Refraction, total internal reflection and optical fibresAQA A-Level Physics: Subtopic test

1. 1
   A narrow beam from a laser pointer travels through air and strikes the flat surface of a rectangular glass block. The glass has a refractive index of 1.50. Take the speed of light in air as 3.00 × 10⁸ m s⁻¹ and the refractive index of air as 1.00.

   (a)

   What is the speed of light in the glass?

   [1 mark]

   • A4.50 × 10⁸ m s⁻¹
   • B3.00 × 10⁸ m s⁻¹
   • C2.00 × 10⁸ m s⁻¹
   • D1.50 × 10⁸ m s⁻¹

   (b)

   The beam meets the glass at an angle of incidence of 40° to the normal. What is the angle of refraction in the glass?

   [1 mark]

   • A19°
   • B25°
   • C27°
   • D40°

   (c)

   The laser beam is later directed from inside the glass towards the glass–air surface at an angle of incidence of 50°. Show, by calculation, what happens to the light at this surface.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A tank contains a layer of water of refractive index 1.33 with a layer of oil of refractive index 1.47 floating on top, separated by a flat horizontal boundary. A ray of light travels upwards through the water and meets this boundary at an angle of incidence of 30°.

   (a)

   Which statement correctly describes the ray and its speed as it enters the oil?

   [1 mark]

   • AIt bends away from the normal and its speed increases.
   • BIt bends away from the normal and its speed decreases.
   • CIt bends towards the normal and its speed increases.
   • DIt bends towards the normal and its speed decreases.

   (b)

   What is the angle of refraction of the ray in the oil?

   [1 mark]

   • A20°
   • B23°
   • C27°
   • D34°

   (c)

   Calculate the speed of light in the oil, and explain why it is less than the speed of light in the water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A step-index optical fibre has a core of refractive index 1.48 surrounded by cladding of refractive index 1.46. A laser sends pulses of light along the fibre to carry digital data.

   (a)

   Explain why light entering the core at a small angle to the axis stays inside the core for the whole length of the fibre.

   [3 marks]

   (b)

   A ray travelling in the core makes an angle of 15° with the axis of the fibre. Determine whether this ray is totally internally reflected at the core–cladding boundary.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A telecommunications company is planning a data link of 80 km using optical fibre. The signal is a series of short pulses of light, and a receiver must be able to tell each pulse from the next.

   (a)

   Explain how pulse broadening and absorption affect the pulses as they travel along the fibre, and the consequences for the data link.

   [6 marks]

   (b)

   The company can choose between two designs. Design X uses a fibre with a wide core and an LED that emits a range of wavelengths. Design Y uses a fibre with a very narrow core and a laser that emits a single wavelength. Evaluate which design is better suited to the 80 km link.

   [6 marks]

   Total for question 4: 12 marks

End of questions