The Earth and Universe

Characteristic features

	Description	Ray diagram	Wavefront / Ripple tank diagram	Notes
Reflection	Waves bounce back from a boundary so that the angle of incidence equals the angle of reflection			There is always some reflection.
Refraction	Waves bend at a boundary so that the angle in the 'slow medium' is less than the angle in the 'fast' medium.			There is refraction only if the application of n=sinq_air/sinq_medium does not lead to the sin of an angle being greater than 1, and if the wave has different speeds in the two media. The critical angle is on the cusp of what is and is not possible. In this case q_airis 90^o, q_medium is called the critical angle and n=1/sinq_critical. Refraction is caused by the light travelling at different apeeds on the two media. It turns out that n=c_air/c_medium, where c stands for speed.
Diffraction	Waves bend as they pass through relatively narrow openings, tending to spread out in all direction as they do so.			The emerging wave is semicircular (i.e. it bends all the way round) if the wavelength is equal to the size of the opening or obstacle. Shorter wavelengths bend less, and look more like the traditional 'light travels in straight lines' scenario.
Interference / Superposition	Not required at GCSE level
Polarisation	Not required at GCSE level

General Notes

'Rays' are lines perpendicular to 'wavefronts', and indicate the direction in which the wavefront travels.
All angles are measured between the ray and the normal (a line perpendicular to the surface, starting from the point where the ray hits the surface).