Refraction and total internal reflectionEdexcel A-Level Physics: Subtopic test

1. 1
   A ray of light in air strikes the flat surface of a glass block of refractive index 1.50 at an angle of incidence of 40°. The speed of light in air is 3.00 × 10⁸ m s⁻¹ and the refractive index of air may be taken as 1.00.

   (a)

   What is the speed of light in the glass?

   [1 mark]

   • A2.0 × 10⁸ m s⁻¹
   • B4.5 × 10⁸ m s⁻¹
   • C3.0 × 10⁸ m s⁻¹
   • D1.3 × 10⁸ m s⁻¹

   (b)

   What is the critical angle for light travelling in the glass towards a glass–air boundary?

   [1 mark]

   • A48°
   • B34°
   • C42°
   • D56°

   (c)

   Calculate the angle of refraction of the ray inside the glass.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A lamp at the bottom of a swimming pool shines light upwards on to the surface of the water. The refractive index of water is 1.33 and that of air is 1.00.

   (a)

   Which pair of conditions is needed for total internal reflection of the light at the water surface?

   [1 mark]

   • AThe light travels from air to water, at any angle of incidence
   • BThe light travels from water to air, at an angle of incidence greater than the critical angle
   • CThe light travels from air to water, at an angle of incidence greater than the critical angle
   • DThe light travels from water to air, at an angle of incidence less than the critical angle

   (b)

   A ray from the lamp reaches the water surface at an angle of incidence of 45°. What happens?

   [1 mark]

   • AThe light is totally internally reflected because it moves from a denser to a less dense medium.
   • BNo light emerges because 45° is less than the critical angle.
   • CThe light is refracted into the air, bending towards the normal.
   • DMost of the light is refracted into the air, bending away from the normal, because 45° is less than the critical angle of 49°.

   (c)

   Calculate the angle between the normal and the refracted ray in the air for the ray in (b).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student determines the refractive index of a rectangular glass block. She uses a ray box with a single slit to direct a narrow ray of light on to the block, and measures the angle of incidence θ₁ in air and the angle of refraction θ₂ inside the block for several different rays. She plots sin θ₁ against sin θ₂ and obtains a straight line through the origin with a gradient of 1.52.

   (a)

   Explain why the graph is a straight line through the origin with a gradient equal to the refractive index of the glass, and calculate the speed of light in the block.

   [3 marks]

   (b)

   Describe how the student should take her measurements to obtain accurate values of θ₁ and θ₂, and explain why she should use a range of angles rather than a single ray.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A telecommunications company makes optical fibres with a glass core of refractive index 1.50. The core is surrounded by a thin glass cladding of refractive index 1.45. The speed of light in air is 3.00 × 10⁸ m s⁻¹.

   (a)

   Explain how light travels along the core of a fibre without escaping, and calculate the critical angle at the boundary between the core and the cladding. State what this tells you about the directions of the rays that can be carried, and why the cladding is needed.

   [6 marks]

   (b)

   An engineer suggests removing the cladding and leaving the glass core directly in air to cut costs. Evaluate this proposal, using calculations to compare the range of ray directions that would be carried with and without the cladding.

   [6 marks]

   Total for question 4: 12 marks

End of questions