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Question:

Light with an energy flux of 18 W/cm² falls on a non-reflecting surface at normal incidence. The pressure exerted on the surface is:

2×10−4N/m2

2N/m2

6N/m2

6×10−4N/m2

Solution:

Pressure exerted on the surface=P=F/A=dp/dt/A
Momentum p=E/c
where E is the energy of incident light, c is the speed of light.
Thus P=(1/c)(1/A)(dE/dt)=Flux/c=18×10⁴/3×10⁸=6×10⁻⁴N/m²