GATE 2025 AR — Architecture – Question 64

Building Services · Principles of electrification and illumination and special lighting for buildings · 2 marks · Numerical answer

Point light C is 1.75 m vertically above A. B is 1 m horizontally from A on the same receiving plane. If illumination at A is 300 lux, find illumination at B, to nearest lux.

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Correct answer: 185 to 205

Explanation

At A, normal incidence gives source intensity I=300×1.75²=918.75 cd. At B, distance r=√(1.75²+1²) and incidence cosine=1.75/r. Therefore EB=I×1.75/r³≈196 lux. Both increased distance and oblique incidence reduce the illumination; inverse square alone omits the latter.

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