Colour & Reflection of Light (AQA GCSE Physics)

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Colour Filters

  • 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

refraction-by-prism, IGCSE & GCSE Physics revision notes

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

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

red-filter, IGCSE & GCSE Physics revision notes

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

Colour & Reflection of Light

  • 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

green-surface, IGCSE & GCSE Physics revision notes

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

Examiner Tip

Colour and colour filters often cause confusion. The trick is to not think about what you are seeing but to think about the behaviour of light and how the eye works.

We know that white light contains all the colours of the rainbow. Most objects will absorb the majority of those wavelengths and just reflect a small section of it. That reflected light is what our eyes detect and our brains perceive that as colour. So a green leaf absorbs all the colours except green, and reflects the green light. Our eyes detect those green wavelengths, and our brains tell us that the leaf is green.

Filters only transmit light of certain wavelengths. So if you shine white light through a blue filter, only the blue wavelengths will pass through, the rest will be absorbed. A good understanding of colour depends on a good understanding of the behaviour of light to either be absorbed, reflected or transmitted (refraction is a type of transmission) when it reaches the surface of a substance.

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.