A point source emitting light uniformly in all directions is placed 60 cm above a table-top. The illuminance at a point on the table-top, directly below the source, is 15 lux. Find the illuminance at a point on the table-top 80 cm away from the first point.

Answer

B, C, D

Explanation

Given that :
`EA=15lux=I_0/((60)^2)`
`rarr I_0=15xx(0.6)^2`
`5.4 candela
so, `EB=10 costheta/OB^2`
`-5.4(3/5)1^2=3.24lux`