Edexcel A Level Physics:复习笔记5.13 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 change speed or direction

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