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Momentum of a photon
« on: July 13, 2019, 01:06:10 pm »
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Hey guys, I was just wondering, I was given a certain frequency of light, and I was asked to calculate the momentum of the photon. Do I use plancks constant in joules or eV? How do you know when to use eV or J in other cases?
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Re: Momentum of a photon
« Reply #1 on: July 13, 2019, 02:20:52 pm »
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Hey guys, I was just wondering, I was given a certain frequency of light, and I was asked to calculate the momentum of the photon. Do I use plancks constant in joules or eV? How do you know when to use eV or J in other cases?

I would actually suggest using de Broglie wavelength equation to find the momentum of a photon. Planck's equation is usually used to find the energy of a photon. (In the end it is really ur choice though ).

The energy of a photon is in joules. If you are going to use planck's equation it would possibly be in joules. Hope that answered ur question.

P.S. In case u were wondering what is de brogile wavelength equation :

p = h / lambda

where h is planck's constant and lambda is wavelength of the certain frequency of light ( u can find wavelength of light through frequency as speed of light = wavelength  * frequency
« Last Edit: July 13, 2019, 02:27:01 pm by ^^^111^^^ »

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Re: Momentum of a photon
« Reply #2 on: July 14, 2019, 01:24:03 pm »
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I would actually suggest using de Broglie wavelength equation to find the momentum of a photon. Planck's equation is usually used to find the energy of a photon. (In the end it is really ur choice though ).

The energy of a photon is in joules. If you are going to use planck's equation it would possibly be in joules. Hope that answered ur question.

P.S. In case u were wondering what is de brogile wavelength equation :

p = h / lambda

where h is planck's constant and lambda is wavelength of the certain frequency of light ( u can find wavelength of light through frequency as speed of light = wavelength  * frequency

Sorry maybe I worded it incorrectly, I am using de Broglie’s equation, but with the h in that equation, do you sub in h in J or eV?
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Re: Momentum of a photon
« Reply #3 on: July 14, 2019, 02:47:54 pm »
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Sorry maybe I worded it incorrectly, I am using de Broglie’s equation, but with the h in that equation, do you sub in h in J or eV?

Planck's constant is measured in joule seconds. The unit for the momentum of a photon is actually kilogram metres per second. This is because de broglie wavelength is measured in meters and (as I said before) planck's constant is measured in joule seconds.

If this sounds confusing, please refer to an example of find the momentum of a photon I found on the internet for you
http://www.softschools.com/formulas/physics/photon_momentum_formula/542/