Colour, Filters and the Visible Spectrum — AQA GCSE Physics (Separate)
The colour of an object depends on which wavelengths of light it reflects, transmits or absorbs.
The visible spectrum
White light is made up of all the colours of the visible spectrum (red, orange, yellow, green, blue, indigo, violet), each with a different wavelength. Red has the longest wavelength and violet the shortest.
Why objects appear coloured
The colour of an opaque object depends on which wavelengths of light it reflects and which it absorbs:
- A red object reflects red light and absorbs the other colours.
- A white object reflects all colours.
- A black object absorbs all colours (reflects none), so it looks black.
Transparent and translucent objects
These objects transmit (let through) and absorb light. Filters work by transmitting particular colours and absorbing the rest:
- A red filter only transmits red light.
- A red object viewed through a green filter looks black (the object reflects no green, and the filter blocks the red).
Specular and diffuse reflection
Light reflecting off a smooth surface gives a clear image (specular); off a rough surface it scatters (diffuse).
Exam tips
- White light contains all colours (a range of wavelengths).
- An opaque object's colour = the wavelengths it reflects (the rest are absorbed).
- Black absorbs all; white reflects all.
- A filter transmits its own colour and absorbs the others.