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Q & A: Can you turn green light orange?

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Most recent answer: 05/15/2013
Q:
hello, today we did an experiment in school it was about light shining through a prism first we used white light and i was okay everything went good but when we tried the Green Laser something weird happend, when the green laser went inside the prism it produced red light and bended and then again exit the prism with same green light ! what is that red light 'inside' the prism and how it is produced ! thnx....... Hope i get the answer =)
- san Ari (age 16)
los angelos
A:
That's a great observation and question.

Let's think about what you see when you look at that beam. You don't see the main light of the beam, which continues on a path that better not be headed toward your eyes. You only see little bits of the light that have bounced off something ("scattered"), heading in a new direction. The vast majority of the beam is staying green, and that's why you see green come out the other side.

What does light bounce off of? One thing is an unevenness in the glass. Think of how light travels through water or clear oil, mostly just straight through. Now shake them up together, and there's a lot of little unevenness for the light to scatter off. And lots of it will scatter.

In glass, there's a little bit of unevenness in the positions of the atoms, and some of the green light will scatter off it. I bet if you look through a green filter designed for that laser color you'll see a very faint green beam in the prism.

That you see red means that there's something else going on, scattering more light. One of the ways light can scatter dumps some of its energy and momentum in the material. There's a sort of recoil, with (typically) a tiny sound wave left in the material. The little "photon" of light that gave up that energy now bounces out with a little less energy than it had going in. That means its frequency shifts down, because the energy is just a constant times the frequency. (That's a basic part of quantum mechanics.) So this type of scattered light is shifted toward the red. That's what you're seeing. It's called "Raman scattering".

You should also be able to see Raman scattering from very clean water. If I remember right, it looks orange if you start with a green laser.  Once a medical doctor who was trying to do some experiments told me he was about to replace his very expensive green laser because it was giving orange light from water. Fortunately we talked before the order went through.

By the way, you may wonder why you didn't see any Raman scattering from the white light. Actually, you did, but since that light has a whole range of frequencies shifting the frequencies a bit isn't so easy to notice.

Mike W.

(published on 02/24/2013)

Follow-Up #1: Why does blue light look green in glass?

Q:
Why is it that when a blue ray hits glass the color is green and once it's through and hits another objects the color remains blue?
- Dulce (age 18)
Phoenix, AZ, USA
A:
This probably has the same answer as why a green beam looks orange. You can have a look at the other answer in this thread.

Mike W.

(published on 05/15/2013)

Follow-up on this answer.