momentum transfer

[nels]

do we have to know this? is it still in the study design? totally tripped out when i saw it in checkpoints. theyre the questions with the mirrors and photons if you dont know what im talking about.

[dejan91]

Just know that when a photon is reflected, its change in momentum = 2 x momentum before impact.

[nels]

how would i go about answering this question: (216 checkpoints) "all these photons are focused onto a perfectly reflecting mirror of area 0.1m^2. calculate the average force on the mirror." i know i havnt written the relevant data down but i hope you get the idea.

[appianway]

Force is the derivative of the change in momentum over time (dp/dt). The area of the mirror doesn't matter - you're calculating the force, not the pressure (which is the force per unit area). The change in momentum would be twice the initial momentum (which is h/wavelength), as the change in momentum is defined as the final momentum (h/wavelength in the direction of travel) minus the initial momentum (h/wavelength in the initial direction of travel). You'd then go about it by finding the number of photons colliding with the mirror per second (if you're given the power, you're given the energy per second, and then to find the number of photons you divide by the energy of each photon), and multiply this by the change in momentum for each photon (which you've already calculated). You then divide by time (which is 1, in this case as you've looked at the number colliding per second), and you've got your average force.

[NE2000]

Is the de Broglie wavelength a vector quantity?

[appianway]

I'm not too sure, but it seems like it should be. Wavelength is usually a vector quantity because it's often expressed in terms of the velocity of the wave, and the de Broglie wavelength is h/p, where p is a vector.

[NE2000]

Yeah it would be then because Planck's constant is just a scalar constant (energy x time). lol good to know I guess