Reflection, Refraction and Diffraction

Three things a wave does when it meets a boundary or an obstacle. All three apply to every wave, not only to light.

Reflection — the wave bounces off a surface.

The law of reflection:

angle of incidence = angle of reflection

Both angles are measured from the normal — the line drawn at right angles to the surface at the point where the ray strikes. Measuring from the surface itself gives the complement of the right answer, and is the most common mistake in ray diagrams.

The image in a plane mirror is virtual (the rays only appear to come from it), upright, the same size as the object, laterally inverted (left and right swapped), and as far behind the mirror as the object is in front.

Refraction — the wave changes direction when it crosses a boundary between two materials, because its speed changes.

The rule for light:

• Entering a denser medium, light slows down and bends towards the normal.
• Entering a less dense medium, light speeds up and bends away from the normal.
• Along the normal (angle of incidence zero), it does not bend at all, though it still changes speed.

Why refraction bends the wave is easiest to see with wavefronts: one edge of the wavefront enters the slower medium first and is held back while the other edge is still moving quickly, so the whole front swings round. The frequency is unchanged, so the wavelength shortens in step with the speed.

Diffraction — the spreading out of a wave as it passes through a gap or around an obstacle.

Diffraction is greatest when the gap is about the same size as the wavelength. A wide gap gives almost no spreading; a gap comparable to the wavelength spreads the wave through nearly a semicircle.

This is why you can hear someone round a corner but not see them: sound has a wavelength of roughly a metre and diffracts around everyday objects easily, while visible light has a wavelength of under a millionth of a metre and barely spreads at all.

Example: long-wave radio reaches valleys that line-of-sight television signals cannot, because its longer wavelength diffracts around hills far more readily.