Explain what happens to the speed, wavelength, and direction of a ray of white light as it passes from glass into air, and explain why white light separates into a spectrum of colours during this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The speed of light increases when it moves from glass into air because the refractive index of air is lower than that of glass.
The wavelength also increases as the light enters air.
The ray bends away from the normal as it enters the less dense medium.
Different wavelengths of white light are slowed by different amounts in glass and therefore speed up by different amounts when leaving glass.
Because each colour refracts by a different amount, they emerge at different angles, spreading out to form a spectrum.
The wavelength also increases as the light enters air.
The ray bends away from the normal as it enters the less dense medium.
Different wavelengths of white light are slowed by different amounts in glass and therefore speed up by different amounts when leaving glass.
Because each colour refracts by a different amount, they emerge at different angles, spreading out to form a spectrum.
Examiner tips
- Use the term ‘refractive index’ to justify the change in speed and wavelength. State the direction change relative to the normal. Explain that dispersion causes the spectrum.
- Mention that the spectrum forms because of varying refraction angles for each wavelength.
Common mistakes
- Confusing the direction of bending (towards or away from the normal). Forgetting that wavelength changes in proportion to speed. Not linking dispersion to the spread of colours.
Mark scheme (5 marks)
- The light speeds up when moving from glass (denser) into air (less dense).
- The wavelength increases as the light enters air.
- The ray bends away from the normal as it enters air.
- Different wavelengths (colours) of light slow down by different amounts in glass / speed up by different amounts when leaving glass.
- Because each colour refracts by a different amount, they emerge at different angles, spreading out to form a spectrum.
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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