First slide
Quantum Theory of Light

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-1

For a perfectly reflecting surface, the average force exerted on the surface is

F=2IAc=2×25×104Wm-2×15×10-4 m23×108 m s-1

    =250×10-8 N=2.50×10-6 N

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