AQA AS Physics

Topic Questions

3.5 Refraction

1a3 marks

An optical fibre used for communications has a core of refractive index 1.65 which is surrounded by cladding of refractive index 1.55.

Figure 1

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Figure 1 shows a light ray P inside the core of the fibre. The light ray strikes the core cladding boundary at Q at an angle of incidence theta and angle of refraction of 55.0°. 

Calculate the critical angle of the core-cladding boundary.

1b3 marks

Calculate the angle of incidence, theta.   

1c2 marks

Explain why the light ray enters the cladding at Q and therefore why total internal reflection does not occur.          

1d3 marks

A different optical fibre is used for the communication. The speed of monochromatic light in the core is 1.64 × 108 m s–1 and the speed in the cladding is 2.51 × 108 m s–1. 

Calculate the critical angle for this light at the interference between the core and the cladding.

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2a2 marks

Figure 1 below shows three wavefronts of light directed towards a glass block in the air. The direction of travel of these wavefronts is also shown.

Figure 1

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Complete Figure 1 to show the position of these three wavefronts after refraction at the surface of the glass block.

2b3 marks

Figure 2 shows a liquid droplet placed on the glass block. A ray of light from air, incident normally on to the droplet, continues in a straight line and is refracted at the liquid to glass boundary as shown. 

Figure 2

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The refractive index of the glass is 1.37. 

Calculate the fraction of the speed of the light ray in the glass block to the speed of light in a vacuum.

2c2 marks

The speed of light in the liquid droplet is equal to 2.35 × 108 m s-1. 

Calculate the refractive index of the liquid droplet.

2d2 marks

Calculate the angle of refraction of the light ray when it enters the glass block.

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3a2 marks

Figure 1 below shows a ray of light passing from air into glass at the top face of glass block A and into another glass block B. 

Figure 1

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The refractive index of block A is 1.55 and the speed of light in block B is 2.27 × 108 m s-1. 

Show that the refractive index of block B is 1.32.

3b4 marks

Calculate the value of theta subscript 1 and theta subscript 2.

3c2 marks

On Figure 1, draw the path of the ray between block B and air.

3d3 marks

Explain qualitatively how the angle the incidence at the block A–block B boundary should change in order for the light ray not to travel into block B at all.

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4a2 marks

Figure 1 shows a cross-section through a step-index optical fibre.

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Beam A is incident at the end of the optical fibre at an angle of 13.8° to the normal and refracts into the core at 7.54° to the normal. 

Calculate the refractive index of the core.

4b2 marks

Beam A travels through the air-core boundary and experiences total internal reflection. 

On the diagram, show the path of this ray down the fibre. Label the angle of reflection.

4c2 marks

Beam B is incident on the end of the fibre, refracts through the air-core boundary and then refracts again when it hits the core-cladding boundary at an angle of 52.3°, travelling along the boundary.

Calculate the refractive index of the cladding.

4d3 marks

A different step-index optical fibre is built with the same core as that in Figure 1 but replacing the cladding. 

The speed of light in the new cladding material is 1.54 × 108 m s–1. 

Explain why this new cladding material would not be suitable for sending signals through the step-index optical fibre. 

Use a calculation to support your answer.

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5a3 marks

A regular glass fish tank containing water is shown in Figure 1. Three light rays A, B and C emerge from the same point on a small object O at the bottom of the tank as shown.

Figure 1

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The critical angle is shown to be from the glass fish tank to light ray B emitted from object O. 

On Figure 1 draw the continuation of the paths taken by the three rays shown. No further calculations are required.

5b2 marks

The speed of the light rays in the water is 72 % of the speed of light in vacuum. 

Calculate the refractive index of the water.

5c2 marks

Calculate the maximum angle of incidence the light rays can have at the surface of the water before total internal reflection.

5d3 marks

The refractive index of the glass the tank is made from is 1.50.

State which of the light rays A, B or C hits the glass and explain whether or not the ray undergoes total internal reflection at the water-glass surface.

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