word problem

Question

the illuminance of a surface varies inversely with the square of its dintance from light source. If the illuminance of a surface is 120 lumens per square meter when its distance from certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?

Answer 1

Carol,

 

Illuminance is inversely proportional to sq. of distance

 

So if illuminance is to be decreased by 4 times (i.e.120/30), the distance should be increased by 2 times

 

New distance is 12 meters or an increase by 6 m.