DeBroglie

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How do you know when to use the DeBroglie equation, wavelength=h/mv, and when to use wavelength=c/v?

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Use the De Broglie equation when calculating the wavelength of particles with mass, like electrons or other subatomic particles, where m is mass and v is velocity.
Use c = (wavelength) * v when dealing with light or other electromagnetic radiation, where c is the speed of light and v is the frequency.

[Alyssa Truong 1L]

You use the DeBroglie equation when we are talking about anything in motion/with momentum (p) and when mass (m) is involved. So, this can be referring to actual items (like a ball, person, etc…but later their wavelengths will just be too small to detect because of the large scale/mass) and electrons, protons, and neutrons. Wavelength = c/v is used when we are trying to calculate in a scenario without mass. That is why, when we calculate wavelengths emitted by photons (that don’t have mass), we’d use this equation. I think it’s also important to note that if the question mentions specifically “non-relativistic” velocity (which is part of mv in Wavelength = h/mv), this is a hint that this velocity isn’t associated/can’t be worked around or calculated using speed of light. In other words, in this case, you’d can’t mix between these two equations to find what you’re looking for if “non-relativistic” is mentioned.