Colour & Reflection of Light (AQA GCSE Physics)
Revision Note
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
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
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
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 Tips and Tricks
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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