Colour & Light (OCR GCSE Physics A (Gateway)): Revision Note

Colour & Light

• White light is a mixture of all the colours of the spectrum

• Each colour has a different wavelength (and frequency), making up a very narrow part of the electromagnetic spectrum

• White light may be separated into all its colours by passing it through a prism

  • This is done by refraction

  • Violet light is refracted the most, whilst red light is refracted the least

  • This splits up the colours to form a spectrum

• This process is similar to how a rainbow is created

White light may be separated into all its colours by passing it through a prism

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

Transmission & Absorption – 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

Reflection

• The colour of an opaque object is determined by which wavelengths of light are more strongly reflected

• Wavelengths that are not reflected are absorbed

  • Hence, this is why different objects appear to be different colours

• For example, white light upon a green surface will only have green light reflected and the others absorbed

  • This light is reflected into our eyes to see the surface in that colour

A green surface reflects green light and absorbs all other colours

• An object will appear white if:

  • All wavelengths are reflected equally

• An object will appear black if:

  • All wavelengths are absorbed

• An object will appear transparent if:

  • All the light is transmitted, and only a small amount is reflected or absorbed

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

Scattering

• Scattering is defined as:

  Reflection from a rough surface in many directions

• 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