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GCSE questions on refraction and TIR

Q7. The diagram shows a ray of light going from air into an optical fibre and then travelling through part of the optical fibre.

(a) Which one of the following pairs of angles correctly identifies the angle of incidence and the angle of refraction?

Angle of incidence
Angle of refraction
Tick here
A
C
  
A
D
B
C
  
B
D
  

[1 mark]

(b) The glass used to make this optical fibre has a refractive index of 1.5

Calculate the critical angle of the glass. Give your answer to 2 significant figures.

sin (∠c) = 1/1.5 = 0.67

∠c = 41.8°

∠c = 42° ( to 2 sig figs)

 

[3 marks]

(c) Short pulses of light can carry information through optical fibres. Each pulse is made of lots of light rays. The light rays refract into the optical fibres at different angles when the pulse is sent.

The paths of two possible rays, A and B, are shown in the diagram above.

These rays make up part of the pulse of light energy that can be detected as it enters and leaves the fibre - shown in the diagram below.

By considering the paths of rays A and B, explain why the pulse of light changed as it passed through the optical fibre.

The pulse leaving the fibre takes longer to exit than it did on entering. This is because some of the rays travelled a longer distance through the fibre because they were refracted and totally internally reflected - like ray B - whereas rays like ray A suffer no TIR and take a straight shorter path.

[2 marks]

(Total 6 marks)