Lesson focus: The colour we perceive depends on the spectrumThe band of colours that white light splits into: red, orange, yellow, green, blue, indigo and violet. of the illumination and the wavelengthThe distance from one point on a wave to the same point on the next wave, such as peak to peak. transmitted or reflected to the eye.
Explain dispersionWhen white light splits into a rainbow of colours, like light through a prism., colour filterA coloured see-through material that lets only some colours of light through., additive light mixing and the appearance of coloured objects.
Build on earlier learning
The eye and the cameraThe eye’s cone cells respond to different wavelength ranges. This lesson follows how the light reaching those cells is selected by sources, filters and objects.
You will be successful when you can…
- Explain dispersion of white light by a prism.
- Describe how a coloured filter transmits and absorbs light.
- Use red, green and blue as additive primary lights.
- Predict object appearance under different illumination.
Words that carry the science
- spectrum
- The continuous range of visible wavelengths or colours.
- dispersion
- Separation of white light because different wavelengths refract by different amounts.
- filter
- A material that transmits selected wavelengths and absorbs others.
- additive mixing
- Combining coloured light so more wavelengths reach the eye.
- primary light
- Red, green or blue light used as the basis of additive colour mixing.
- wavelength
- The distance between equivalent points on successive waves; in visible light it relates to perceived colour.
Track which wavelengths reach the eye
01White light contains a visible spectrum
A prism refracts visible wavelengthThe distance from one point on a wave to the same point on the next wave, such as peak to peak. by different amounts, separating white light into a spectrumThe band of colours that white light splits into: red, orange, yellow, green, blue, indigo and violet.. This is dispersionWhen white light splits into a rainbow of colours, like light through a prism.. Red light bends least and violet bends most in a typical glass prism.
The colours form a continuous range, even though they are often named in bands. The prism does not add colour; it separates wavelengths already present.
02Filters remove light rather than recolouring it
An ideal red filter transmits red wavelengths and absorbs much of the other visible light. Placing filterA coloured see-through material that lets only some colours of light through. together can only transmit wavelengths that pass through all of them.
Filters do not turn incoming green light into red. If a red filter receives only green light, little light emerges.
03Object colour depends on illumination
Under white light, a green object appears green because it reflects green wavelengths more strongly and absorbs many others. Under red light, the same object may appear very dark because no green illumination is available to reflect.
Red, green and blue are additive primary lightRed, green or blue light, which can be mixed to make all other colours of light.. Red + green produces yellow light; green + blue produces cyan; blue + red produces magenta; all three at suitable intensities can appear white.
Coloured filters and coloured objects
White light is a mixture of colours. A filter lets only its own colours through; an object reflects its own colours and absorbs the rest.
Red Green Blue light
Controls
Yellow, cyan and magenta filters each let two of the primary colours through. An object looks black when none of the light reaching it is a colour it reflects.
Predict a shirt under coloured light
A green shirt is illuminated only by red light. Predict its appearance.
- Identify the available illumination: red wavelengths only.
- A green surface usually reflects green wavelengths strongly.
- No green light is available to reflect.
- Most incident red light is absorbed, so little light reaches the eye from the shirt.
How to make the reasoning visible: The green shirt appears very dark or black under red illumination.
Track available and reflected light
Use an idealised model. Real pigments and filters have broader, imperfect spectra.
| Illumination | Object or filter | Light reaching eye | Appearance or result |
|---|---|---|---|
| White | Red object | Mostly red reflected | Red |
| Red | Green object | Very little reflected | Dark |
| White | Blue filter | Mostly blue transmitted | Blue transmitted beam |
| Red + green light | White screen | Red and green reflected | Yellow |
| Red + green + blue light | White screen | All three reflected | White or near-white |
Why does a blue filter placed after an ideal red filter transmit almost no light?
Distinguish yellow light made by mixing red and green from yellow pigment.
Check your understanding
Answer from memory, then check against the suggested answer.
What is dispersion?
Which visible colour bends least in a typical glass prism?
What are the additive primary lights?
A red object is lit by white light and then by blue light. Predict each appearance.
Explain why the spectrum is continuous even though school diagrams show coloured bands.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
White light passes through a red filter and then shines on a green object.
Explain what happens to the white light at the red filter.
(3)Explain what colour the object appears.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., follow the light in order: source spectrum → filter selection → object interaction → eye.