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Man, if I only had a nickel for every photon I shot out of my laser....

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Ok, so you're shining your pointer around photons everywhere, but if you were bored enough (as I am), you just might care to know approximately how many photons you are blasting out per second? Sounds kind of cool anyway.

Well to do this you need some simple algebra, and a couple of facts about your laser pointer, it's wavelength and power output. Now of course an exact answer here is ridiculous because the laser output and wavelength are just close approximations and the constants being used in calculation are rounded off. Oh, and a scientific calculator might help too.

For my example I will use my ~380mw 405 nm from O-like.

The first thing we need to know is how much energy each photon in a beam of 405nm light has. The power of the pointer here does not matter.

the energy of each photon is given by E = (hc/λ)

where h= Planck's constant = 6.626*10 ^(-34) J*s

c= speed of light = roughly 3.0*10^(8) (m/s)

and λ= wavelength of light in meters, so 405 nm = 405*10^(-9) m, or in proper scientific notation 4.05*10^(-7)m

units should cancel and leave you with Joules, so the energy per photon of a 405 nm light source is

E= [(6.626*10 ^(-34) J*s)(3.0*10^(8) (m/s))]/4.05*10^(-7) =

4.908*10^(-19) Joules


OK, so now how to get the photons per second as promised.......well your laser output is generally in mW and each mW is a (1/1000) of a Watt so just take you mW power and divide by a thousand. In my case 380mW/1000 =
0.380 W

now remember that 1 Watt = J*s ! so if we know how many joules per second of energy is coming from the laser and we then know the energy of each photon, all we have to do is simply divide the output of the laser in Watts (J*s) by the energy per photon (J) to find out how many photons per second we are emitting.

in my case:

(0.380)J*s/ 4.908*10^(-19)J = 7.742*10^(17) OR roughly 774 Quadrillion photons per second!!!!!
:beer:
 
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I'm not sure what use this information has other than novelty, and what use is a high-powered laser pointer anyway other than novelty? Sometimes you do things just to do them.
 

Cyparagon

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I did that once (was equally bored). A formula I came up with is

Photons per second = (P*λ*10^17)/19.86


Where P is the power in watts, and λ is wavelength in nanometers.
 
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Yep, gives you the same number, I just showed the units and whatnot so people didn't think I was doing the ever so frowned upon "magic math" that I was accused of in high school.
 
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