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Reflection & Refraction


Waves can be reflected at a surface. 

Law of Reflection:
Angle of incidence = Angle of Reflection

In reflection, there are two rays: the incident ray and the reflected ray. 
Normal is an imaginary line which is perpendicular to the surface. 

The angle of incidence is measured between the incident ray and the normal.
The angle of reflection is measured between the reflected ray and the normal.

In the image, you can see that the angle of incidence (angle between incident ray and the normal) is 57˚. 
Therefore, the angle of reflection (angle between reflected ray and the normal) is also 57˚. 

The normal is the dashed line 90˚ to the surface. 


When light travels from one medium (e.g. air) to another medium (e.g. glass), its speed changes. This causes its direction in the second medium to change as well. 

Look at the diagram below:

The incident ray’s direction changes when it travels from air to glass. 

When light travels from a less dense medium such as air to a more dense medium such as glass, its speed decreases. This causes the light to bend towards the normal (as seen in diagram). 
angles of incidence (i) > angle of refraction (r)

When light travels from a more dense medium such as glass to a more less medium such as air, its speed increases. This causes the light to bend away from the normal (as seen in diagram).

The emergent ray bends away from the normal when the light travels from the glass back into the air. 

Total Internal Reflection

At a certain angle of incidence, the refracted ray of light travels along the boundary of the two media. 
This angle of incidence is called the critical angle. The critical angle is different for every medium. 
For glass, the critical angle is 42˚. 

If the angle is increased further than the critical angle, then the light will be totally internally reflected. 

Conditions for Total Internal Reflection:

  1. Light must be travelling from a more dense medium to a less dense medium
  2. The angle of incidence must be greater than critical angle. 

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