The energy efficiency of an incandescent light bulb (= the percentage of consumed power that is actually converted into radiated light) is typically around 5%. Suppose a certain brand of 75 W incandescent bulb emits light primarily at wavelength = 570 nm.

Calculate the total number of photons emitted by one of those bulbs if it has been shining for a 6-hour period.

4*10^22 photons

Explanation:

To find the number of photons is necessary to calculate the total energy of the light emitted in one hour = 3600s:

Furthermore, is necessary to find the  associated energy to the photon 0f 570nm with following formula:

h: Planck's constant = 6.62*10^-34 Js

c: speed of light = 3*10^8 m/s

wavelength = 570*10^-9 m

Finally you divide E' between Ep to find the number of photons:

the number of emitted photons is 4*10^22