The questions are on the refractive index test. Take it under real conditions before reading the mark scheme.
Quick answer: Refraction is the bending of light as it changes speed between materials, and its size is given by the refractive index n = sin i ÷ sin r. This mark scheme gives the model answers and the marks for the 8 questions of the Physics 9-10 test on refractive index, from law of reflection to real or virtual, with the equivalent answers to accept.
The model answers include mark allocations. Award method marks even if the final answer is slightly wrong, and apply error carried forward when a student uses their own earlier result consistently.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: IGCSE 3.2
Question 1 Law of reflection (2 marks)
- The angle of incidence equals the angle of reflection. i = r
- Angle of incidence = 90° − 28° = 62°, so the angle of reflection is 62°.
Marking: 1 mark for the law; 1 mark for using the normal to get 62°.
Question 2 Refractive index (3 marks)
- n = sin 35° ÷ sin 22° = 0.574 ÷ 0.375 = 1.53
- v = c ÷ n = 3.0 × 10⁸ ÷ 1.53 = 2.0 × 10⁸ m/s
Marking: 1 mark for substitution, 1 for answer; 1 mark for the speed.
Question 3 Bending of light (2 marks)
- It bends towards the normal.
- Light travels more slowly in water than in air, so the wavefront turns towards the normal.
Marking: 1 mark for the direction, 1 mark for the slower speed in water.
Question 4 Critical angle (3 marks)
- The angle of incidence in the denser material for which the angle of refraction is 90°.
- sin c = 1 ÷ 1.62 = 0.617, so c = 38.1°
Marking: 1 mark for the definition; 1 for sin c = 1 ÷ n with numbers, 1 for the answer. (Total 3 marks.)
Question 5 Optical fibre (3 marks)
Light enters one end and strikes the glass-air boundary at an angle of incidence greater than the critical angle. Light is travelling from the denser glass to less dense air, so it undergoes total internal reflection. It repeats this at each boundary down the fibre.
Marking: 1 mark for denser to less dense, 1 for angle greater than c, 1 for repeated total internal reflection along the fibre.
Question 6 Image in a lens (4 marks)
- A ray parallel to the axis refracts through the far F; a ray through the centre is not deviated. The two rays cross below the axis, to the right of the lens.
- The image is real, inverted and larger than the object.
Marking: 1 mark for each correct ray; 2 marks for the image description (1 for real and inverted, 1 for larger).
Question 7 Magnification (2 marks)
- m = 1.8 ÷ 6.0 = 0.30
- image = 0.30 × 9.0 = 2.7 cm
Marking: 1 mark each part.
Question 8 Real or virtual (1 mark)
A real image can be shown on a screen because light rays actually meet there; a virtual image cannot, because the rays only appear to come from it.
Marking: 1 mark for a correct difference.
