# A monochromatic source of light operating at 200 W emits 4 x 1020 photons per second. Find the wavelength of the light.

Arun
25758 Points
4 years ago
The energy of each photon=
E = (200 J/sec)/(4 * 10^20 /sec)
= 5 * 10^-19 J

Now $\lambda = hc/ E$
$\lambda$ = (6.6* 10^-34) *(3* 10^8)/(5 * 10^-19)
= 400 nano metre