O-LevelPhysicsLight5054/21 · May/June 2016 · Q67 Marks

Explain what is meant by critical angle.

(a)Explain what is meant by critical angle.[2]
(b)(i)Diamonds are attractive because of their ability to reflect light. The critical angle for diamond in air is 24°. Calculate the refractive index of diamond.[2]
(b)(ii)Fig. 6.1 shows two diamonds of different shapes. A ray of light enters each diamond, as shown. [Figure 6.1] On Fig. 6.1 continue the path of each ray of light until it emerges into the air.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[B1] angle of incidence
[B1] smallest angle for light to be totally internally reflected or largest angle (of incidence) for ray to be refracted/emerge or when light emerges along surface or when angle of refraction is 90°
[C1] n = 1/sinC algebraic or numerical
[A1] 2.5 or 2.46 or 2.458(59)
[B1] left hand diagram ray refracts away from normal and emerges into air at bottom left surface
[B1] right hand diagram reflected horizontal ray (by eye)
[B1] right hand diagram rest of ray completely correct to emerge into air at top face without refraction (by eye)

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