Question

Calculate the number of photons emitted by a 100W yellow lamp in 1 s. Take the wavelength of yellow light as 560nm and assume 100 percent efficiency.

• A. $1.6\times {10}^{20}$
• B. $1.4\times {10}^{20}$
• C. $2.8\times {10}^{20}$
• D. $2.1\times {10}^{20}$

Answer 1

The correct option is C $2.8\times {10}^{20}$

Energgy of 1 photon:
$E_p=\frac{hc}{\lambda }{E}_p=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{560\times {10}^{-9}}$
$E_p=3.55\times {10}^{-19}J$
Total energy emitted is $100J$
Number of photons:
$n_p=\frac{\text{Total energy emitted}}{\text{Energy of 1 photon}}=\frac{100}{3.55\times {10}^{-19}}=2.8\times {10}^{20}$