# Photon's emmited by a 100w bulb.

1. Sep 4, 2008

### Seda

This is my question verbatim:

"If 5% of the power of a 100w bulb is radiated in the visible spectrum, how many visible photons are radiated per second?"

My work:

Obviously, 5% of 100w is 5w, or obviously, 5 J/s.

So the bulb is emanating 5 J of visble light per second.

Now, I could easily solve this if I knew exactly what frequency (or wavelength) of visible light this bulb was emanating, but I don't. If I knew the frequency, I could simply use

E (per photon) = hf where h is planck's constant.

And then simply do 5 J / E (per photon) = # of photons.

But I'm not given a frequency, and wavelength, or even what type of bulb this is.

Any ideas?

Mabye I should use the lowest and highest wavelengths of visible light and just give a range of photons...

2. Sep 4, 2008

### Redbelly98

Staff Emeritus
For a problem of this sort, just using the wavelength in the middle of the visible range (as an "average" visible photon) is probably sufficient.