A student shines a beam of white light onto a green opaque surface. Explain why the surface appears green when viewed in white light, and describe what would happen to the appearance of the surface if it were viewed under red light only.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
White light is made up of all the colours of the visible spectrum. Opaque materials absorb some wavelengths and reflect others.
Model answer (5 marks)
The green surface absorbs all wavelengths of white light except green.
Only green light is reflected from the surface.
Our eyes detect the reflected green light, so the surface appears green.
Under red light only, the green surface absorbs the red light and no green light is present to be reflected.
The surface appears black under red light because no light is reflected to the observer.
Only green light is reflected from the surface.
Our eyes detect the reflected green light, so the surface appears green.
Under red light only, the green surface absorbs the red light and no green light is present to be reflected.
The surface appears black under red light because no light is reflected to the observer.
Examiner tips
- Use the word ‘absorb’ and ‘reflect’ to show understanding of light–matter interaction.
- Explain the role of the eye’s photoreceptors in detecting reflected light.
- Mention that the absence of reflected light gives a black appearance.
Common mistakes
- Saying the surface ‘glows’ instead of ‘reflects’. Confusing absorption with transmission. Forgetting to state that no light is reflected under red illumination.
Mark scheme (5 marks)
- The green surface absorbs all colours/wavelengths of white light except green
- Only green light is reflected from the surface
- Our eyes detect the reflected green light, so the surface appears green
- Under red light only, the green surface absorbs the red light / no green light is present to be reflected
- The surface appears black under red light because no light is reflected to the observer
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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