In this video, David (and surfer Dan) explain how to determine the momentum of a photon.
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- 0:47What is the name of the book shown?(21 votes)
- Serway and Jewett's "Physics for Scientists and Engineers"(17 votes)
- 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!(6 votes)
- 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.(3 votes)
- How did we figure out that a photon is massless?(9 votes)
- if you look outside, there is sunlight, and if it had mass, it would just stick to the sun, because of gravity(7 votes)
- How to understand the difference between E=mc^2 and E^2=p^2c^2+m^2c^4(4 votes)
- 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.(5 votes)
- 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?(2 votes)
- 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.(7 votes)
- Why is it so important to use lambda. Isn't there a specific number we can use?(3 votes)
- 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.(2 votes)
- 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 :)(1 vote)
- I assume you mean photon, not proton.
Photons always move at the speed of light. Always.(6 votes)
- 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.(2 votes)
- Any videos on that specific formula of special relativity? at1:30? How it was derived and what it means?(2 votes)
- I believe Einstein created it. Basically, when an object goes close to or the speed of light its mass becomes infinite and can't go any faster than the speed of light.(2 votes)
- When we consider the formula momentum = h/wavelength for light isn't it's value very small? Doesn't it make momentum of light negligible?(1 vote)
- You're right, the momentum of each individual photon is tiny. But when you have many trillions of photons striking an object per second, then the net momentum transfer can be significant.(3 votes)
- [Narrator] A while back, I was teaching physics in California. I got to class and I was all like, hey class, you wanna hear a physics joke? Yeah, okay, totally. Does light hold mass? I don't know, does it? No, it's not even Catholic. Oh man. Shoulda went to the beach today. Dude, really? Totally lame. But wait, doesn't light really have mass? It should. It's got momentum so it's gotta have mass. It has momentum so it must have mass. Whoa, hey! Everyone just calm the heck down. You see, the worst part of all this wasn't that my joke bombed, it was that I actually managed to confuse people by telling it. My students had read that light has momentum and they were right, light really does have momentum. But then I come to class and I'm all like, hey, guess what, light has no mass. Now my students are thinking to themselves, dude, but P equals MV. In other words, if momentum equals mass times velocity, how could light, which has no mass, ever have momentum? Well, I had to break it to my students that P equals MV isn't really accurate for things that travel close to the speed of light. For things going that fast, you have to use special relativity. I don't wanna waste a lot of time talking about special relativity in this video so you're just gonna have to take my word for it that the rules of special relativity allow for a loophole. This loophole provides a way for massless objects to have momentum. Alright, so the bad news is that we cannot use P equals MV to find the momentum of a photon. The good news is that the formula for the momentum of a photon is simple, the momentum of a photon equals H over lambda. H is Planck's constant, 6.626 times 10 to the negative 34 joule-seconds. Lambda is the wavelength of the light in meters. Be careful, don't use nanometers, you have to convert to meters. The momentum of a single photon is gonna be extremely small, that's why it doesn't feel like we're getting pushed on very much when light shines on us, but theoretically, if you had a big enough solar sail, the light bouncing off of that solar sail could propel it forward due to the impulse imparted by the momentum of the light. Okay, so very long story short, you should never ever use P equals MV to find the momentum of a photon. To find the momentum of a photon, you should always use momentum equals H over lambda. (ambient techno music)