Question

A bulb of 40W is producing a light of wavelength 620 nm with 80% of eficiency then no of photons emitted by the bulb in 20 sec are

• A. $2\times (10{)}^{18}$
• B. $10\times (10{)}^{18}$
• C. $10\times (10{)}^{21}$
• D. $2\times (10{)}^{20}$

Answer 1

The correct option is D $2\times (10{)}^{20}$

Energy of photon emitted by a bulb,

$E=\frac{80}{100}\times 40\times 20=640$

Wavelength of light, $\lambda =620nm$

Energy of photons $E^{\prime }=\frac{hc}{\lambda }$

$=3.2\times {10}^{-19}J$

$\therefore$ No of photons emitted , $n=\frac{E}{E^{\prime }}$

$=\frac{640}{32\times {10}^{-19}}$

$=2\times {10}^{20}$