Question

A bulb of $40W$ is producing a light of $\lambda 620nm$ with $80\%$ oif efficiency then the number of photons emitted by the bulb in $20\sec [1ev=1.6\times {10}^{-19}J,hc=12400ev{A}^{\circ }]$.

• A. $2\times {10}^{18}$
• B. $1\times {10}^{18}$
• C. $1\times {10}^{21}$
• D. $2\times {10}^{21}$

Answer 1

Answer: (D)

$2\times {10}^{21}$

$Power=\frac{nhc}{\lambda \times t}\Rightarrow 40\times \frac{80}{100}=\frac{n\times 6.62\times {10}^{-32}\times 3\times {10}^8}{620\times {10}^{-9}\times 20}\Rightarrow n=2\times {10}^{21}$