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Course: Physics archive>Unit 17

Lesson 1: Photons

Photon Momentum

In this video, David (and surfer Dan) explain how to determine the momentum of a photon.

Want to join the conversation?

• What is the name of the book shown?
• Serway and Jewett's "Physics for Scientists and Engineers"
• How did we figure out that a photon is massless?
• if you look outside, there is sunlight, and if it had mass, it would just stick to the sun, because of gravity
• If I write momentum of photon=Energy of photon/velocity of light, is that incorrect? I got my marks deducted in a physics exam for writing that!
• That's correct! For a massless object, the energy and momentum carried by the massless object are proportional to one another, and the proportionality constant (to get the units right) is c.
• How to understand the difference between E=mc^2 and E^2=p^2c^2+m^2c^4
• E=mc^2 is for objects that have mass and that aren't moving, whereas the full formula takes the reverse of those into account as well.
• Then what is the range of velocity for the formula p=mv?
Like David said you cannot use it for objects which move at the speed of light
So we start at 0 then what? till where?
• It depends on how precise you need to be.
For speeds below about 0.2 c the relativistic and non-relativistic calculations are nearly indistinguishable. As you get closer to c they diverge, and the rate of divergence grows very rapidly as you get closer and closer to c.
• If a photon bounces off a surface or object, would it still move at light speed, but we just can't see it? Or does it dissipitate somehow? Also, what is the song playing in the background? I like it :)
• I assume you mean photon, not proton.
Photons always move at the speed of light. Always.
• But Dan you said in the last video you said that the energy
of a photon is E=hf but now you subbed E with hc/λ why is that.
(1 vote)
• f=c/λ
• Why is it so important to use lambda. Isn't there a specific number we can use?
• Lambda refers to the wavelength of the light. So it depends on what light you're talking about. It's not a constant, but used to represent the wavelength of the light we're talking about.