Reflection, Refraction & Transmission (SL IB Physics)

• When waves arrive at a boundary between two materials, they can be:
  • Reflected
  • Refracted
  • Transmitted
  • Absorbed

• In optics, a transparent material is called a medium
  • When referring to more than one medium these are called media

Reflection

• Reflection occurs when:

A wave hits a boundary between two media and does not pass through, but instead bounces back to the original medium

• The law of reflection states:

The angle of incidence, i = The angle of reflection, r

Reflection of a wave at a boundary

• When a wave is reflected, some of it may also be absorbed or transmitted through the medium
• At a boundary between two media, the incident ray is the ray that travels towards the boundary
• During reflection, the frequency, wavelength and speed of the wave does not change

Refraction

• Refraction is:

The change in direction of a wave when it passes through a boundary between mediums of different density

• This change of direction is caused by a change in the speed of different parts of the wavefront as they hit the boundary
  • In optics, the word medium is used to describe a transparent material

A wavefront changes direction when part of it hits a boundary and changes speed before the other parts

Conditions for refraction

• When a wave travels from a less dense medium into a denser medium:

• • The more optically dense the medium
  • The slower the waves travel
  • The smaller the angle of refraction
  • The light bends towards the normal
• When a wave travels from a denser medium into a less dense medium:

• • The less optically dense the medium
  • The faster the waves travel
  • The greater the angle of refraction
  • The light bends away from the normal

• The amount of refraction that takes place is determined by the difference between the angles of incidence (i) and refraction (r) of the waves at the boundary
• The angles of incidence and refraction are measured from the normal line 
  • This is drawn at 90° to the boundary between the two media

The angle of incidence is the angle between the approaching incident ray and the normal. The angle of refraction is the angle between the ray leaving the boundary and the normal.

• The amount of change in direction that takes place depends on the difference in optical density between the two media

• When light passes from a less dense medium to a more dense medium, (e.g. air → glass):
  • The refracted light has a lower speed and a shorter wavelength than the incident light 

• When light passes from a more dense medium to a less dense medium (e.g. glass → air):
  • The refracted light has a higher speed and a longer wavelength than the incident light

• When a wave refracts, its speed and wavelength change, but its frequency remains the same
  • This is noticeable by the fact that the colour of the wave does not change

When light passes from a less to a more dense material the ray refracts towards the normal

• When the light ray is incident on the boundary at 90°:
  • The wave passes straight through without a change in direction
  • This is because the whole wavefront enters the boundary at the same time at the same speed

Light travelling along the normal to the boundary between material 1 and material 2

Refraction of Water Waves

• Refraction can also occur between materials of different depths
  • You may be asked to explain the behaviour of water waves when they refract between deep and shallow areas
• When waves pass from deep to shallow water there is more friction between the sea bed and the wave and less space for the wave to oscillate so the waves:
  • Slow down
  • The wavelength of the wave decreases
  • So the distance between wave peaks is reduced
  • Angle of refraction is less than angle of incidence, r < i
• If the waves hit the boundary between the change in depth at an angle then they refract towards the normal
  • So the angle of refraction < angle of incidence

When water waves travel from deep areas to shallow areas they slow down

Transmission

• Transmission occurs when:

A wave passes through a substance

• Refraction is a type of transmission
  • Transmission is the more general term for a wave appearing on the opposite side of a boundary (the opposite of reflection)
  • Refraction is specifically the change in direction of a wave when it crosses a boundary between two materials that have a different density
•  When passing through a material, waves can be partially absorbed
• The transmitted wave will have a lower amplitude if some absorption has occurred

When a wave passes through a boundary it may be absorbed and transmitted

• During transmission, the frequency or speed of the wave does not change

• Reflection, refraction and transmission occur for all types of waves, both transverse and  longitudinal