AQA GCSE Combined Science: Trilogy Physics Paper 2 (Higher), 2019: Question 7

9 marks · Standard Demand difficulty · Extended Answer

Explain and complete ray and wavefront diagrams to describe refraction of light in glass and a prism, and use wavelength-speed data to explain why violet light refracts most in water.

Practise this question

Question

The question page contains three linked parts labelled 07.1, 07.2 and 07.3, each worth 3 marks. Figure 10 shows wavefronts and a ray crossing a horizontal boundary from air into a shaded glass block; the wavefront spacing is smaller in the glass and the question asks to explain why light refracts as it passes from air into glass. Figure 11 shows a triangular glass prism with an incident ray entering the left face and a dashed normal at the entry point; students are asked to complete the ray diagram to show the ray emerging from the prism. Figure 12 is a graph of speed of light in water, in units of ×10^6 m/s, against wavelength from 400 to 700 nm; the curve rises from about 222 at 400 nm to about 226 at 700 nm, and the question asks why violet light is refracted the most as it enters water.
Question text

07 Wave front diagrams are used to explain why light refracts when it passes from air

into glass.

Figure 10

07.1 Explain why the light refracts as it passes from air into glass.

[3 marks]

07.2 Figure 11 shows a ray of red light entering a glass prism.

Figure 11

Complete the ray diagram to show the ray emerging from the glass prism.

[3 marks]

*0197.*3 White light is made up of a continuous spectrum of different wavelengths that all travel

at 3 × 108 m/s in air.

Rainbows are produced because different wavelengths of light travel at different

speeds in water.

Figure 12 shows the speed of different wavelengths of light in water.

Figure 12

Explain why violet light is refracted the most as it enters water.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme is a table for questions 7.1 to 7.3 with answer points and marks. For 7.1 it awards marks for stating that light travels slower in glass, that the part of the wavefront entering glass slows first, and that the part still in air continues faster causing a change in direction. For 7.2 it gives marks for a ray inside the prism bent towards the normal, a second normal at the exit face, and an emergent ray bent away from the normal. For 7.3 it gives marks for identifying violet as the shortest wavelength at about 400 nm, stating that violet light travels slowest in water, and concluding that it has the greatest change in speed and therefore direction.

AO /

Question Answers Extra information Mark ID

Spec. Ref.

7.1 speed / velocity in the glass is speed / velocity changes is 1 AO1 E

lower insufficient 6.6.2.2

allow the refractive index of

glass is higher than that of air

allow glass has a higher optical

density than air

so the edge of the wave(front) 1

entering the glass slows down

but the part of the wave(front) in 1

the air continues at the higher

speed / velocity (causing a

change in direction)

7.2 correct ray in the prism bent 1 AO1 E

towards the normal 6.2.2.2

second normal at 90° at the 1

point the ray emerges

correct emergent ray bent away this mark can be awarded 1

from the normal without a normal line drawn

7.3 violet has the shortest 1 AO3 E

wavelength (400 nm) 6.2.2.2

violet light travels the slowest in 1

water

violet light undergoes the 1

greatest change in speed (and

direction)

Total 9

How to answer it

Refraction, Prisms and White Light

What this question tests
Using wave fronts to explain refraction and why violet light bends the most

This question checks whether you can explain refraction using wave speed, apply the rules for drawing a ray diagram through a prism, and use a graph to link wavelength to speed in water. Full marks come from using the correct physics ideas in the right order.

Question 7.1 — Explain why the light refracts as it passes from air into glass.

3 marks

✅ Correct answer

  • The speed of light is lower in glass than in air.
  • So the part of the wavefront entering the glass slows down first.
  • The part still in the air continues at the higher speed, so the wavefront changes direction and the light refracts.

💡 Key knowledge

  • Glass has a higher refractive index than air.
  • Glass is more optically dense than air.
  • Refraction happens because a wave changes speed when it enters a different medium.

🧠 Exam technique

  • To get all 3 marks, link speed → wavefront bends → direction changes.
  • Use the wavefront diagram idea: one side slows first, so the wavefront rotates.
  • Avoid vague statements like “the light changes because it hits glass”.

❌ Common errors

  • Saying only “the speed changes” is not enough.
  • Writing “glass has a higher density” can be misleading unless you mean optical density.
  • Some students forgot to mention that the part still in air continues faster.
How marks are awarded: one mark for lower speed in glass, one for the wavefront entering glass slowing down, and one for the part still in air moving faster and causing a change in direction.

Question 7.2 — Complete the ray diagram to show the ray emerging from the glass prism.

3 marks

✅ Correct answer

  • Draw the ray bending towards the normal as it enters the prism.
  • Draw a second normal at 90° to the prism face where the ray leaves.
  • Draw the emergent ray bending away from the normal as it leaves the glass.

💡 Key knowledge

  • When light enters glass from air, it slows down and bends towards the normal.
  • When it leaves glass into air, it speeds up and bends away from the normal.
  • A normal must be perpendicular to the surface.

🧠 Exam technique

  • Use a ruler and keep ray lines straight between surfaces.
  • Check the prism shape carefully before drawing the emergent ray.
  • The mark scheme allows the emergent ray mark even if the second normal is not drawn, but drawing it is the safest way to score full marks.

❌ Common errors

  • Drawing the ray the wrong way at entry, especially bending away from the normal in glass.
  • Forgetting the second normal.
  • Making the emergent ray parallel to the incident ray when the diagram does not support that.
What examiners wanted: a clear ray inside the prism bent towards the first normal, then a second normal at the exit point, then a ray emerging away from that normal.

Question 7.3 — Explain why violet light is refracted the most as it enters water.

3 marks

✅ Correct answer

  • Violet light has the shortest wavelength in visible light, about 400 nm.
  • From the graph, violet light travels the slowest in water.
  • Because it has the greatest change in speed when entering water, it is refracted the most.

💡 Key knowledge

  • White light contains a range of wavelengths.
  • Different wavelengths travel at different speeds in water.
  • The greater the change in speed at a boundary, the greater the refraction.

🧠 Exam technique

  • Use the graph directly: the shortest wavelength on the left is around 400 nm, and that line is at the lowest speed.
  • For full marks, make the chain clear: shortest wavelength → slowest speed → biggest change in speed → greatest refraction.
  • Top answers often mention both the graph and the physics reason.

❌ Common errors

  • Saying violet is refracted most just because it is “the darkest” or “the strongest colour”.
  • Confusing wavelength with speed.
  • Not linking the answer to the idea of change in speed.

📐 Calculations / graph reading

  1. Read the graph: violet light is at the shortest wavelength, about 400 nm.
  2. Find the speed at 400 nm: about 222 × 10⁶ m/s.
  3. Compare this with red light near 700 nm: about 226 × 10⁶ m/s.
  4. Violet therefore has the largest speed decrease when entering water.
  5. That is why violet light bends the most.

❌ Common calculation / graph traps

  • Reading the y-axis wrongly: the scale is × 10⁶ m/s.
  • Quoting only the wavelength without using the speed data from the graph.
  • Forgetting that “refracted the most” means the greatest change in direction due to the biggest change in speed.
Examiner insight: the best answers joined together the graph reading and the physics explanation. Students lost marks when they only said violet has the shortest wavelength, without explaining that it also travels the slowest in water and so changes speed the most.

Quick revision summary

Key ideas to remember

  • Light slows down when it enters glass or water from air.
  • Slower speed in the new medium causes refraction.
  • Enter denser medium → towards the normal.
  • Leave denser medium → away from the normal.
  • Violet light bends most because it has the shortest wavelength and slowest speed in water.

How to score full marks

  • Use the correct physics words: speed, normal, refract, wavelength.
  • Describe the process in order.
  • When drawing, be precise and neat.

Topics

Physics · P6: Waves

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Physics Paper 2 (Higher), 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.