Critical angle and total internal reflectionEdexcel International A Level Physics: Subtopic test

1. 1
   A ray of light travels inside a rectangular block of glass of refractive index 1.60 and strikes a flat glass–air boundary from inside the glass. Take the refractive index of air as 1.00.

   (a)

   What is the critical angle for the glass–air boundary?

   [1 mark]

   • A23°
   • B32°
   • C51°
   • D39°

   (b)

   A ray in the glass reaches the glass–air boundary at an angle of incidence of 45°. What happens to the ray?

   [1 mark]

   • ASome of it is refracted into the air, bending towards the normal
   • BIt is totally internally reflected back into the glass
   • CIt is refracted into the air at an angle of 45° to the normal
   • DIt travels along the surface of the glass

   (c)

   The block is now immersed in water of refractive index 1.33. Calculate the critical angle for the glass–water boundary.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A bare optical fibre made of glass with refractive index 1.52 is surrounded by air. Light signals travel along the fibre by repeatedly meeting the side wall of the fibre, which is a glass–air boundary.

   (a)

   What is the critical angle for the glass–air boundary of the fibre?

   [1 mark]

   • A49°
   • B33°
   • C41°
   • D26°

   (b)

   Which pair of conditions must both be met for total internal reflection to occur at a boundary?

   [1 mark]

   • ALight travels towards a medium of lower refractive index, and the angle of incidence is greater than the critical angle
   • BLight travels towards a medium of higher refractive index, and the angle of incidence is greater than the critical angle
   • CLight travels towards a medium of lower refractive index, and the angle of incidence is less than the critical angle
   • DLight travels towards a medium of higher refractive index, and the angle of incidence is less than the critical angle

   (c)

   A ray inside the fibre meets the side wall at an angle of incidence of 38°. Determine whether the ray is totally internally reflected.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A small lamp is fixed at the bottom of a swimming pool, 2.0 m below the water surface, and shines light in all directions upwards. The refractive index of the water is 1.33 and that of air is 1.00. A swimmer looks at the surface from above.

   (a)

   Calculate the critical angle for the water–air surface. Explain why light from the lamp that reaches the surface at an angle of incidence of 60° does not emerge into the air.

   [3 marks]

   (b)

   The swimmer sees a circle of light on the surface above the lamp. Explain why the edge of the circle is formed by rays at the critical angle, and calculate the radius of the circle. State the radius when the lamp is raised to a depth of 1.0 m.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A jeweller is comparing a cut diamond of refractive index 2.42 with a glass imitation of refractive index 1.52. Both gems are viewed in air, and each has flat polished back faces on which light inside the gem can reflect.

   (a)

   Calculate the critical angle for each gem in air. Use your values to explain why the diamond appears to sparkle more than the glass imitation.

   [6 marks]

   (b)

   A ray inside each gem meets a back face at an angle of incidence of 30°. Predict whether total internal reflection occurs in each gem in air. The diamond is then placed in oil of refractive index 1.50. Calculate the new critical angle and explain why the diamond loses much of its sparkle.

   [6 marks]

   Total for question 4: 12 marks

End of questions