Question

A 60-Watt bulb illuminates a surface a distance d away with
an illuminance of 3600 lux. What would be the illuminance
on a surface a distance 0.5•d away (i.e., one-half the
distance away) when illuminated by a 120-Watt bulb?
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Answer 1

Answer:

Explanation:

Relation between power and illumination and distance  is as follows

$I = \frac{P}{R^2}$

For first  case

$3600 = \frac{60}{d^2}$

for second case

$I = \frac{120}{(0.5d)^2}$

$\frac{I}{3600} = \frac{d^2}{60} \times \frac{120}{.25d^2}$

I = 3600 x 8

= 28800 lux .