TE Enriched Sample (E)

29 Measuring illumination 1.3 B A 60° A. 16 : 1 B. 8 : 1 C. 4 : 1 D. 2 : 1 5. Briefly explain why the inverse-square law for illuminance is not applicable in the following cases. (a) A small surface on a desk illuminated by a desk FTL (b) A small surface on a wall illuminated by a parallel beam of light 6. You are given an incandescent lamp, an illuminance meter and a metre rule. incandescent lamp illuminance meter metre rule (a) Design an experiment to verify the inverse- square law of illuminance. (b) State one precaution that we should take to obtain accurate results. 7. Amercury vapour lamp has a luminous efficacy of 55 lm W - 1 . The power rating of the lamp is 1200 W. (a) How much luminous flux is given out by the lamp during normal operation? (b) Estimate the maximum illuminance of a small surface at a distance of 4 m from the lamp. What should be the orientation of the surface in order to obtain the maximum illuminance? 8. Michelle is reading a book under an LED lamp. The lamp produces a luminous flux of 640 lm and is 0.8 m vertically above the centre of the book. Initially, the book is normal to the incident light. Treat the lamp as a point light source. (a) (i) Find the illuminance around the centre of the book. (ii) Michelle finds that the light reflected from the book is disturbing while she is reading. Suggest one reason. (b) Michelle rotates the book by 20° while the separation between the lamp and the book remains unchanged. What is the illuminance around the centre of the book now? 9. A lamp producing a luminous flux of 2200 lm is at the centre of the ceiling of a room as shown. P , Q and R are three small surfaces on the floor. P is directly under the lamp, Q is next to the centre of the wider wall and R is next to the corner. Suppose the lamp and the floor are 2.5 m apart. 10 m R P Q 2.5 m 8 m (a) Without calculations, explain which surface has the smallest illuminance. (b) Estimate the illuminance on each surface. Neglect reflection of light. 10. The sun produces a luminous flux of 3.75 × 10 28 lm and is 1.5 × 10 11 m from the Earth. (a) Find the maximum illuminance on the Earth due to sunlight. (b) At a certain time, the sun is right above a place on the equator. Neglecting the atmospheric reflection and absorption of sunlight, find the illuminance on the place (i) at this moment, and (ii) 3 hours later. Earth’s rotation North pole position after 3 hours sunlight position at a certain time P : 28.0 lx, Q : 4.17 lx; R : 1.35 lx 1.33 × 10 5 lx 1.33 × 10 5 lx 9.38 × 10 4 lx 328 lx; normal 66 000 lm B R 74.8 lx 79.6 lx Further question (Q9) (c) If the entire wall at the back was a perfect mirror, what would be the illuminance on Q ? Answer: Doubled to 8.34 lx (Based on DSE 2022 Q3.2) Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.