Reflection and Refraction Model

Angle of incidence i40°

Object distance from the mirror 4.0 cm

light ray normal where rays seem to come from angle of incidence angle of reflection or refraction critical angle glass water

Lengths and angles on the paper are to scale (1 square = 1 cm). Angles are drawn true size when you look straight down at the paper: ⟲ resets the view. Not to scale: the ray box is drawn about 3 times smaller than a real one, and the optical fibre is drawn about 200 times thicker than a real one (a real fibre is about 0.1 mm across). The thin plastic walls of the water containers are left out. The brightness of each ray is only a rough guide.

How to write it in the exam

The normal is a line at 90° to the surface at the point where the ray meets it. The angles of incidence, reflection and refraction are all measured between the ray and the normal.
For reflection, the angle of incidence is equal to the angle of reflection.
The image in a plane mirror is virtual, the same size as the object, and the same distance behind the mirror as the object is in front of it.
Light refracts because its speed changes. Going from air into glass it slows down and bends towards the normal. Going from glass into air it speeds up and bends away from the normal.
Refractive index is the ratio of the speeds of light in two regions. n = sin i / sin r, where i is the angle in air and r is the angle in the material.
The critical angle is the angle of incidence for which the angle of refraction is 90°. n = 1 / sin c.
Total internal reflection happens when light travels from glass or water towards air and the angle of incidence is greater than the critical angle. All the light is reflected.
In an optical fibre, light hits the side at more than the critical angle, so it is totally internally reflected. This repeats all along the fibre. Fibres carry data as pulses of light in telecommunications.