# Homework Help: Photons per second hitting an aperture

1. Dec 1, 2008

### SammC

1. The problem statement, all variables and given/known data

>An aperture is 3m away from a light bulb, with an area of .09m^2.
>The light bulb is 100 Watts, and 2.5% of the energy is converted to visible light
> From the previous part: The average wavelength of the light is 520 nm, meaning that each photon has an energy of 2.386 eV (this answer is correct in the system)

How many photons per second will hit the aperture.

[The hint for this is "Remember to convert eV to Joules, because the intensity is given in W/m^2"
I'm not using intensity in my answer, nor am i sure how to, but the answer i'm getting seems to make sense anyway.]

2. Relevant equations

3. The attempt at a solution

First, i convert 2.386 eV to Joules, getting 3.82279*10^-19 Joules/Photon.
Now, given that its a 100 Watt bulb, converting 2.5% to light, i assume that means that it emits 2.5 Joules/Second of light.

So, 2.5 (J/S) / 3.82279*10^-19 (J/P) will give an answer in Photons/Second, which is
6.5397 * 10^18 Photons/Second. Emitted from the bulb (I assume spherically)

So, then I find what percentage of those photons will hit the aperture by dividing Aperture area by the surface area of a 3m radius sphere: .09m^2 / (4*pi*3^2) = 7.9577*10^-4 m^2/m^2

Multiplying that percentage by the number of photons per second: 5.204*10^15 photons/second, which is incorrect.

Last edited: Dec 1, 2008
2. Dec 1, 2008

### turin

Hmm... I haven't read the numerical details of your calculation, but the way you describe it sounds correct.

OK, I just did the calculation, and I agree with your result except for the last decimal place. Are you entering this into one of those online homework systems that is picky about sig figs? If so, post the numbers EXACTLY how they are given, and I will check for sig figs.

Last edited: Dec 1, 2008