Refraction & Refractive Index (Edexcel International A Level Physics)

Refraction & Refractive Index

• Refraction occurs when light passes a boundary between two different transparent media

• At the boundary, the rays of light undergo a change in direction and a change in speed

• The change in direction is caused by the change in speed

  • Entering a more dense medium slows the light down and it bends towards the normal

    • In the denser medium there are more particles closer together providing more friction to the passing of the light through the material

  • Entering a less dense medium speeds the light up and it bends away from the normal

  • When passing along the normal (perpendicular) the light does not direction

  • Its speed does still change, as it is passing through a medium with a different refractive index

Refraction of light through a glass block

Calculating Refractive Index

• The refractive index, n, is a property of a material which measures how much light slows down when passing through it

• Where:

  • c = the speed of light in a vacuum (m s^–1)

  • v = the speed of light in a substance (m s^–1)

• Light travels at different speeds within different substances depending on their refractive index

  • A material with a high refractive index is called optically dense, such material causes light to travel slower

• Since the speed of light in a substance will always be less than the speed of light in a vacuum, the value of the n is always greater than 1

• In calculations, the refractive index of air can be taken to be approximately 1

  • This is because light does not slow down significantly when travelling through air (as opposed to travelling through a vacuum)

Snell's Law

• Snell’s law relates the angle of incidence to the angle of refraction, it is given by:

n₁ sin θ₁ = n₂ sin θ₂

• Where:

  • n₁ = the refractive index of material 1

  • n₂ = the refractive index of material 2

  • θ₁ = the angle of incidence of the ray in material 1 (°)

  • θ₂ = the angle of refraction of the ray in material 2 (°)

Snell's Law is used to find the refractive indices or the angles to the normal at a boundary

• θ₁ and θ₂ are always taken from the normal

• Material 1 is always the material in which the ray goes through first

• Material 2 is always the material in which the ray goes through second