O-LevelPhysicsLight5054/22 · May/June 2019 · Q36 Marks
Fig. 3.1 shows three rays of light travelling in water from a light source S. One ray of light is totally internally reflected at the boundary between water and air. [Figure 3.1] (not to scale)
- (a)On Fig. 3.1, mark the critical angle and label it c.[1]
- (b)State why only one of the three rays is totally internally reflected.[1]
- (c)(i)Calculate: the refractive index for water[2]
- (c)(ii)Calculate: the critical angle for the boundary between water and air.[2]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [B1] middle ray angle of incidence labelled c
- [B1] angle of incidence (of right-hand ray only) greater than the critical angle
- [C1] (n=) sin i / sin r algebraic or numerical
- [A1] 1.3
- [C1] sin c = 1/n algebraic or numerical
- [A1] answer in range 49° to 50.3°
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