Light with an energy flux of  25x104W m-2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15cm2, the average force exerted on the surface is

Energy flux, I=25×104 W m-2 Area, A=15 cm2=15×10-4 m2 Speed of light, c=3×108 m s-1For a perfectly reflecting surface, the average force exerted on the surface isF=2IAc=2×25×104Wm-2×15×10-4 m23×108 m s-1    =250×10-8 N=2.50×10-6 N