yego.me
💡 Stop wasting time. Read Youtube instead of watch. Download Chrome Extension

Photon Momentum | Quantum physics | Physics | Khan Academy


2m read
·Nov 11, 2024

[Music]

A while back I was teaching physics in California. I got to class and I was all like, "Hey class, you want to 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, she should have went to the beach. Really totally lame.

But wait, doesn't light really have mass? It should! It's called momentum, so it's gotta have mass. It adds 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 want to 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.

All right, 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 34th 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 going to 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.

More Articles

View All
Jessica Livingston at Startup School 2012
Hi everyone! This is so big league this year! I can’t believe it. We have like this team of people in the back helping. There’s real chairs, and look how many seats there are! This is so exciting. Um, I’m Jessica Livingston. I’m one of the founders of Y …
Safari Live - Day 372 | National Geographic
Program features live coverage of an African safari and may include animal kills and carcasses. Viewer discretion is advised. Well, good afternoon everybody and welcome to a spectacular start to the afternoon safari with one of our leopards we don’t see …
Julia Hartz at Female Founders Conference 2014
Our next speaker is Julia Hearts. She’s the founder of Eventbrite, which is such a big deal. I know you’ve all used Eventbrite, and I’ve known Julia for years. Actually, we kind of started our companies around the same time, and I’ve wanted for years to p…
Fishing Tips: How to Rig a Harpoon | Wicked Tuna: Outer Banks
[Applause] [Music] Captain TJ out of the Hot Tuna, and today I’m going to show you how we like to rig our harpoons and board the Hot Tuna. So what we have here is an 8ft scourge of the sea harpoon, our Lily dart on the end here. What I like to do is tak…
Calculating a confidence interval for the difference of proportions | AP Statistics | Khan Academy
Duncan is investigating if residents of a city support the construction of a new high school. He’s curious about the difference of opinion between residents in the north and south parts of the city. He obtained separate random samples of voters from each …
The Cost of Living Crisis Isn't What You Think
Is the cost of living crisis actually real? Hear me out on this. According to the Survey of Household Economics and Decision-Making, and as reported by the Financial Times, when people are asked about the health of the US economy year by year, more people…