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### When do you use light equations?

Posted: **Mon Nov 04, 2019 9:19 pm**

by **Grecia Velasco 4D**

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!

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

Posted: **Mon Nov 04, 2019 9:43 pm**

by **905416023**

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.

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

Posted: **Mon Nov 04, 2019 9:52 pm**

by **Sophia Shaka 3L**

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.

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

Posted: **Tue Nov 05, 2019 9:54 pm**

by **Hannah Lee 2F**

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).