AQA A-Level Physics Paper 1, November 2020: Question 14
1 mark · Medium difficulty · Multiple Choice
Identify how the wavelength, speed and photon energy change when a monochromatic light wave travels from glass into air.
Practise this questionQuestion
Question text
14 A monochromatic light wave travels from glass into air.
Which row shows what happens to the wavelength, speed and photon energy?
[1 mark]
Wavelength Speed Photon energy
A increases increases increases
B does not change decreases does not change
C does not change decreases increases
D increases increases does not change
Mark scheme
Show the mark scheme
14 D
How to answer it
Light Wave Properties Across Boundaries
What this question tests
This question assesses your understanding of wave behavior during refraction—specifically how wave speed, wavelength, and photon energy (frequency) change when monochromatic light transitions from an optically denser medium (glass) to a less dense medium (air).
Exam Breakdown & Analysis
✅ Correct Answer: Row D
Wavelength: Increases
Speed: Increases
Photon Energy: Does not change
💡 Key Knowledge
- Frequency (f): Determined entirely by the source. It does not change when a wave crosses a boundary.
- Wave Equation ( c = fλ ): Since speed increases entering air, wavelength must increase proportionally.
- Photon Energy ( E = hf ): Directly proportional to frequency. Since frequency is constant, energy remains unchanged.
🧠 Exam Technique
- Start by locking down what stays constant: frequency and photon energy never change during refraction. This immediately eliminates options A and C.
- Recall optical densities: glass is more dense than air, so light speeds up as it enters the air. Eliminate option B.
❌ Common Errors
- Assuming photon energy changes because the speed or environment changes. Remember that E = hf depends only on frequency.
- Confusing the direction of speed change: forgetting that light speeds up when entering a vacuum or air from glass.
Topics
Physics · 3.2 Particles and radiation · 3.3 Waves
Question and mark scheme from the AQA A-Level Physics examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.