The illumination of a lamp on a surface varies directly as its power and inversely as the square of its distance from the surface. If a 60 W lamp placed 2 m above a desk provides an illumination of 15 lux, find the illumination when a 100 W lamp is placed 3 m above the desk.
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Correct answer: B
Explanation
I ∝ P/d²
I₂/I₁ = (P₂/P₁) × (d₁²/d₂²)
I₁ = 15 lux, P₁ = 60 W, d₁ = 2 m
P₂ = 100 W, d₂ = 3 m
I₂ = 15 × (100/60) × (2²/3²)
= 15 × (5/3) × (4/9)
= 15 × 20/27
= 300/27
= 11.11 lux