Physics - Modern Physics (5 of 26) Momentum of Light

[@GRBtutorials]

Or you could save the extra calculations and use E = pc (because photons have no rest mass), then p = E/c = 1.5 * 10^(-3)/3 * 10^8 = 5 * 10^(-12) Ns, which is the same result. After all, it's this equation, along with E = hf = hc/λ, that the momentum formula you used is derived from: E = pc = hc/λ → pc/c = (hc/λ)/c → p = h/λ.

[@josefhrdlicka2251]

I'm just wondering what would be the equivalent momentum for everyday objects, it is hard to imagine. Also for pulsed lasers that are much more powerful, are they going to be able to move for exmple a 1kg target? My calculations for ns laser said the momentum is around 20*10^-4 kg*m/s. However then for force if I do F = p/t, not sure now which time to use as I have already calculated the Energy of the pulse in ns, do I use the ns time again for the force? Thanks

[@MichelvanBiezen]

It would be very difficult to move a 1 kg object with a laser, even a powerful one. But it has been shown that spacecraft can be pushed by the sunlight if they have big "light sails".

[@karelknightmare6712]

Excuse me if I'm wrong, but lambda and h are useless because they are canceling Einp/(hc/lambda)X(h/lambda)... Ptotal=Eimp/c=(1.5X10^-3)/(3X10^8)=5X10^-12 !!!! Whatever the color of light.

[@MichelvanBiezen]

I am not sure what you mean by "useless". We are simply showing how to go from one form of the equation to another form of the equation.

[@karelknightmare6712]

Michel van Biezen i am sorry, English is not my mother tongue, I mean that this could have been yellow or blue photons, it would only have changed the number and energy of each of them but not the total momentum. Which is total energy divided by c. Maybe keeping mass.speed as a unit of momentum for light is confusing. Maybe using force.time for momentum would be more intuitive (N.s) and energy in force.distance (N.m), c (m/s) would be the conversion between them. Your channel is great I am sorry useless seems inappropriate maybe accessory ?

[@MichelvanBiezen]

Now I get your question. Note the the energy of a photon does depend on the frequency of photon (or the wavelength). A purple light photon will therefore have almost twice as much energy as a red light photon. Also the momentum depends in the same way on the frequency of the photon. A purple light photon has almost twice as much momentum as a red light photon.

[@kalvenbonin920]

If it is monochromatic light you can say that total energy is the energy of one photon times the number of photons N*E_1 = (N*P_1) * c = E_total = P_total * c

[@johnm.v709]

@@MichelvanBiezen Watch "Quantum Mechanics" on... kzbin.info/www/bejne/i6rQi4WVg5l6f68

[@Sandysan11111]

I was wondering if this lectures for high school of college level?

[@MichelvanBiezen]

There is a mix for both high school and college. This particular video could easily be taught at high school, however they usually do not get this far in the physics curriculum.

[@johntindell9591]

So interesting!

[@taxifaninternational7636]

And can anybody tell me what would be the direction of the momentum of light, and why? thanks :)

[@MichelvanBiezen]

The direction of the momentum is in the same direction as the direction of travel. The interaction of the photon with a particle must be in the same direction of travel of the photon for the momentum to be conserved during the collision.

[@taxifaninternational7636]

@@MichelvanBiezen Thank you very much Michel.

[@marcosmimenza]

Thus, the food cost of a white board is diluted by the number of observers.

[@MichelvanBiezen]

An interesting analogy.