# Formula we can use to calculate the number of photons

1. Sep 17, 2008

### 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?

2. Sep 17, 2008

### Redbelly98

Staff Emeritus
Re: Photons

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

3. Sep 17, 2008

### Staff: Mentor

Re: Photons

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.

4. Sep 18, 2008

### a.a

Re: Photons

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

5. Sep 18, 2008

### a.a

Re: Photons

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

6. Sep 18, 2008

### Redbelly98

Staff Emeritus
Re: Photons

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 https://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:
https://www.physicsforums.com/blog.php?u=122961 [Broken]
For example:
E = hc/λ

Last edited by a moderator: May 3, 2017