Snell's Law (Edexcel IGCSE Science (Double Award) (Modular): Physics Unit 2): Revision Note

Snell's law

• The angles of incidence and refraction are related to the refractive index of a medium by an equation known as snell's Law:

$n=\frac{\sin i}{\sin r}$

• Where:

  • n = the refractive index of the material

  • i = angle of incidence of the light (°)

  • r = angle of refraction of the light (°)

• 'Sin' is the trigonometric function 'sine' which is on a scientific calculator

• You can revise the concept of refraction using the revision notes Reflection & refraction

A formula triangle can help rearrange the snell's law equation

Snell's law formula triangle

• For more information on how to use a formula triangle refer to the revision note on Speed

Refractive index

• The refractive index is a number which is related to the speed of light in the material (which is always less than the speed of light in a vacuum):

$\mathrm{refractive}\mathrm{index},n=\frac{\mathrm{speed}\mathrm{of}\mathrm{light}\mathrm{in}\mathrm{a}\mathrm{vacuum}}{\mathrm{speed}\mathrm{of}\mathrm{light}\mathrm{in}\mathrm{material}}$

• The refractive index is a number that is always larger than 1 and is different for different materials

  • Objects which are more optically dense have a higher refractive index, eg. n is about 2.4 for diamond

  • Objects which are less optically dense have a lower refractive index, eg. n is about 1.5 for glass

• Since the refractive index is a ratio, it has no units