Light and momentum

$\lambda=\frac{h}{p}$

Melody Haratian 2J
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Joined: Wed Sep 30, 2020 9:48 pm
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Light and momentum

Hi everyone!
I have a couple of questions about light and De Broglie.
How does light have a momentum if it doesn’t have a mass? Can light be used in the De Broglie equation if its momentum is given, or can it not be used because light doesn’t have a mass?
Thank you!

Chem_Mod
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Re: Light and momentum

Although light does not carry momentum due to its mass, it does carry momentum in the form of energy. This can be seen through the equation E=pc (found on the equation sheet)!

Funmi Baruwa
Posts: 108
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Re: Light and momentum

I am pretty sure Dr. Lavelle said that light does not actually have any momentum, but it has energy which takes the place of momentum.

edward_brodell_2I
Posts: 58
Joined: Wed Sep 30, 2020 9:59 pm

Re: Light and momentum

Dr.Lavelle mentioned that light exists as specific packets of energy, is quantized and exists in discrete amounts of energy. So since light has energy, is has to have a velocity which can be related to the speed of light 3.00x10^8 m.s^-1. So, light does have a kinetic energy, however it is massless since it is only a flow of packets of energy through space, an oscillating electromagnetic field in which energy moves through.

Jason_John_2F
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Re: Light and momentum

The equation E=pc shows how momentum p and energy E are related where c is the speed of light.

Brandon Le 3C
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Joined: Wed Sep 30, 2020 9:49 pm

Re: Light and momentum

Using the given equation E = pc, we know that light does not actually carry any momentum (since it does not have any mass). However, we know that light carries energy, which takes the form of momentum. The energy carried by light is momentum multiplied by the speed of light. Therefore, if we know the amount of energy in light, we can calculate the momentum.

Alex Benson
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Re: Light and momentum

Hi! I believe that the momentum is due to the energy that it has as opposed to the mass. This is because mass is not a requirement in the E=pc formula.

Mackenzie Stockton 2H
Posts: 92
Joined: Wed Sep 30, 2020 10:11 pm

Re: Light and momentum

Hi! Confirming what has already been said, light has momentum in the form of energy even though it does not have a mass. So, photons do have momentum but because they do not have mass, you cannot use the debroglie equation for a photon/EMR