De borgie relation and photon energy

[siromar]

I was playing around with equations last night while waiting on a friend, when I thought I stumbled on something strange.

I came up with the equation energy(e) = mass(m) x speed of light(c) x velocity(v) from the de Borgie relation wavelength(l)= plank's constant (h)/mv and the formula for the photon's electromagnetic energy e= hc/l. I tried plugging in numbers to see if I can solve for the wavelength of an electron without using the de Borgie relation, and it worked.

If the electron travels at 3.65 x 10^6 and using the invariant mass of the electron, e= mcv would give 7.24 x 10^-16

Plugging in that number in e = hc/l would give the correct answer of 2.74 x 10^-10 even though, according to my professor, the photon energy is completely unrelated to the electron.

Upon showing it to the professor to inquire as why such method would give a correct answer, he quickly dismissed me and said I got the correct answer because I went in a circle.

Can someone explain to me how what I did was going in a circle?

 

[Drakkith]

Aren't you simply using the de broglie relation in a different way?

 

[siromar]

Aren't you simply using the de broglie relation in a different way?

Wouldn't that be true only if e= hc/l is relevant for the electron? I have been told that the equation can only be used for the energy of a photon, and that the electron kinetic energy is something completely different.

 

[Drakkith]

Wouldn't that be true only if e= hc/l is relevant for the electron? I have been told that the equation can only be used for the energy of a photon, and that the electron kinetic energy is something completely different.

I believe the energy of any particle can be defined by using the wavelength. But don't quote me on that.

 

[sophiecentaur]

It's the momentum that's related to wavelength.
P=h/λ