# How does light get absorbed if its a changing electric/mag field?

1. Aug 18, 2013

### helpmeplz!

Hey guys, I was just stuck on this idea. I know that in the classical sense, light or any kind of electromagnetic radiation is a continuously changing electric and magnetic field. As e field changes it creates a changing magnetic field which creates a changing electric field and so on.

However, how does light get absorbed by atoms? Take the electric field for example, if there's an electric field I get how a charged particle or an atom would react in response to that field. But why would that field simply get "absorbed" or stop behind that point. If I have an electric field (due to a charged rod that I hang somewhere let's say), then any number of charged object that I bring in that position will move in accordance with the field but the field will not really change or disappear.

How come light does?

2. Aug 18, 2013

### Staff: Mentor

Think about some classical macroscopic waves:
Tie one end of a length of rope to a tree, hold on to the other end, and wave your arm to send waves down the rope. If you hang a weight of the right size onto the middle of the rope you will find that it absorbs some of the waves - when the wave in the rope reaches the weight it lifts the weight but there's no energy left to move the rope on the far side of the weight.

Or you can think about how water waves interact with a floating object: A ocean swell with a wavelength of 10 meters will raise and then drop a small boat as it flows happily past; but a smaller ripple will hit the boat, transfer all its energy to the boat, and disappear instead of propagating on beyond the boat.

3. Aug 18, 2013

### helpmeplz!

Yeah I get what you're saying but since in the case of light it is the electric and magnetic fields that will get "absorbed", how do you get rid of electric and magnetic fields? How is it that an atom can stop the propogation of light if all it's supposed to do is move in relation to the field's strenght and direction? And if an atom can cancel the electric field, wouldn't an atom be able to do that with any old electric field. Say if you had a charged balloon and a distance 1 metre away there was a certain electric field, just by placing another charged object there and having it move, the electric field would go to 0?

4. Aug 18, 2013

### Staff: Mentor

The charged particles in the atom move just enough in response to the time-varying electrical and magnetic fields of the wave so that the changing fields produced by the moving charged particles exactly cancel out the electrical fields of the wave. This can only happen if the masses and field strengths are just right, which is why you don't only get absorption at some frequencies for any given material.

No, you cannot cancel a static field with a time varying one (except at a single moment perhaps). But we aren't talking about static fields - there is no absorption of the wave without interaction with the absorber, and there is no interaction without something changing on both sides.

(There's only so far that handwaving like I'm doing now will take you in analyzing wave phenomena, unfortunately).

5. Aug 18, 2013

### helpmeplz!

Thank you, that was sort of my instinct as well. So basically if an atom is hit with light of exactly the right frequency to get absorbed, then it will move in a certain direction depending on the electric/magnetic field strenght and direction at that point. It's movement creates another changing electromagnetic field that completely cancels out the original light, which results in complete absorbtion. However, If there isn't perfect absorption, then the remaining field is just the net difference between the field the electron "created" when it moved (due to the original light that struck it) and the field of the original light?

And reflection works the same way I guess. Am I on the right track?

6. Aug 18, 2013

### Staff: Mentor

I would say, rather, that the remaining field is just the sum (superposition) of the incoming field and the field radiated by the electron(s).

Right. In general, you have both reflection and transmission, and some amount of absorption.