Question

Moonlight has an intensity of 0.84$\times {10}^{-}3W/m^2$. The average wavelength of moonlight is 5000$A^0$.
A)The force exerted by moonlight on a perfectly absorbing surface of area 100$m^2$ is 0.28$\times {10}^{-}9$ N.
B) The force exerted by moonlight on a perfectly absorbing surface of area 100$m^2$ is 0.84${\times }^{-}2$N.
C)The number of photons/$m^3$ in the beam is $7\times {10}^6$
D)The number of photons/$m^3$ in the beam is 7$\times {10}^3$

Answer 1

I=$\frac{energy}{area}$

I=$\frac{power}{area}$

=$\frac{energy}{time\times area}$

=$\frac{numberofphoton\times energyof1photon}{time\times area}$

$\frac{numberofphotons}{time\times area}$=$\frac{I}{energyof1photon}$

=$\frac{0.84\times {10}^{-}3\times 5000\times {10}^{-}10}{6.626\times {10}^{-}34\times 3\times {10}^8}$

=$211.28\times {10}^{13}$