Question

A blue lamp mainly emits light of wavelength 4500 $\mathring{A}$. The lamp is rated at 150 W and 8% of the energy is emitted as visible light. The number of photons emitted by the lamp per second is:

• A. $3\times {10}^{19}$
• B. $3\times {10}^{24}$
• C. $3\times {10}^{20}$
• D. $3\times {10}^{18}$

Answer 1

Answer: (A)

$3\times {10}^{19}$

Number of photons emitted per second

$=\frac{8}{E}=\frac{8P\lambda }{100hc}=\frac{8\times 150\times 4500\times {10}^{-10}}{100\times 6.63\times {10}^{-34}\times 3\times {10}^8}$

$=3\times {10}^{19}$ photons per second