How do you change the distance from a point light source to reduce the illumination of a surface

How do you change the distance from a point light source to reduce the illumination of a surface perpendicular to the light rays by 4 times?

To find the required change in the distance from the specified point light source to the surface, we use the equality (we take into account that the surface is perpendicular to the light rays): k = En / Ek = (I / rn ^ 2) / (I / rk ^ 2) = (rk / rн) ^ 2.

Variables: k – decrease in illumination (k = 4 p).

Calculation: k = 4 = (rk / rn) ^ 2 and rk / rn = √4 = 2 and rk = 2rn.

Answer: To reduce the illumination, the distance from the specified point light source must be increased by 2 times.