A 100 W point source emits monochromatic light of wavelength `6000 A`
Q. Calculate the photon flux (in SI unit) at a distance of 5m from the source. Given `h=6.6xx10^(34)`J s and `c=3xx10^(8)ms^(-1)`

Options

A. `10^(15)`
B. `10^(18)`
C. `10^(20)`
D. `10^(22)`

Answer

B

Explanation

Intensity (i.e., energy flux) at a distance of 5m from the source of power P,
`I=(P)/(4pir^2)=(100)/(4pixx5^2)=(1)/(pi)(W)/(m^2)`
The number of photons passing normally per unit area per second i.e.,
Photon flux `=(I)/(E)=((1)/(pi))/(3.3xx10^(-19))approx10^(18)` photons `m^(-2)s^(-1)`