1. Apr 11, 2008

### Air

1. The problem statement, all variables and given/known data
Scientists put a lot of effort into developing suncreams which absorb the damaging wavelength of ultraviolet radiation. Suggest what might be happening to the atoms within the suncream when they absorb ultraviolet photons.

2. Relevant equations
None. This is an explain question.

3. The attempt at a solution
I believe the photons are excited to higher energy levels.

^What more could I write to explain this?

2. Apr 11, 2008

### Tom Mattson

Staff Emeritus
It sounds to me like you're supposed to be doing some research. Have you tried to dig up anything online?

3. Apr 11, 2008

### Air

No, it's a past exam question. One mark was obtained with what I had written about energy levels and the second mark says 'related to electron energy levels' which I don't know how to explain.

Surely, what I have written that photons are excited to different energy level is the same concept?

4. Apr 11, 2008

### Tom Mattson

Staff Emeritus
No, not really. Photons aren't excited to different energy levels, atomic electrons are. And you can be more detailed. Remember that UV photons are more energetic than visible photons, which is why they cause damage. So you've got a powerful UV photon coming in to penetrate deep into your skin tissue. But you've protected yourself with this cream that somehow softens the blow. Let's think about how that could happen.

The UV photon slams into you and gets absorbed by an atom in the sunscreen. When this happens, an electron gets excited from the ground state to a high energy level (skipping some intermediate energy levels).

Can you briefly describe a deexcitation process in which the emitted photons are less energetic than the initial UV photon?

5. Apr 11, 2008

### Air

Thanks for the great explanation. It was very clear. Erm, I didn't quite understand the question, but I think you wanted me to explain the process when electrons fall back levels.

When the electrons de-excite, they emit photons corresponding to the change in energy levels. These photons are by definition travelling at $3.00 \times 10^8 \ \mathrm{ms^{-1}}$ and release light.

Is that correct?

6. Apr 13, 2008

### Tom Mattson

Staff Emeritus
Right, and if the UV photon promotes an atomic electron up several energy levels, can you think of a way that it could come back down by (perhaps more than one, hint hint) photons that are all less energetic than UV photons?