Question

A monochromatic source of light of wavelength $\lambda$ operating at 400 W emits $2\times {10}^{16}$ photons per second. Now source operates at 600 W & wavelength changes to $3\lambda$, number of photons emitted per second are (Both wavelength are less than threshold wavelength ${\lambda }_0$)

• A. $4\times {10}^{16}$
• B. $3\times {10}^{16}$
• C. $9\times {10}^{16}$
• D. $12\times {10}^{16}$

Answer 1

The correct option is C $9\times {10}^{16}$

$P=\frac{Nhc}{\lambda }N=\frac{p.\lambda }{hc}\frac{N_1}{N_2}=\frac{P_1{\lambda }_1}{P_2{\lambda }_2}=\frac{\text{/}400\times \text{/}\lambda }{\text{/}600\times 3\text{/}\lambda }=\frac{4}{18}=\frac{2}{9}{N}_2=\frac{9}{2}{N}_1=\frac{9}{2}\times 2\times {10}^{16}=9\times {10}^{16}$ photons per second