Radiation pressure on a totally absorbing (non reflecting) surface is Pressure=
F
A
=
I
c
...(i) Where, I= Intensity,c= speed of light it is given that; F=10−6N, A=15cm2=15×10−4m2 Time for which radiation falls on surface continuously, t=20‌min =20×60s=1200s Speed of light, c=3×108m∕s From Eq.(i) we have, I=