## Photons of light

$c=\lambda v$

Chem_Mod
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### Photons of light

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

Chem_Mod
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### Re: Photons of light

First you have to calculate the energy from the lamp, which is simply 32J/s *2s = 64J. Next you are given the wavelength of the light that the lamp emits. You can calculate the energy corresponding to that 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. Notice how the units work out

DavidEcheverri3J
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### Re: Photons of light

I get how you got E from the wavelength, but why do you need to divide 64 by the answer? (I also understand where the 64 came from) Is it to get the photons isolated from the J??

WilliamNguyen_4L
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### Re: Photons of light

DavidEcheverri3J wrote:I get how you got E from the wavelength, but why do you need to divide 64 by the answer? (I also understand where the 64 came from) Is it to get the photons isolated from the J??

So 64 is the total amount of energy there is and since the E calculated from wavelength tells you the amount of energy per photon, dividing 64 by that value will tell u how many photons there are present in the 64 J of energy.
Think of it like you have 30 apples. If you are giving away 2 apples per person how many people will get apples? 15 since 30/2=15
The units also cancel out. J/(J/Photon)=# of Photons

Nina Do 4L
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### Re: Photons of light

First, I converted my given wavelength of 420 nm to m (420 x 10^-9) and multiplied the given 32 W to joules (32 J/s). Since the problem gave us a time, I multiplied the time with the Joules/sec to give me 64 total Joules. That is what I would call the TOTAL Energy, not the energy for one photon. So now I used the E=hc/lambda equation and subbed in lambda for the 420 x 10^-9 m and got 4.73 x 10^-19 J for 1 photon. That is the energy of 1 photon so I then used a conversion table, 64J x 1 photon / 4.73 x 10^-19 J to get 1.4 x 10^20 photons emitted.

To put it into moles, I just divided the 1.4 x 10^20 photons by 6.022 x 10^23 and got 2.3 x 10^-4 moles.

Camille 4I
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### Re: Photons of light

Why do we have to multiply 32J/S by 2?