Question:
The illuminance of a small area changes from 900 lumen/ $\mathrm{m}^{2}$ to 400 lumen/ $\mathrm{m}^{2}$ when it is shifted along its normal by $10 \mathrm{~cm}$. 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.
Solution:
