# Absorption and emission of light

1. Jun 20, 2015

### Scheuerf

What happens on the atomic level when light is reflected or refracted? Also why do some objects absorb light as heat?

2. Jun 20, 2015

### phinds

What do you understand about all that so far on your own?

3. Jun 20, 2015

### Scheuerf

I just finished my first year of HS physics. We learned about what both reflection and refraction are, but not really much about absorption. I'm trying to understand the greenhouse effect, and how/why some things reflect light while others absorb it and turn its energy into heat.

4. Jun 20, 2015

### phinds

Do you think that some things reflect all incident light and others absorb all of it? If not, what sort of factors might make something more prone to one or the other?

5. Jun 25, 2015

### Scheuerf

Some objects absorb certain frequencies and reflect other frequencies based upon their molecular structure don't they?

6. Jun 25, 2015

### phinds

Yes, and what is the result of absorption? You say some objects absorb light as heat. What else does absorption result in if not heat? that is, your statement is open to two interpretations, and I'm asking which one you mean

1) Some objects absorb light and that produces heat but other objects absorb light and it does not produce heat
2) Some objects absorb light, thus producing heat, and other objects do not absorb and light, thus no heat is generated

And by the way, I don't consider either of those to be correct, but before proceeding, I'd like to know which one you are saying.

7. Jun 25, 2015

### Scheuerf

I meant some objects don't absorb certain frequencies of light. For example white objects don't absorb any visible light.

8. Jun 25, 2015

### phinds

Which tells me nothing about what you think regarding the production of heat. You basically did not answer my question about your original post.

9. Jun 25, 2015

### Scheuerf

Sorry I forgot to add, as far as I'm aware objects always gain heat when absorbing light.

10. Jun 25, 2015

### phinds

Good. That's what I was getting at.

As far as white objects not absorbing any visible light, I'm not positive about that. "visible" gets a bit vague at the upper and lower ends and an object that is red-hot certainly emits visible light but it's possible that all of the heat absorption from such objects is in the non-visible part of the spectrum and the red that we see does not contribute anything to the heating of white objects.

Anyway, it had previously been my understanding that it's a simple matter of atoms gaining energy by electrons jumping up one or more energy levels but I was told, and I don't remember the details, that that is an overly simplistic model of what's happening.

So I guess all I've done here is help you clarify your position but not actually answer your question.

11. Jun 25, 2015

### Scheuerf

Thanks, what I'm curious about is electrons jumping to higher energy levels. I'm somewhat familiar with it, but I wasn't really sure about its relation to the absorption of light. Is there anyone that knows what happens after the electron jumps to a higher energy level? Does the electron go back to its ground state giving the extra energy to heat? Also Is there any relation between electrons jumping to higher energy levels and reflection/refraction?

12. Jun 25, 2015

### christopher.s

I guess the simple answer is that yes, usually light that is absorbed is turned into heat. It is not as simple to say that an electron moving from a ground state to an excited state gets "hotter," though. The kinetic theory of temperature can be used to explain what is happening to objects as they get "hotter." The thermal energy contained in a system is not strictly due to electron energy levels, although thermal excitation can raise the energy level of an electron. The opposite is also true, and can usually be explained through electron-phonon coupling. After an electron is excited to a higher energy state (either by lattice vibrations, i.e., temperature and phonons, or by absorption of a photon that corresponds to the energy level of the jump,) it can either; hold on to that energy for a little while, spontaneously emit a photon, be stimulated to emit a photon (think lasers,) or it can give up its energy to the lattice (temperature.)

Intuitively, I like to think of a beam of light hitting a material sort of like a wave in the ocean. If you look at the water, you will notice there are large crests, being made up of smaller crests, with tiny ripples riding all of these. The large crests are analogous to radio frequency waves, the smaller crests infrared, and the tiny ripples visible light. This is a very simple explanation, but I find it to work well. The properties of the material will determine how much of that wave is absorbed, or reflected. Most materials let radio waves (big crests) pass right through, many have strong absorption in the infrared (think resonance, as if the smaller crest waves sync up with the atoms making up the material and transfer their energy to it very well,) and many have high reflectivity in the visible. For reflection, you can think of the atoms making up the material as not being very "compliant" to the visible light radiation. This will cause a lot of the energy to be bounced back. Colors can be explained very simply by the difference in bounce back rates between the different energy waves. Things like metals have electron clouds which can occupy many different energy levels. This is one reason why you can't see through them, and also gives them their distinctive appearance.

These are simple explanations but I hope it gives you a little more insight into what's going on.

13. Jun 25, 2015

### fireflies

Yes, when it absorbs light(photon/energy actually) the electron goes to
higher energy levels, and it comes back to the ground state again by emitting energy. If the wavelength of emitted energy is equal to that of InfraRed(IR) then it is actually heat.

Not sure whether it is related to reflection or refraction, but my assumpion is, no. Those are optical properties, not atomic.

14. Jun 25, 2015

### Scheuerf

I found this video if anybody wants it. It explained that electrons actually do go to a higher energy level before being reflected or refracted if I understood it correctly.

15. Jun 26, 2015

### Scheuerf

Does anybody know what How greenhouse gases interact with infrared light to keep it in the atmosphere? Do they just absorb it?

16. Jun 26, 2015

### rootone

I am pretty sure that is just as simple as that, bearing in mind that quite a lot of the IR which gets absorbed is coming from the ground (or sea), not directly from sunlight.
In the absence of greenhouse gases more the IR would be radiated away in to space, so the gases act in the same war as clothing or blanket will keep your warmth in.
Greenhouse gases are just gases that are particularly 'good' at absorbing photons which have IR wavelengths.
http://scied.ucar.edu/longcontent/greenhouse-effect