-- The energy of one photon is (h · frequency of the light)
' h ' is 6.626 × 10⁻³⁴ m²-kg/s ("Planck's Constant")
-- The question doesn't tell you the frequency of the light from the LED, but it tells you the wavelength, and
Frequency = (speed of light) / (wavelength) .
-- Now you have everything you need to calculate the energy carried by one photon from the LED.
-- The power of the light from the LED is 120 milliwatts. That's 0.120 Joule of energy per second.
Now you should be able to find the number of photons per second. It's going to be (0.120 Joule) / (energy carried by one photon) .
When I scribbled it out on a scrap of scratch paper, I got 3.853 x 10³⁸ photons, but you'd better really check that out.
