## λ=c/v VS. λ=h/mv

Hannah Guo 3D
Posts: 56
Joined: Fri Sep 29, 2017 7:06 am

### λ=c/v VS. λ=h/mv

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?

Sarah Rutzick 1L
Posts: 50
Joined: Tue Oct 10, 2017 7:13 am

### Re: λ=c/v VS. λ=h/mv

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.

Maria1E
Posts: 61
Joined: Sat Jul 22, 2017 3:01 am

### Re: λ=c/v VS. λ=h/mv

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.

Elika Asis 3C
Posts: 19
Joined: Fri Sep 29, 2017 7:07 am

### Re: λ=c/v VS. λ=h/mv

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!!

Yizhou Liu 3L
Posts: 19
Joined: Fri Sep 29, 2017 7:07 am

### Re: λ=c/v VS. λ=h/mv

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.

aaron tang 2K
Posts: 49
Joined: Thu Jul 27, 2017 3:01 am

### Re: λ=c/v VS. λ=h/mv

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.

Sydney Briggs 1B
Posts: 6
Joined: Fri Sep 29, 2017 7:03 am

### Re: λ=c/v VS. λ=h/mv

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.

Richard Braun 1I
Posts: 65
Joined: Fri Sep 29, 2017 7:05 am

### Re: λ=c/v VS. λ=h/mv

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.

nanditasundarapandian1D
Posts: 21
Joined: Sat Jul 22, 2017 3:01 am

### Re: λ=c/v VS. λ=h/mv

DeBroglie's equation is used because it is the only equation that is a direct link between wavelength and momentum.

welcometochillis
Posts: 20
Joined: Fri Mar 17, 2017 3:00 am

### Re: λ=c/v VS. λ=h/mv

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.