Predict first: if white light is a mixture of all colours, what colour will a red jumper appear under pure blue light? The answer is black — because blue light contains no red wavelengths for the jumper to reflect, so no light reaches your eyes. This predict-then-explain approach unlocks every colour question in KS3 physics.
What is white light and why does it contain colour?
White light is a mixture of all the visible colours of the electromagnetic spectrum. When a beam of white light passes through a glass prism, the different wavelengths are refracted (bent) by different amounts, separating into a continuous band of colours — the visible spectrum:
Red → Orange → Yellow → Green → Blue → Indigo → Violet (often remembered as ROYGBIV)
Red light has the longest wavelength (~700 nm) and is refracted least; violet has the shortest wavelength (~400 nm) and is refracted most. This is why white light splits into a rainbow of colours through a prism (or in a real rainbow, where raindrops act as tiny prisms).
The full visible spectrum is just one small part of the electromagnetic spectrum; infrared lies just beyond red and ultraviolet lies just beyond violet.
Why do objects appear to have colour?
An object's colour is determined by which wavelengths of light it reflects and which it absorbs:
- The wavelengths that are reflected reach your eyes — those are the colours you see.
- The wavelengths that are absorbed convert to heat energy and are not seen.
| Object | Reflects | Absorbs | Appears |
|---|---|---|---|
| Red apple | Red | Orange, yellow, green, blue, violet | Red |
| Green leaf | Green | Red, orange, yellow, blue, violet | Green |
| White paper | All wavelengths | None | White |
| Black car | None (absorbs all) | All wavelengths | Black |
| Yellow banana | Red + green | Blue, violet | Yellow |
Key rules:
- A white object reflects all colours of light equally.
- A black object absorbs all colours of light.
- A coloured object reflects only certain wavelengths.
How do colour filters work?
A colour filter is a transparent material that allows only certain wavelengths of light to pass through, absorbing the rest.
A red filter, for example:
- Transmits red light.
- Absorbs (blocks) all other colours — orange, yellow, green, blue, violet.
So if you hold a red filter in front of your eyes and look at a white surface, it appears red because only the red component of white light passes through. If you look at a blue object through a red filter, the object appears black — no blue light can pass through the filter.
Worked example: A white LED torch shines on a stage with a blue filter in front of it. What colour does a red dress appear?
- Blue light leaves the torch.
- Red dress: reflects red light (normally), absorbs blue light.
- Blue light hits the red dress — but red objects absorb blue light.
- No light is reflected towards the audience.
- The dress appears black.
What are the primary colours of light and how do they mix?
The primary colours of light are red, green, and blue (RGB). Mixing these colours of light is called additive colour mixing because you are adding wavelengths together.
| Colours mixed | Result |
|---|---|
| Red + Green | Yellow |
| Red + Blue | Magenta |
| Green + Blue | Cyan |
| Red + Green + Blue | White |
| None | Black (no light) |
This is different from mixing paints or pigments, which is subtractive mixing (each pigment absorbs certain colours, leaving fewer wavelengths to reflect). Adding all paint colours together gives a muddy dark brown/black, not white.
Additive mixing is used in TV screens and computer monitors — every pixel is made of tiny red, green, and blue sub-pixels that light up at different intensities to produce any colour.
How can you predict the colour of an object under coloured light?
Use a two-step method:
- Work out which wavelengths are in the light source (after any filters).
- Work out which of those wavelengths the object can reflect.
If the light contains a wavelength the object can reflect → that colour is seen. If the light contains only wavelengths the object absorbs → the object appears black.
Worked example: A yellow banana is lit by red light. What colour does it appear?
- Yellow banana reflects red and green light; absorbs blue.
- Red light contains only red wavelengths.
- Red is one of the wavelengths the banana reflects.
- The banana appears red (only red is present to be reflected).
Frequently asked questions
Why does a red filter block blue light but let red light through?
A red filter is made of a material that absorbs blue (and green and other) wavelengths but allows red wavelengths to pass through because the molecular structure of the filter material interacts selectively with different frequencies of light. At a particle level, photons of blue light have more energy than red photons; the dye molecules in the filter absorb this energy and re-radiate it as heat, while red photons pass through without being absorbed. The result is that only red light emerges on the other side.
Why do leaves appear green?
Leaves contain the pigment chlorophyll, which absorbs red and blue light and uses the energy for photosynthesis, but reflects green light. This is why leaves appear green — green is the wavelength that chlorophyll does not use (it is reflected rather than absorbed). Interestingly, this means photosynthesis is driven mainly by red and blue light; green light is largely "wasted" by plants, which is why plants grow well under pink or purple LED grow-lights that emphasise those wavelengths.
What is the difference between additive and subtractive colour mixing?
Additive mixing combines light of different colours: the more colours you add, the closer to white you get. It is used in screens and stage lighting. Subtractive mixing combines pigments or filters: each pigment absorbs (subtracts) more wavelengths from white light, so the more pigments mixed, the darker the result. Mixing all paint colours together removes nearly all wavelengths, giving a near-black. Artists use subtractive mixing; screen designers use additive mixing.
Why do stage lighting designers use red, green, and blue spotlights?
Red, green, and blue are the three primary colours of light — the minimum set from which all other colours can be produced by additive mixing. By controlling the intensity of each colour independently, a lighting designer can produce any colour in the visible spectrum on stage. Combining all three at full intensity gives white; turning all three off gives no light (black). This flexibility from three primary colours is why RGB is the standard for all electronic display technologies.
For KS3 physics that predicts first and explains second — try Professor Newton at aitutors.me.