Question

Monochromatic light of wavelength 3000 is incident on a surface area 4 cm 2. If intensity of light is 150 mW/m2, then rate at which photons strike the target is

• A. $3\times {10}^{10}/sec$
• B. $9\times {10}^{13}/sec$
• C. $7\times {10}^{15}/sec$
• D. $6\times {10}^{19}/sec$

Answer 1

The correct option is B $9\times {10}^{13}/sec$

Area of the surface $A=4c{m}^2=4\times {10}^{-4}{m}^2$
Intensity of incident light $E=150mW/m^2$
Thus total energy incident on the surface $E_i=0.150\times 4\times {10}^{-4}=6\times {10}^{-5}J$ in one second
Wavelength of photon $\lambda =3000\times {10}^{-10}m$
Energy of incident photon $E_p=\frac{hc}{\lambda }=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{3000\times {10}^{-10}}=6.63\times {10}^{-19}J$
Number of photons $N=\frac{E_i}{E_p}=\frac{6\times {10}^{-5}}{6.63\times {10}^{-19}}=9\times {10}^{13}$ photons/ second