Question

A $100$ watt bulb emits monochromatic light of wavelength $400\,nm$. Calculate the number of photons emitted per second by the bulb.

• A. $3 \times 10^{20}\, s^{-1}$
• B. $2 \times 10^{-20}\, s^{-1}$
• C. $2 \times 10^{20}\, s^{-1}$
• D. $1 \times 10^{-20}\, s^{-1}$

Answer 1

Answer: (C)

Power of the bulb $= 100$ watt $= 100 \,J\,s^{-1}$
Energy of one photon $E = h\upsilon = hc/\lambda$
$=\frac{6.626 \times 10^{-34}\,J\,s \times 3 \times 10^{8}\,m\,s^{-1}}{400 \times 10^{-9}\,m}$
$=4.969\times10^{-19}\,J$
Number of photons emitted
$=\frac{100\,J\,s^{-1}}{4.969 \times 10^{-19}\,J}$
$=2.012\times10^{20}\,s^{-1}$