A level Physics · Edexcel 9PH0 · Topic 5C

Refraction and polarisation

Key points

Refraction is a change of direction when a wave changes speed crossing a boundary. Refractive index $n = \frac{c}{v}$ .
Snell's law: $n_1\sin\theta_1 = n_2\sin\theta_2$ , with angles measured from the normal.
Total internal reflection happens when light in the denser medium hits the boundary at more than the critical angle $C$ , where $\sin C = \frac{1}{n}$ (going into air). Used in optical fibres.
Lenses: power $P = \frac{1}{f}$ (dioptres); thin lens equation $\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$ (real is positive); magnification $= \frac{\text{image height}}{\text{object height}} = \frac{v}{u}$ .
Plane polarisation restricts oscillations to one plane; only transverse waves can be polarised, which is evidence that light is transverse.
Light reflected from water or glass is partly polarised; polarising filters reduce glare and are used to find stress patterns in transparent plastics.
Practical: trace rays through a glass block, measure angles of incidence and refraction and use Snell's law to find $n$ .

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Light changes speed and direction when it crosses into a new medium. These notes cover refraction, total internal reflection, lenses and polarisation, the evidence that light is a transverse wave.

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