Light - Reflection

Reflection

In this section, we will focus on light energy and how it interacts with the World around us. Light moves as a wave in straight lines at a very high speed (300 million ms^-1).

Humans use light energy to 'see', it is detected by special cells in the back of the eye in a part called the retina. In order to see an object, the object must either emit (give out) light or reflect light.

Reflection

When a wave 'hits' an object, the wave can change direction and 'bounce off' the object. This is known as Reflection.

There are two types of reflection:-

1. Mirror Reflection - If an object has a very smooth surface, then all of the light is reflected in the same way, making a mirror Image.

2. Diffuse Reflection - If the object has a rough surface, then each ray of light will be reflected in different directions, scattering the light. This is how most objects reflect light.

The Law of Reflection

When light hits a mirror, the ray of light is reflected from its surface. The diagram below shows an example of this:-

Where:-

• • • θ_i = Incident angle (angle going in) in degrees.

    • θ_R = Reflected angle (angle coming out) in degrees.

In Optics, all angles are measured from the same place, a line called the Normal. This is a line at a right angle to the surface of the mirror. Never measure an angle from the mirror itself.

By experiment, we can show that:-

" For a mirrored surface, the angle of incidence of a ray of light is equal to the angle of reflection."

This is known as the Law of Reflection.

Curved Reflectors

If the mirror is curved instead of flat (plane), then an unusual effect can be seen. Each part of the curved surface acts as a tiny plane mirror, following the Law of Reflection. With the right shape, all of the light hitting the mirror will be focussed to a single point.

This is how satellite dishes work. By reflecting the signal all to the same point, the signal will be made stronger:-