14
Law of Reflection

The Law of Reflection states that the angle of incidence is equal to the angle of reflection.

${\theta }_{i}={\theta }_{r}$

The simplest example of the law of incidence is if the angle of incidence is ${0}^{\circ }$ . In this case, the angle of reflection is also ${0}^{\circ }$ . You see this when you look straight into a mirror. When a wave strikes a surface at right angles to the surface, then the wave is reflected directly back.

If the angle of incidence is not ${0}^{\circ }$ , then the angle of reflection is also not ${0}^{\circ }$ . For example, if a light strikes a surface at ${60}^{\circ }$ to the surface normal, then the angle that the reflected ray makes with the surface normal is also ${60}^{\circ }$ as shown in [link] . Ray diagram showing angle of incidence and angle of reflection. The Law of Reflection states that when a light ray reflects off a surface, the angle of reflection θ r is the same as the angle of incidence θ i .

An incident ray strikes a smooth reflective surface at an angle of 33 ${}^{\circ }$ to the surface normal. Calculate the angle of reflection.

1. We are given the angle between the incident ray and the surface normal. This is the angle of incidence.

We are required to calculate the angle of reflection.

2. We can use the Law of Reflection, which states that the angle of incidence is equal to the angle of reflection.

3. We are given the angle of incidence to be 33 ${}^{\circ }$ . Therefore, the angle of reflection is also 33 ${}^{\circ }$ .

## Types of reflection

The Law of Reflection holds for every light ray. Does this mean that when parallel rays approach a surface, the reflected rays will also be parallel? This depends on the texture of the reflecting surface.

## Specular reflection

[link] (a) shows a surface that is flat and even. Parallel incident light rays hit the smooth surface and parallel reflected light rays leave the surface. This type of reflection is called specular reflection . Specular reflection occurs when rays are reflected from a smooth, shiny surface. The normal to the surface is the same at every point on the surface. Parallel incident rays become parallel reflected rays. When you look in a mirror, the image you see is formed by specular reflection.

## Diffuse reflection

[link] (b) shows a surface with bumps and curves. When multiple rays hit this uneven surface, diffuse reflection occurs. The incident rays are parallel but the reflected rays are not. Each point on the surface has a different normal. This means the angle of incidence is different at each point. Then according to the Law of Reflection, each angle of reflection is different. Diffuse reflection occurs when light rays are reflected from bumpy surfaces. You can still see a reflection as long as the surface is not too bumpy. Diffuse reflection enables us to see all objects that are not sources of light.

## Experiment : specular and diffuse reflection

A bouncing ball can be used to demonstrate the basic difference between specular and diffuse reflection.

Aim:

To demonstrate and compare specular and diffuse reflection.

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