A student places a glass prism in the path of a ray of white light. The white light enters one face of the prism and a spectrum of colours emerges from the opposite face. Explain why white light produces a spectrum of colours when it passes through a glass prism.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
White light is a mixture of many colours, each with a different wavelength.
When the light enters the glass prism it is refracted; the amount of refraction depends on the wavelength.
Violet light, having the shortest wavelength, is bent the most, while red light, with the longest wavelength, is bent the least.
Because the different colours slow down by different amounts in the glass, they follow different paths.
The result is that the colours are separated and spread out, producing a visible spectrum.
When the light enters the glass prism it is refracted; the amount of refraction depends on the wavelength.
Violet light, having the shortest wavelength, is bent the most, while red light, with the longest wavelength, is bent the least.
Because the different colours slow down by different amounts in the glass, they follow different paths.
The result is that the colours are separated and spread out, producing a visible spectrum.
Examiner tips
- Use the word "refracted" and mention wavelength. State the comparative bending of violet vs red. Explain the speed difference in glass. Show the separation leads to a spectrum.
Common mistakes
- Saying white light is a single colour. Forgetting to mention wavelength or speed. Confusing refraction with reflection.
Mark scheme (5 marks)
- White light is made up of a mixture of different colours (frequencies/wavelengths)
- Each colour (wavelength/frequency) refracts by a different amount when passing from air into glass
- Violet light refracts the most and red light refracts the least (or correct comparative statement)
- The different colours slow down by different amounts when entering the glass (different speeds in glass)
- The colours are separated / spread out, producing a visible spectrum
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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