# Energy in packets

1. Jul 25, 2008

### vipulsilwal

i have just started studying quantum physics, please clear this doubt

energy is in small packets called quanta. when a body absorbs heat(form of energy) these packets are absorbed. so after being absorbed are they destroyed (or disintegrated) or remain in form of packets.
i believe it should remain in form of packets.
please correct me, if i am wrong.

2. Jul 25, 2008

### mathman

When photons are absorbed, the results are more energetic electrons, atoms, or molecules, depending on the photon energy. The photons are gone.

3. Jul 26, 2008

### vipulsilwal

'gone', does that means they are destroyed??
if that is the case, then for again making a photon(when body radiates energy) there should be some driving force or something that keeps that photon integrated(intact).
are there some constituents of photons too?

4. Jul 26, 2008

### mathman

"destroyed" is a misnomer, they are absorbed. An example - microwave oven. The energy is absorbed by the water and it gets hot.

Photons don't have constituents - they are fundamental particles of energy.

Last edited: Jul 26, 2008
5. Jul 27, 2008

### vipulsilwal

something which gets absorbed does it loses its existence?
if not, then number of photons in universe will always remain constant?
if no, how they are created?

6. Jul 28, 2008

### mathman

Absorption means going out of existence. No. of photons in universe is not constant. Photons are created whenever some process generates them. Examples: microwave oven, TV transmitter, flashlight, etc.

7. Aug 1, 2008

### map19

we don't know why you can only add or subtract energy from an electromagnetic wave in fixed amounts (quanta) but we know the amount in a packet is E = hf. So a photon is one packet and contains hf. So it's frequency dependent.
If the amount of energy in a photon is close to the amount of energy required to move a bound electron into a higher orbit it is absorbed.
At low frequencies the photon has small energy and may be absorbed into the quantised vibration of a molecule. This is infra red radiation causing the molecule to warm up. That is, vibrate faster.

8. Aug 1, 2008

### whynothis

Is "destroyed" really a misnomer? When you say "absorbed" I think of a feynman diagrams with an electron and photon coming in and an electron going out. I say it is destroyed. The energy and other defining properties may be conserved but the photon doesn't exist anymore. This seems true from a QFT point of view and from a non-relativistic QM point of view where we quantize the electromagnetic field.

9. Aug 2, 2008

### mathman

I think it is simply a matter of definition. To my mind absorbed is better, since the energy goes into the energy of motion of the electron or something similar. Destroyed to me means it is no longer there at all.

10. Aug 2, 2008

### map19

A photon is not a little ball bearing with energy. It's a quantum of energy. Energy can't be destroyed, but after a photon is absorbed it no longer exists. If absorbed by a bound electron the electron may, after a brief time delay, relax to the original state by emitting a photon. This is not the original photon because its direction of motion is likely to be random.

11. Aug 23, 2010

### curioso

I would like to extend the question. When a photon is absorbed by an electron, making the electron more energetic, does this increase the mass of the electron?

Curioso

12. Aug 23, 2010

### map19

Yes.
What applies to matter in general does not always apply to sub-atomic particles. For matter any increase in temp, angular momentum, density, (that is, any energy addition) increases the mass. and thus the the gravitation.