Reflection & Total Internal Reflection


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Reflection & Total Internal Reflection

Yes! When light leaves a vacuum or travels from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.
Refraction refers to the way light changes both direction and speed as it travels from one transparent medium into another.
As light travels from a fast medium such as air to a slow medium such as water it bends toward 'the normal' and slows down. As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from 'the normal' and speeds up.

About the diagram

Have you already checked out An Introduction to Reflection, Refraction and Dispersion?

It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:

The diagram

  • In this diagram sunlight or artificial light travelling through water reflects upwards off the body of the fish.
  • For clarities sake,  the diagram doesn’t show light travelling towards the fish.
  • Check out our diagram dealing with the way parallel incident light rays scatter in different directions as they are reflected off the body of the fish:  Sunlight Reflects off a Fish in Water
  • Notice how the light reflected off the fish and towards the surface is incident to the boundary between water and air.
  • As the incident light strikes the surface a proportion is refracted as it crosses the boundary into the air and a proportion is reflected off the surface back into the water.
  • The diagram demonstrates the paths taken for a ray travelling parallel to the normal and striking the boundary at right angles and for rays at angles of 150, 300,  45to the normal.
  • Notice that the amount of light that is reflected increases as the angle increases but that above 48.60 total internal reflection takes place and all the light is reflected.
  • Depending on the angle at which the light strikes the surface, different proportions are refracted or reflected.
  • The ratio of the reflected intensity to the incident intensity is called the reflectance (R) and the ratio of the transmitted intensity to the incident intensity is called the transmittance (T).  Energy conservation requires that R + T = 1 (if there is no absorption).
    • The reflectance of the surface of a material is its effectiveness at reflecting radiant energy.
    • The transmittance of the surface of a material is its effectiveness at transmitting radiant energy.

Some key terms


Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the ...

Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media ...


If one line is normal to another, then it is at right angles. So in geometry, the normal is a ...

Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface. The angle of incidence is measured ...

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