Photons

[a.a]

Is there a formula we can use to calculate the number of photons needed to provide a certian energy given only the wavelength of the photons?

[Redbelly98]

Yes there is. Is this homework? What's the energy of one photon, if you're given its wavelength?

[berkeman]

This does look like coursework, so I'll move the thread to the Homework Help forums.

And as Redbelly says, a.a, are you familiar with the standard formula for the energy of a photon in terms of its wavelength (or frequency)? It should be in your textbook, or you could find it by searching on "photon" at wikipedia.org.

[a.a]

when i searched on wikki i only found E= hc all ever lambda
but all these are given, we have the energy, the wave length and both constants

[a.a]

nevermind.. just realized i was being stupid, so if wavelength is 656nm abd we need 10 J then this would be what we need to do, can someone please check this? thx.

E = hc/lanbda = ...= 3.0299 * 10^-19 J
then that means one photon gives 3.0299 * 10^-19 J
so to get 10 J: 10 J --> 10/3.0299 * 10^-19 = 3.3 * 10^19 photons
ANS: 3.3*10^19 photons needed to produce 10 J

sorry im not familiar with how you post eqn the proper way

[Redbelly98]

Looks good!

There are 2 ways to write equations.

You can use LaTex. Click on this equation, to see what I typed to generate it:

E=\frac{hc}{\lambda}

More info on LaTex is at http://www.physicsforums.com/misc/howtolatex.pdf . Also, you can click on the Σ symbol in "advanced edit" mode to get a menu of Latex symbols.

If you don't need the full features of Latex, you can also copy-and-paste a lot of math symbols from here:
http://www.physicsforums.com/blog.php?u=122961
For example:
E = hc/λ