[Foobarz]

Ok so I'm kind of confused as to what determine the energy of an EM wave. According to Planck's quantum theory, the energy of 1 photon is E=hv and is therefore determined by frequency. However, the principle by which lasers work is that the light waves are in phase and enhance each other, creating a large amplitude. In other words, which has more energy: One photon at gamma ray frequency or many photons at visible light frequency, and why?

[Borek]

Every photon has its own energy, described by the frequency. Laser doesn't emit different photons, it just emits lots of them. They are in phase, but it doesn't make them one. So what you are trying to compare is a single high energy gamma photon, with many small photons.

Which is basically the same as asking what is heavier - one 50kg dumbbell, or 100 0.5kg dumbbells 

[Foobarz]

Yeah I understand that a laser emits a lot of photons to be high powered

So using the dumbbell comparison, I'm assuming that many low-frequency photons have the same energy as one high frequency photon? If so, then how come the photoelectric effect is not explained with low frequencies. It says in my text:

"The number of electrons ejected was proportional to the intensity (or brightness) of the light, but the energies of the ejected electrons were not. Below the threshold frequency no electrons were ejected no matter how intense the light."

I am confused.

[fledarmus]

This is due to the quantum nature of molecular orbitals. There is a work function that can be calculated for a metal, which describes the amount of energy reguired to remove an electron from the surface - a single photon has to have sufficient energy to overcome this work function or no electron is released. Photons having lower energy may be able to raise an electron into a higher molecular orbital, but the electron will simply drop back down and re-emit the energy. Even lower amounts of energy can be absorbed into different vibrational and spin states, but no many of these low-energy photons you produce, you won't get an electron released. You have to hit the system with a single photon of high enough energy to release an electron.

You can think of it almost like an elastic force against a spring loaded button - like throwing ping-pong balls at the buttons on the inside of an elevator. It doesn't matter how many ping-pong balls you throw at the buttons, you just aren't going to be able to activate one. The forces are not cumulative - the energy of each ball hitting is absorbed and released before the next ball can hit. If you throw golf balls, it's a different story - a single golf ball can activate a button, where a hundred ping-pong balls, with the same total amount of energy, can't.

[Foobarz]

Thank you Borek that cleared everything up. I understand now that a high amplitude just means there's more photons and a higher photon density. So although the total energy of the high intensity, low frequency photons is the same as just one high frequency photon, the energy of ONE photon from the low frequency light still has much lower energy that the high frequency photon. HOORAY FOR E=hv! And now I understand quantum mechanics. . .