Reflection and refraction · GCSE Physics

What can happen at a boundary

When a wave meets a boundary between two different materials, it can be reflected, refracted (transmitted with a change of direction), transmitted without changing direction, or absorbed.
What happens depends on the wavelength of the wave and the materials.

The angle of incidence equals the angle of reflection. Both are measured from the normal: a line at right angles to the surface.
A smooth surface, such as a mirror, gives specular reflection in a single direction. A rough surface scatters light in many directions (diffuse reflection).

Refraction is the change in direction of a wave when it passes from one material into another at an angle, because its speed changes.
When light enters a denser material such as glass, it slows down and bends towards the normal. When it leaves, it speeds up and bends away from the normal.
A wave travelling along the normal (at 90° to the surface) changes speed but not direction.
(Higher) On a wave front diagram, the part of the wave front that enters the new material first changes speed first, so the wave front turns.

When light travels from glass or water towards air, it speeds up and bends away from the normal.
The critical angle is the angle of incidence at which the refracted ray just runs along the boundary (the angle of refraction is 90°). For glass it is about 42°.
If the angle of incidence is bigger than the critical angle, all the light is reflected back inside: total internal reflection.
Optical fibres and endoscopes use total internal reflection to carry light along the fibre, even round bends.

Shine a ray of light from a ray box at a surface or a glass block on paper, and trace the rays with a pencil.
Draw the normal at the point where the ray hits the surface and measure the angles of incidence, reflection and refraction with a protractor.
Repeat for different angles and different materials. Carry this out in a darkened room with a narrow ray for accuracy.

Reflection: When a wave bounces off a surface.
Refraction: The change in direction of a wave as it passes into a different material at an angle, caused by a change in speed.
Normal: A line at 90° to a surface at the point where a ray meets it.
Angle of incidence: The angle between the incoming ray and the normal.
Angle of reflection: The angle between the reflected ray and the normal.
Angle of refraction: The angle between the refracted ray and the normal.
Specular reflection: Reflection from a smooth surface in a single direction.
Diffuse reflection: Reflection from a rough surface that scatters light in many directions.
Critical angle: The angle of incidence at which light leaving a denser material travels along the boundary.
Total internal reflection: When all the light is reflected back inside a material because it meets the boundary at more than the critical angle.