A point source is kept at a distance of 1000 m has an illumination I. To change the illumination to 16I the new distance should become

Options

A. 250 m
B. 500 m
C. 750 m
D. 800 m

Answer

A

Explanation

For a point source, illuminance at a distance r is given by
Illuminance `I = ("luminous flux " xx phi)/(4pi r^(2))`
`therefore (l)/(I')=((r')/(r ))^(2)`
`(l)/(I6I)=(r'^(2))/((1000 m)^(2))`
or `r'^(2)=((1000 m)^(2))/(16)` or `r' =((1000 m))/(4)`
`r' = 250 m`