## Calculate Number of Photons

Duby3L
Posts: 72
Joined: Fri Sep 28, 2018 12:26 am

### Calculate Number of Photons

Question 1B.9:A lamp rated at 32W emits violet light of wavelength 420nm. How many photons of violet light can the lamp generate in 2 seconds?

So I calculated the energy per photon which turned out to 4.7x10^-19 J, but I'm not sure what are the next steps I need to take to find the number of photons.

Sydney Tay 2B
Posts: 64
Joined: Fri Sep 28, 2018 12:20 am

### Re: Calculate Number of Photons

First you have to calculate the energy from the lamp, which is 32J/s *2s = 64J. Then you can calculate the energy corresponding to the wavelength (using E=hc/wavelength), which would give the the energy per photon of light. Once you have that, you can divide 64J by your answer which is in J/photon to give you the # of photons.

Duby3L
Posts: 72
Joined: Fri Sep 28, 2018 12:26 am

### Re: Calculate Number of Photons

Why do we have to divide energy by the energy of a photon? I know its to calculate the number of photons, but how did you know we had to divide?

Sophia Ding 1B
Posts: 62
Joined: Fri Sep 28, 2018 12:16 am

### Re: Calculate Number of Photons

I like to think of it in terms of the units; when you calculate the energy from the lamp, you get your answer in Joules. Then, when you calculate the energy corresponding to the wavelength and therefore the photon, you are left with Joules/photon. Thus, if you divide Joules by Joules/photon, you are left with the Joules cancelling out and photons on top, giving you the number of photons generated.