Question

In 5.0s, a 75-watt light source emits 9.91×1020 photons of a monochromatic (single wavelength) radiation. What is the color of the emitted light?

Answer 1

find the energy of one of the photons (in joules) knowing that 1 watt = 1 J/s
(i.e. divide the total energy by the number of photons)

then use \(E_{photon}=\dfrac{hc}{\lambda}\)

knowing the wavelength you can find the respective colour.