Question

Monochromatic light of wavelength $3000$ is incident on a surface area $4c{m}^2$. If intensity of light is $150mW/m^2$, 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

Answer: (B)

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