## When do you use light equations?

Grecia Velasco 4D
Posts: 56
Joined: Thu Jul 11, 2019 12:17 am

### When do you use light equations?

Hey guys,
I'm confused as to when use light equations (c=wavelengthxfrequency, and E=hc/wavelength) because some questions provide wavelengths already and it's technically not speed of light being used. Thank you!

905416023
Posts: 54
Joined: Thu Jul 25, 2019 12:17 am

### Re: When do you use light equations?

I'm confused about this too. in some of the review sessions, they mentioned that there were only certain instances where the equation could be used. And you needed to know when.

Sophia Shaka 3L
Posts: 57
Joined: Wed Sep 18, 2019 12:20 am
Been upvoted: 1 time

### Re: When do you use light equations?

If the value given is wavelength for an electron, frequency cannot be calculated with this equation because the constant c is the speed of light, and it is impossible for an electron to move at the speed of light. If the value given is wavelength for a photon, go ahead and use it. For electrons, we use de Broglie's equation.

Hannah Lee 2F
Posts: 117
Joined: Thu Jul 11, 2019 12:15 am

### Re: When do you use light equations?

Whenever you're dealing with photons and electromagnetic radiation, you use E = hv, c = lambda x frequency, etc. However, when you're dealing with particles WITH mass (like electrons, protons, neutrons, etc), then you'd use de Broglie's equation, λ = h / p --> λ = h / mv.

You can't use E = hv or c = λv to work with particles with mass considering that particles with mass have their own unique velocities, while the speed of light remains constant (c = 2.98 x 10^8 m/s).