- #1
lulzury
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Homework Statement
A sodium lamp emits light at the power P = 130 W and at the wavelength λ = 570 nm, and the emission is uniformly in all directions. (b) At what distance from the lamp will a totally absorbing screen absorb photons at the rate of 1.00 photon /cm^2s?
Homework Equations
1. (Rate of emission/absorption) ## R= \frac {P\lambda} {hc} ##
2. (Intensity) ## I=\frac {P}{A} = \frac {P}{4\pi r^2} ##
3. (Photon Flux) ## I= \frac {R} {A} =\frac {R}{4\pi r^2} ##
The Attempt at a Solution
My confusion here comes from the units of Intensity vs. Photon Flux
From the equations above, I get that
P = R, but this is not the case right?
Here is how I had set up my solution:
Since they give us I = 1.0/cm^2, this means I = 1e4 photons/m^2s
IF, I use the second equation
## I=\frac {P}{A} ##
## I=\frac {P}{4\pi r^2} ##
## r=\sqrt{\frac{P}{4\pi I}} ##
## r = \sqrt{\frac{130}{4\pi 1e4}} ##
this yields 0.0321 m, which is NOT the answer.
I get the correct answer using the third equation, but I don't understand why the first equation doesn't work in this case.