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λ=c/v VS. λ=h/mv

Posted: Mon Oct 16, 2017 9:39 pm
by Hannah Guo 3D
What's the difference between λ=c/v and λ=h/mv (De Broglie's wave equation λ=h/p)? Do we use λ=c/v when calculating the wavelength of photons and λ=h/mv when calculating the wavelengths of other moving particles?
Please help. Thank you!

Re: λ=c/v VS. λ=h/mv

Posted: Mon Oct 16, 2017 9:59 pm
by Sarah Rutzick 1L
I think you are correct that you want to use λ=c/v when dealing with a photon of light, and λ=h/mv for other particles. What also helps is that when you are given a velocity in the problem, you know you must use λ=h/mv, but if you are not given a velocity, you can assume that the velocity is the speed of light, c.

Re: λ=c/v VS. λ=h/mv

Posted: Tue Oct 17, 2017 2:04 pm
by Maria1E
You decide what equation to use based on the information you are given in the question. If the frequency is given in the equation and you want to find wavelength, use λ=c/v. In addition, if you are given energy in J and you need to find wavelength, you would use E = hv to find the frequency and then apply that value to λ=c/v to find the wavelength. If you are given velocity (v) in the question, it would be better to use the equation λ=h/mv to find the wavelength.

Re: λ=c/v VS. λ=h/mv

Posted: Wed Oct 18, 2017 11:09 am
by Elika Asis 3C
De Broglie's equation is used for particles with mass, like electrons, protons, or bigger items, while the equation of λ=c/v, would only be used for massless particles, such as photons. If you were to use De Broglie's equation for a massless particle, you would end up with a 0 in the denominator. Hopefully that helps!!

Re: λ=c/v VS. λ=h/mv

Posted: Thu Oct 19, 2017 12:12 pm
by Yizhou Liu 3L
In the equation λ=h/mv, v represents the velocity of any particle with momentum; while in the equation λ=c/v, c is a constant number represents the speed of light, which means this equation is only used for photon.

Re: λ=c/v VS. λ=h/mv

Posted: Thu Oct 19, 2017 11:57 pm
by aaron tang 2K
λ=c/v : this equation means λ (wavelength) is equal to c (speed of light) divided by v (frequency). c will always be 3.00 x 10^8 m/s.
λ=h/mv : this equations means that λ (wavelength) is equal to h (Plank's constant) divided by m (momentum) * v (velocity).

The biggest difference between the that λ=c/v is used for finding the energy of a photon because λ=c/v is derived from E=hv, while λ=h/mv is used when the question is asking for the velocity or mass of an electron.

Re: λ=c/v VS. λ=h/mv

Posted: Fri Oct 20, 2017 1:11 pm
by Sydney Briggs 1B
Maria1E wrote:You decide what equation to use based on the information you are given in the question. If the frequency is given in the equation and you want to find wavelength, use λ=c/v. In addition, if you are given energy in J and you need to find wavelength, you would use E = hv to find the frequency and then apply that value to λ=c/v to find the wavelength. If you are given velocity (v) in the question, it would be better to use the equation λ=h/mv to find the wavelength.


If E is equal to hv, how would we convert the hv back into the equation λ=h/mv? I think I miss a couple steps when switching.

Re: λ=c/v VS. λ=h/mv

Posted: Sat Oct 21, 2017 10:01 pm
by Richard Braun 1I
Use c/v for something with no mass, like a photon or electromagnetic waves. Use h/mv for problems involving mass, such as a proton.

Re: λ=c/v VS. λ=h/mv

Posted: Sat Oct 21, 2017 10:17 pm
by nanditasundarapandian1D
DeBroglie's equation is used because it is the only equation that is a direct link between wavelength and momentum.

Re: λ=c/v VS. λ=h/mv

Posted: Sun Oct 22, 2017 3:56 pm
by welcometochillis
The v's in the equations are different. in the c/v equation "v" is actually a greek letter nu which represents frequency of things with no mass. In the h/mv equation V represents velocity of a particle with mass. That's why mass is usually somewhere in the equation, you are usually calculating how fast an object with mass is going.