O-LevelPhysicsLight5054/22 · Oct/Nov 2021 · Q39 Marks

Fig. 3.1 shows light entering a transparent block. [Figure 3.1 (not to scale)] The light enters the block at an angle θ to the normal and travels through the block until it meets the bottom surface. The angle between the ray in the block and the vertical side of the block is φ.

(a)(i)Light travels more slowly in the block than in air. Explain how Fig. 3.1 shows this.[2]
(a)(ii)State what happens to the wavelength of the light and what happens to the frequency of the light as it enters the block. wavelength frequency[2]
(b)(i)The refractive index of the transparent material is 1.6. Angle θ is 45°. Determine angle φ. φ =[3]
(b)(ii)The angle of incidence i at the bottom surface is equal to φ and the critical angle for the material of the block in air is 39°. Explain what happens to the light after it meets the bottom surface.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[B1] it/light refracts / bends / moves towards normal or angle of incidence greater than angle of refraction
[B1] change in direction of the ray / refraction shows a change in speed (at the boundary) or sin(i) / sin(r) is ratio of speeds or top of ray slows down first and enters the block first
[B1] (wavelength) decreases
[B1] (frequency) does not change.
[C1] (sin(r) =) sin(i) / n or sin(45°) / 1.6 or 0.71 / 1.6 or 0.44 or (r =) 21(°)
[C1] 26(°)
[A1] 64°
[B1] 64° / angle of incidence / i is greater than the critical angle / 39° or angle of incidence = 64°
[B1] it undergoes total internal reflection

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