AQA AS Physics

Topic Questions

3.5 Refraction

1a2 marks

Monochromatic light refracts as it passes from air to glass. 

State two properties of light that change when it refracts.

1b1 mark

State one property of the light that stays the same when it refracts.

1c2 marks

Figure 1a shows the light entering the glass from the air.

Figure 1a

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Figure 1b shows the light leaving the glass into the air. 

Figure 1b

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Sketch on Figure 1a and Figure 1b the light ray when it enters and leaves the glass respectively.

1d2 marks

Explain why the light rays in your answer to part (c) for 

   (i)         Figure 1a 

   (ii)        Figure 1b 

   were drawn in that direction.

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2a3 marks

State how the following properties of a laser light change as it passes from glass to air: 

            (i)         Velocity 

            (ii)        Wavelength 

            (iii)       Frequency

2b2 marks

The refractive index of a substance n is given by the equation 

           n =c over c subscript s   

State the definition of the following variables and an appropriate unit for each: 

            (i)         c 

            (ii)       c subscript s

2c4 marks

Table 1 shows the speed of light in different mediums. 

Table 1

Medium

Speed of Light in Medium / × 108 m s–1

Refractive Index

Sodium Chloride

1.95

 

Water

2.26

 

Amber

1.94

 

Liquid Helium

2.91

 

 Complete the refractive index of each medium in Table 1.

2d2 marks

A student incorrectly calculates the refractive index of a medium as 0.78.

State and explain why this value of refractive index is incorrect.

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3a3 marks

Optical fibres are commonly used for transmitting electrical signals for telephones, the internet and cable televisions. The cross–section of an optical fibre is shown in Figure 1. 

Figure 1

3-5-s-q--q3a-easy-aqa-a-level-physics

Complete the missing labels in Figure 1.

3b2 marks

Explain why C is necessary for optical fibres.

3c1 mark

Name the process that allows a signal to travel effectively through the optical fibre.

3d3 marks

A side on view of the optical fibre is shown in Figure 2. A light ray enters the optical fibre from the air.

Figure 2

3-5-s-q--q3d-easy-aqa-a-level-physics

Sketch the path of the light ray as it travels through the optical fibre until it exits the fibre.

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4a2 marks

State the two conditions needed for total internal reflection to happen.

4b2 marks

Material or spectral dispersion and modal dispersion are common phenomenon when white light travels down an optical fibre. 

State how each of these effects of dispersions can be reduced: 

   (i)         Material dispersion 

   (ii)        Modal dispersion

4c4 marks

Complete the sentences by using an answer from the box. 

You may use a word once, more than once or not at all.

Ae7XLkI-_3-5-s-q--q4c-easy-aqa-a-level-physics

During ________, the _________ of a signal is reduced causing a loss of _________. In other words, the signal is ________ by the core.
________ is caused by material and modal dispersion. This ________ the signal through the optical fibre.

4d3 marks

The signal in Figure 1 is from a step–index optical fibre.

Figure 1

3-5-s-q--q4d-easy-aqa-a-level-physics

Sketch the new signal after: 

            (i)         Absorption 

            (ii)        Pulse broadening

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5a4 marks

For two difference substances, the law of refraction is stated as 

             n subscript 1 sin space theta subscript 1 space equals n subscript 2 sin space theta subscript 2

State the definition of the following variables:           

             (i)       n subscript 1

            (ii)       theta subscript 1

            (iii)       n subscript 2

            (iv)      theta subscript 2

5b2 marks

Figure 1 shows a light ray entering water in an outdoor swimming pool from the air. 

Figure 1

3-5-s-q--q5b-easy-aqa-a-level-physics

On Figure 1, label theta subscript 1 and theta subscript 2 as stated in the law of refraction.

5c2 marks

A different light ray now travels from the water to the air. The water has a refractive index of 1.33. 

Calculate the critical angle between the water and the air.

5d1 mark

State the value of an angle of incidence which would create total internal reflection.

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1a3 marks

A ray of light passes from air into a glass prism as shown in Figure 1.

Figure 1

3-5-s-q--q1a-hard-aqa-a-level-physics

As the light ray passes through the prism, it emerges back into the air.

Calculate the critical angle from the glass to the air.

1b2 marks

On Figure 1, draw the continuation of the path of the ray of light until it emerges back into the air. Write on Figure 1 the values of the angles between the ray and any normals you have drawn.

1c4 marks

Figure 2 now shows the prism rotated and one side is coated with a film of transparent gel. A ray of light strikes the prism, at an angle of incidence of 38°, and continues through the glass to strike the glass–gel boundary at the critical angle.

Figure 2

3-5-s-q--q1c-hard-aqa-a-level-physics

Calculate the refractive index of the gel.

1d2 marks

A ray of light now strikes the prism at an angle of incidence which means that it now refracts straight through the gel at the glass–gel boundary. 

Without calculation, explain how the critical angle for the glass–gel boundary differs from the critical angle for the gel–air boundary.

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2a4 marks

A small intense light source is 1.2 m below the surface of the water in a large swimming pool, as shown in Figure 1.

Figure 1

3-5-s-q--q2a-hard-aqa-a-level-physics

Speed of light in water = 2.25 × 108 m s–1 

Calculate the diameter of the disc through which light emerges from the surface of the water.

2b4 marks

Below the surface of the swimming pool there is a glass panel with a light source underneath it. A thin layer of oil ends up on top of the water, as shown in Figure 2. 

Figure 2

3-5-s-q--q2b-hard-aqa-a-level-physics

Refractive index of oil = 1.47

Refractive index of glass = 1.78 

Calculate the speed of light inside the oil.

2c4 marks

Diamond has certain properties which makes it one of the most sought-after gem stones for jewellery. 

Its refractive index is around 2.4, and jewellers cut it in specific ways to maximise its appeal. 

Discuss, referring to an equation, why the refractive index of diamond makes it a popular gem stone for jewellery.

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3a4 marks

Figure 1 below shows two rays of light A and B travelling through a straight optical fibre.

3-5-s-q--q3a-hard-aqa-a-level-physics

The overall length of the optical fibre is 0.70 km. Ray travels down the centre of the core of the optical fibre. The path of ray has an overall length of 0.86 km as it travels through the core. 

The refractive index of the core is 1.6 and ray and ray enter the fibre at the same instant. 

Compare, with a calculation, the differences between the two rays as they exit the core of the optical fibre.

3b4 marks

The rays are replaced with a signal of a square pulse of white light transmitted along the centre of a fibre. 

On Figure 2, sketch the path of the blue and red light through the core of the optical fibre, and label clearly which path is red or blue. Explain, using your sketch, the phenomena that occurs because of this. 

Figure 2

3-5-s-q--q3b-hard-aqa-a-level-physics

3c4 marks

The time taken for the two paths to travel down the optical fibre is 3.82 µs and 3.87 µs. 

Calculate the refractive index of the fibre for the red and blue light.

3d3 marks

The blue light enters the core of the optical fibre at an angle of 20º as shown in Figure 3 and hits the cladding at an angle of incidence of 1º more than the critical angle.

3-5-s-q--q3d-hard-aqa-a-level-physics

Show that the refractive index of the cladding is 1.62.

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4a2 marks

Figure 1 shows two prisms, prism 1 and prism 2, made from transparent material and placed firmly together. A ray of light in air is incident normally on prism 1. 

Figure 1

3-5-s-q--q4a-hard-aqa-a-level-physics

Prism 1 is made from a denser material than prism 2 and the angle incident upon the boundary between them is below the critical angle. 

Sketch, without calculation, the path followed by the refracted ray as it enters prism 1 and prism 2 and then emerges into the air.

4b2 marks

The refractive index of prism 1 is 1.48 and the refractive index for prism 2 is 1.20.

Calculate and label the angles of incidence and refraction on Figure 1 between:           

   (i) The prism 1 and prism 2 bound

   (ii) The prism 2 and air boundary

4c5 marks

Figure 2 shows the same two prisms. However, the light ray enters prism 1 at an angle of 76º to the normal.

Figure 2

3-5-s-q--q4c-hard-aqa-a-level-physics

On Figure 2, draw the continuation of the path of the light ray until it emerges back   into the air. Write the values of the angles between the ray and any normals you have drawn.

4d4 marks

A different right–angled prism is in contact with a transparent substance on one of the faces as shown in Figure 3. One of the other angles in the prism is θ.

Figure 3

3-5-s-q--q4d-hard-aqa-a-level-physics

Refractive index of glass prism = 1.65

Refractive index of transparent substance = 1.10 

The ray enters perpendicularly to one face of the prism. It is refracted at the interface between the glass and the transparent substance. The ray eventually leaves the transparent substance at an angle of 8º to the surface of the transparent substance. 

Determine the angle θ.

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5a2 marks

The tube of an endoscope behaves like an optical fibre to examine the interior of the body for medical diagnosis. One end of the fibre is illuminated and an image of the inside of the stomach is viewed by the doctor as shown in Figure 1. 

Figure 1

3-5-s-q--q5a-hard-aqa-a-level-physics

Draw on Figure 1 the complete path of the light from the illumination until it emerges to the doctor.

5b4 marks

Figure 2 shows a cross–section through an optical fibre used in an endoscope. The critical angle is a 7% decrease from the 75º angle to the normal at the core–cladding boundary. The refractive index of the cladding is 1.4. 

Figure 2

3-5-s-q--q5b-hard-aqa-a-level-physics

Calculate the angle of incidence theta subscript i at the air–core boundary.

5c3 marks

Figure 3

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Complete the graph shown in Figure 3 to show how the refractive index changes with radial distance along the line ABCD in Figure 2.

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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

3-5-s-q--q1a-medium-aqa-a-level-physics

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

3-5-s-q--q2a-medium-aqa-a-level-physics

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

3-5-s-q--q2b-medium-aqa-a-level-physics

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

3-5-s-q--q3a-medium-aqa-a-level-physics

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.

3-5-s-q--q4a-medium-aqa-a-level-physics

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

3-5-s-q--q5a-medium-aqa-a-level-physics

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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