Light & Surfaces (Edexcel GCSE Physics): Revision Note

Specular & Diffuse Reflection

Specular Reflection

• Specular reflection is defined as:

  Reflection from a smooth surface in a single direction

• When light reflects off a smooth surface, such as a mirror, specular reflection occurs

  • This is what gives a mirror its shiny appearance

• This is why a reflection can be seen clearly in a mirror

• In this case, the angle of reflection r is equal to the angle of incidence i

When reflecting off a mirror, the specular reflection occurs. The angle of incidence is equal to the angle of reflection

Diffuse Reflection

• Diffuse reflection is defined as:

  Reflection from a rough surface causes scattering

• When light reflects off a rough surface, which applies to the majority of surfaces, diffuse reflection occurs

  • This is what gives objects a dull or matt appearance

• This is why a reflection cannot be seen clearly from a table surface, for example

  • Even though a table's surface may look smooth from afar, it is actually made up of many tiny ridges which the light rays are scattered off

• When light scatters, it leaves the surface in all directions

When light is reflected from the majority of surfaces it is scattered – a process known as diffuse reflection

Colour

Differential Absorption of Colour

• Absorption occurs when:

  Energy is transferred from the wave into the particles of a substance

• Waves can be partially or completely absorbed

  • Sound waves are absorbed by brick or concrete in houses

• Light will be absorbed if the frequency of light matches the energy levels of the electrons

  • The light will be absorbed, and then reemitted over time as heat

• If an object appears red, this means:

  • Only red light has been reflected

  • All the other frequencies of visible light have been absorbed

The object is seen as red since the red light is reflected whilst the other colours are absorbed

Colour Filters

• Colour filters work by absorbing certain wavelengths and transmitting other wavelengths

  • These certain wavelengths correspond to certain colours

• When white light passes through a coloured filter, some colours are absorbed whilst others are able to pass straight through

• For example, when white light passes through a red filter:

  • Red light is transmitted

  • All the other colours are absorbed

• The colour that is transmitted is the same colour as the filter

A red filter transmits only red light and absorbs all other colours of light