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

Step-by-step solution

For a point source, illuminance at a distance (r ) is given by Illuminance (I= luminous flux 4 r^2 ) ( aligned & I I^ = ( r^ r )^2 & I 16 I = r^ 2 (1000 ~m )^2 & or r^ 2 = (1000 ~m )^2 16 or r^ = (1000 ~m ) 4 & r^ =250 ~m & aligned )