Question

The illuminance of a small area changes from 900 lumen/m² to 400 lumen/m² when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position

Answer 1

Let, I = Luminous intensity of a source

So, The distance between the source and the original position is 20cm.