refraction + critical angle.

• Refraction


  When light waves change direction at the boundary between two transparent materials

  
• Incident ray


  The incoming ray that hits the boundary between the two materials

  
• Angle of incidence


  The angle between the incident ray and the normal line

  
• Refracted ray


  The ray that changes direction after passing through the boundary between the two materials

  
• Angle of refraction


  The angle between the refracted ray and the normal line

  
• (higher-tier only)

  Effect of density on the speed of a wave

  The denser the transparent material, the more slowly the wave travels through it as there's more density per unit of volume
• (higher-tier only)

  What happens to speed when light enters a more dense material


  The light ray slows down and bends towards the normal line

  
• (higher-tier only)

  What happens to speed when light enters a less dense material

  The light ray speeds up and bends away from normal line

  
• (higher-tier only)

  Effect of density on the wavelength of a wave

  The denser the transparent material, the more slowly the wave travels through it so the wavelength decreases

  
• (higher-tier only)

  Effect of density on the frequency of a wave

  The frequency of a wave is unaffected
• (higher-tier only)

  Total internal reflection (TIR)

  When the light emerging from a boundary will refract 90 degrees and travel along the boundary between the two media

  
• (higher-tier only)

  Critical angle


  The angle at which refracted ray will start to travel along the boundary between the two media

  
• When the angle of incidence is less than the critical angle


  Most of the light passes out

  
• When the angle of incidence is greater than the critical angle

  All light is totally internally reflected

  
• Conditions required for TIR to occur

  The light must be travelling from a more dense medium into a less dense medium and the angle of incidence must be greater than the critical angle
• Uses of TIR


  Optical fibres, long-distance communication