1. Journal 2/11/15 Tell me everything you know about lasers Objective Tonight’s Homework To learn how a laser functions read pp 586 to 590. Do 2.1 and 2.2 on 591

2. Notes on Lasers We wrap up our discussion on light by talking about modern applications and theories. The first of these beings with lasers. But to understand lasers, we have to understand where light comes from, first.

3. Notes on Lasers Light comes from electrons in atoms Here is our atom. Electrons can sit at different energy levels

4. Notes on Lasers With enough heat the electrons have the energy to jump up to higher levels. It's like how food can give you the energy to climb a ladder

5. Notes on Lasers Electrons are unstable at high levels, so they drop back down releasing energy as light as they go. Its just like how if you jump off a ladder, you'll release energy as sound and broken bones

6. Notes on Lasers The farther down an electron jumps, the more energetic the light it releases. Big jumps make light that is more blue, ultraviolet, or x-ray. Small jumps make light that is more red, infrared or radio. Simply put, bigger jumps release more energy.

7. Notes on Lasers So what? In normal objects, light of all frequencies and wavelengths is emitted randomly, or “incoherently”. In a laser, all the light is directed to have the same wavelength and phase.

8. Notes on Lasers So how is this done? By exploiting a trick. We said that with energy, you can get electrons in atoms to jump up a level. When they jump back down, they emit light. If we fill a tube with just one element, it will be able to produce only a few specific wavelengths of light when the electrons jump back down.

9. Notes on Lasers But this still isn’t a laser, our light may be all the same color (wavelength), but it isn’t in phase or heading the same direction. In 1917, Einstein described stimulated emission, in which we hit the same atom twice in a row with the exact same kind of light. If you do that, the second hit can drop the electron back down, but put out a new beam of light exactly in step with the second one, which continues.

10. Notes on Lasers Any material or object can do this is said to “lase”. Since we can turn one beam of light into two, after this process starts, it does a chain reaction, hitting all the atoms in the gas. After every atom has been hit, they’re all “used up” and our laser pulse is done. If you want this process to keep going, you have to have a power source keep stimulating the “used up” atoms to get their electrons excited again. That’s why most lasers take battery power.

11. Notes on Lasers To build up laser power, both ends of the tube are usually covered in glass. The back end will be 100% reflective, and the front will be 99% reflective. The 1% that gets out is the laser we see. The rest bounces back and forth, stimulating more atoms to release light. As a note, “laser” is an acronym. It stands for Light Amplification by Stimulated Emission of Radiation.

12. Notes on Lasers What about lasers as weapons? Well… they’re being developed. But you need a lot of energy to get the light from a laser to be hot enough to actually melt or burn. Essentially, the heat from a laser weapon would melt a bit of the target, which would quickly evaporate and send shockwaves through the rest of the object. Such weapons exist, but only a few, and only on a very large scale.

13. Worktime We’ve been busy the last week or so. Take the rest of today for worktime to get done whatever you need to get done.

14. Exit Question Why would it be more efficient to try and use a laser as a weapon as opposed to just a really, really bright flashlight? a) The laser can be used at longer range b) The laser could be aimed more carefully c) The laser would be invisible, except where it hits d) The laser could be built to only emit high energy light e) All of the above f) It wouldn’t be. Both could work equally well