Question

A bulb emits light of wavelength $4500A^0$. The bulb is rated as 150 watt and 8% of the energy is emitted as light. How many photons are emitted by the bulb per second?

• A. $27.2\times {10}^{18}$
• B. $2.72\times {10}^{16}$
• C. $27.5\times {10}^4$
• D. $27.2\times {10}^8$

Answer 1

The correct option is A $27.2\times {10}^{18}$

The energy of 1 photon is $E=\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{4500\times {10}^{-10}}=4.42\times {10}^{-19}J$

Total energy (in the form of light) emitted in 1 second is $=\frac{150\times 8}{100}=12J$

The number of photons emitted is $\frac{12}{4.42\times {10}^{-19}}=27.2\times {10}^{18}$.