O-LevelPhysicsLight5054/22 · May/June 2017 · Q915 Marks

Fig. 9.1 shows a man in a room looking into a mirror, as viewed from above. The man can see an image of part of the wall AB in the mirror. Point C is the point on the wall closest to A that the man can see by reflection in the mirror. This point is not marked on Fig. 9.1. [Figure 9.1]

(a)(i)On Fig. 9.1, find point C and draw a ray of light from C which enables the man to see the image of C.[2]
(a)(ii)On the ray that you have drawn on Fig. 9.1, label the angle of incidence i and the angle of reflection r.[2]
(a)(iii)1One characteristic of the image formed by the mirror is that it is virtual. Explain what is meant by a virtual image.[2]
(a)(iii)2State one other characteristic of an image formed by a plane mirror.[1]
(b)(i)Light travels along optical fibres by total internal reflection. State two differences between total internal reflection and reflection in a mirror.[2]
(b)(ii)The critical angle for light travelling from the material of the fibre to air is 44°. Calculate the refractive index of the material in the fibre.[2]
(b)(iii)Light enters the optical fibre from air with an angle of incidence of 50°. Calculate the angle of refraction.[2]
(b)(iv)Optical fibres are used in telecommunications to send large amounts of data. Pulses of light travel at almost the same speed along a fibre as electrical pulses travel along a metal wire. State two advantages of using optical fibres rather than wires for transmitting data.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] ray from right-hand corner of mirror to eye
[1] any incident and corresponding reflected ray correct by eye
[1] normal drawn at any intersection of incident and reflected ray
[1] both r and i labelled correctly with normal
[1] cannot be formed on a screen or nothing at the image (position)
[1] rays do not come (all the way) from the image or rays only appear to come from image
[1] (same distance) behind the mirror or same size (as object) or upright/erect or laterally inverted
[1] reflection in mirror occurs at any angle or total internal reflection (TIR) only occurs for i > critical angle or there is no critical angle for the mirror
[1] TIR occurs from dense to less dense medium or in the dense(r) medium or from glass to air or inside / does not escape glass or from slow to fast (media) or mirror reflection from air to glass
[1] (mirror) reflection is not total, e.g., not all reflected or better quality of image or multiple images from a mirror
[1] (n=) 1/sinC or 1/sin44
[1] 1.4
[1] n = sini / sinr in any form, e.g. sinr = sin50/n
[1] 32° - 33°
[1] more data per second or per unit time
[1] less decrease in strength / amplitude / attenuation
[1] less heat/power produced / wasted
[1] less need for repeating or amplification stations
[1] less interference / noise
[1] more secure / less chance of cross-talk
[1] lighter/less heavy

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