## How do I know which formula to use to find photons?

Antonio Melgoza 2K
Posts: 33
Joined: Fri Sep 25, 2015 3:00 am

### How do I know which formula to use to find photons?

This is question #25 on the HW. Which formula do I use and why?
Sodium vapor lamps, used for public lighting, emit yellow
light of wavelength 589 nm. How much energy is emitted by
(a) an excited sodium atom when it generates a photon

Grace_Aspinwall4J
Posts: 26
Joined: Fri Sep 25, 2015 3:00 am

### Re: How do I know which formula to use to find photons?

To figure out the energy given off by a sodium atom when it generates a photon you use the formula: E=hc/λ. This equation comes from a combination of λν = c and E = hν. You use a combination of these equations because in the problem you are not given the frequency(v) so you can not simply use E = hv which is the equation used to find the energy of a photon. You also have to use λν = c to first find the frequency or you can do some simple algebra, combine the equations to get E = hc/λ and skip the step of finding the frequency. I hope this was helpfull!
- Grace Aspinwall

Kiara1F
Posts: 64
Joined: Fri Sep 25, 2015 3:00 am

### Re: How do I know which formula to use to find photons?

Wow, Grace I forgot about the combination of the two equations. That's very convenient and so helpful.

I used the light equation to find the frequency c= lambda/ nu ..and then I solved for nu (the frequency) to get the new equation : nu=speed of light/lambda.

Once I have the frequency I then use the Energy of a photon equation: E=hv