# Why is deBroglie λ for electrons the same as λ for photons?

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1. Apr 28, 2017

### Tommy R

Hi, I got the following question in my textbook: [translated]"Compare the wavelength of a photon and an electron where the photon and the electron have the same momentum".
My thinking is the following:
Firstly, pp (photon) = pe (electron).
My textbook briefly mentions the extention of the mass-energy equivalence E2 = p2 c2 + m2 c4, so I go by this since the particles have different speeds. The de Broglie wavelength of the electron is given by fe = E/h = √(p2 c2 + m2 c4) / h. The wavelength of the photon is the same except that the mass is 0, so it reduces to fp = √(p2 c2) / h. Since m2 c4 ≥ 0 it follows that fe ≥ fp.

My textbook says that the answer is that fe = fp tho. Their argument is that p = h/λ holds for both the electron and the photon. But they previously state that it only applies for massless particles. They derive it from E2 = p2 c2 + m2 c4 by setting m to 0. [translated]"[...]E = pc which holds for massless particles.[...] we find that p=h/λ".

Why is my argument invalid and why does λ=h/p hold for the electron? Thanks!

2. Apr 28, 2017

### Staff: Mentor

No, it isn't. $E/h$ is a frequency, not a wavelength.

Does it? With $f_e$ defined as $E / h$? Or does it have a different formula for the de Broglie wavelength?

And that is correct. Or, rearranging the formula, $\lambda = p / h$. Not $E / h$.

3. Apr 28, 2017

### Tommy R

The deBroglie wavelength as fe was a typo, I meant frequency. I now notice that I without really thinking about it assumed E=hf applied for the electron. I cannot this? (The book's definition of deBroglie wavelength is h/p, yes)

4. Apr 28, 2017

### Staff: Mentor

It does. What changes between the electron and the photon is the relationship between E and p; that should be obvious from the equations you wrote down in the OP. That in turn implies a change in the relationship between $f$ and $\lambda$. (This relationship is often called a "dispersion relation" in the literature, and the difference I've just described is called a difference in the dispersion relation between massive and massless particles.)