De Broglie Equation in terms of Light

[Alyssa-Marie Dones]

Good Evening y'all!
I have a question that I need a bit of clarification on. Today in class, Prof. Lavelle was explaining how we can use the De Broglie equation to calculate the length of the wavelength for anything that has momentum and energy such as photons and electrons. However, he also said that when finding the wavelength of light we should just use the wavelength * frequency = c or E= h * frequency. Could someone please explain why we can't use the De Broglie equation to solve for light's wavelength if technically light is made up of photons?

Thank you for your time!!

[Christine Rae 1I]

In today's lecture, Prof Lavelle discussed how the De Broglie equation should be used to calculate wavelength for things that have rest mass (have mass in general). A photon, or a packet of light, has no rest mass, since it is just a unit of energy.

I hope this helps!

[Zeynep Celikkol 2A]

To extend on Christine's point,

The De Broglie equation is Planck's constant divided by momentum, which is λ=h/p. Since "rest mass" is required to find momentum. We can replace it with E/c. Thus the equation will become: λ=h/(E/c)=hc/E. We can switch around the variables to find that E=hc/λ=hv, which is an equation that can be applied to photons, which do not have a "rest mass", as the equation no longer includes momentum.