Light waves reflect from surfaces. When waves reflect, they obey the law of reflection: the angle of incidence equals the angle of reflection.
- The normal is a line drawn at right angles to the reflector.
- The angle of incidence is between the incident (incoming) ray and the normal.
- The angle of reflection is between the reflected ray and the normal.
Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images. In the plane (flat) mirror, the image appears to be behind the mirror.
Light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.
There is one special case you need to know. Refraction doesn't happen if they cross the boundary at an angle of 90° (called the normal) - in that case, they carry straight on.
Total internal reflection
Beyond the critical angle
Waves going from a dense medium to a less dense medium speed up at the boundary. This causes light rays to bend when they pass from glass to air at an angle other than 90º. This is refraction.
Beyond a certain angle, called the critical angle, all the waves reflect back into the glass. We say that they are totally internally reflected. Have a go at the animation to check your understanding of this.
All light waves, which hit the surface beyond this critical angle, are effectively trapped. The critical angle for most glass is about 42 °