Announcements

1. Announcements Exam Tuesday night, 7:30 pm in Muenzinger Psychology E-0046. Exam will emphasize material covered since last exam (HWs 4,5,6,7). Some questions may relate to lab so make sure you refresh yourself on the physics of what was covered in lab. Multiple choice and essays. Bring: Two note cards, calculator, pencils Exam review on Tuesday… email questions to both Kathy and Carl. Pick up little plastic diffraction grating ! Hold by the edge… avoid fingerprints.

2. Why snow and white paint are white. Look at perfect giant block of ice (frozen lake) where beam of sunlight is hitting it. It will look what color? a. blue b. white c. black d. yellow e. red c. black!! If perfect block, light only reflects straight up or goes on through. No light scatters or reflects into eyeball. If no light comes from object, it looks black!!

3. Same thing but add some more ice on top of it in form of snow. Look at where beam of sunlight is hitting it. It will look what color? a. blue b. white c. black d. yellow e. red

4. magnify white where reflect, black if not Same thing but add some more ice on top of it in form of snow. Look at where beam of sunlight is hitting it. It will look what color? a. blue b. white c. black d. yellow e. red b. white, each chunk of frozen ice has many surfaces. Some reflect light into eye. Reflect all colors the same, so look white! So many surfaces that all the light ends up getting reflected. White paint works same way. Scatters all colors of light equally, absorbs none. Grey absorbs all a little.

5. Why do stars and distant lights twinkle? skip, but in notes. Bending of light by air. light slows down in transparent material- “index of refraction, n” reflected refracted. Amount of bend depends on angle of surface and the difference in indices of refraction (speed of light in air vs. material) air, v = .9999c (n = 1.0001) material, v = c/n index of refraction and hence amount of bending from air always small but depends on pressure and temperature.

6. patch of colder air, blowing through, bends light to go into eye for a moment, then blows away. Amount of light increases then drops. twinkling just cause air bends light around and different temp blobs of air moving around bending light a little bit so amount reaching eye increases and decreases. must be long ways away, so goes through large air blob to bend enough.

7. Pick up little plastic diffraction grating ! • Today- lasers (complete with awesome graphics!) • a. What is different/special about laser light. • b. How does a laser work. • review atomic discharge streetlight. • how light interacts with atoms • how these idea used to make laser. lasers- 0.001 W laser pointers 10,000 W metal cutters (infrared) (50 W in my lab, hurts!) all kinds of colors, times--continuous down to 0.000000000000001 sec long pulses. Uses- CD players, surgery, pointers, sending telephone signals, measuring distance to moon, …

8. reading quiz. 1. “stimulated emission of radiation” refers to a. a type of decay of radioactive nuclei, b. the emission of a duplicate photon by an atom when another photon is passing by, c. colliding electrons with atoms causing them to produce light, d. very energetic radioactive particles being emitted. ans. b. 2. “incoherent light” is light that is a. babbling nonsensically, b. made up of a single electromagnetic wave, c. made up of many different electromagnetic waves, d. coming out of multiple widely separated light bulbs, e. light reflected from a punk rock CD. ans b. 3. the “laser medium” mostly a. amplifies light, b. absorbs light, c. calls back ghosts of dead lasers, d. reflects light, e. none of the above ans. a

9. Review of atom discharge lamps-- neon signs. Energy levels in isolated atom. Energy levels metal, bulb filament, or not stuck in atom (like sun). If hot, jump between all diff. levels. Wiggle around, all colors. kick up, only certain wavelengths when come down. In discharge lamps, lots of electrons given bunch of energy (voltage). Bash into atoms. (“discharge tube”) 120 V or more with long tube

10. atoms lazy- always want to go back to lowest energy state. Have to get rid of energy, send it off as light. Excited atom 2 1 3 Fast electron or right color light hits atom Atom back to low energy e or e e Light emitted e

11. look at neon lamp with diffraction gratings. (much more stuff like this in lab next week) Hold grating only by edges...oil from hands ruins grating. Hold close to eye... See rainbow from lights. Turn so rainbow is horizontal. See lines from neon lamp.

12. compare flor. light, neon lamp and laser light from pointer. If you look at light from laser pointer, it will look a. more like from light bulb with a range of colors, b. like from neon lamp, but with only a single color c. will not show up at all when you look at it through diffraction grating. d. will be a single bright color but too intense to look at without discomfort. b. It is light of all exactly the same color, so no spread with grating. Focus flashlight beam and laser beam with lens. a. both will focus to same size spot. b. laser will focus to much smaller spot with much more power in it. c. flashlight will focus to smaller spot with more power in it. d. laser will focus to smaller spot but with less total power. d. flashlight actually has quite a lot more power, but not as intense because spread out over much larger region. All laser light the same so focuses into same spot, (or very nearly)

13. light from lasers are much more likely to damage the retina of the eye than light from a light bulb because a. laser is at a more dangerous color, b. has lots more power in the beam c. light is concentrated to a much smaller spot on the retina, d. light from bulb is turning off and on 60 times per second so light is not as intense. c. focuses to much smaller spot, local burn. Why lasers are good for surgery: can make a spot much smaller than scalpel, get to retina or lens of eye without damaging stuff in front, send down tiny fiber running down artery to get into middle of body with only tiny hole. Why bad-- tend to keep on burning in straight line. laser light is special and useful because all light exactly the same color and direction. Can be controlled much better, focused into smaller spot, sent in more parallel beam etc. HW talks about how not perfect beam. On to how laser works.

14. Laser-- Light Amplification by Stimulated Emission of Radiation repeated cloning of photons to produce LOTS of identical photons of light. Requirements: stimulated emission (always have) population inversion of bunch of atoms (hard) optical feedback (mirror) photon, little piece of wave, we often draw as little ball because less work. or ? Everything to know about interaction of light and atoms. 3 easy steps. atom photon electron in higher energy level 1. absorption of light e 2. Spontaneous emission of light. Electron jumps down from upper level, gives off light. Randomly in any direction.

15. 3. Stimulated emission of light. First realized by A. Einstein Photon hits atom already in higher energy level. What happens? a. gets absorbed atom to even higher energy level. b. goes right on through, atom does not notice, c. original photon continues and second identical one comes out of atom. d. original photon scatters off in random direction. c. second identical photon comes out. Cloning photons. Amplifying amount of light x 2. P2k applet. If send in light faster, what happens?

16. Surprising fact. Chance of stimulated emission of excited atom EXACTLY the same as chance of absorption by ground state atom. Equal chance bottom atom will absorb photon leaving no photon, as top atom will jump down and so have two photons. glass tube full of atoms, discharge lamp I would expect that a. more photons will come out right hand end of tube, b. less come out right, c. same number as go in, d. none will come out.

17. glass tube full of atoms, discharge lamp I would expect that b. less come out right, 2 atoms inside get excited. How do we know it is 2 and not all 3? We don’t. I made it up. A matter of chance, sometimes could be 0, sometimes 1, sometimes 2 or 3, and 3,2,1 or 0 photons make it through. Depends on number of atoms and how close they are together. But most of the time will be less photons coming out because more lower level atoms than upper. To amplify number of photons going through the atoms I need a. more atoms in lower energy level, b. half in lower, half upper, c. more in upper energy level, d. a sufficient number in upper level and it does not matter how many are in the lower.

18. To amplify number of photons going through the atoms I need c. more in upper energy level than in lower. Lower eats them up, upper clones them (adds energy). Equal prob. so amplification or loss is just Nupper-Nlower. Nupper > Nlower, more out than in. (atoms change) Nupper < Nlower, fewer out than in. (and atoms change)

19. so get amplification if more in upper level than lower this condition very unnatural known as “population inversion” how to get population inversion? skip? try getting by shining lots of light on atoms. If turn light way up will get a. population inversion with all atoms excited to upper level, b. no atoms in upper, c. half the atoms in upper.

20. (probably skip this and the next slide) half the atoms in upper. Once in lower level absorb, so start absorbing, going to upper level. But then ones in upper level start getting stimulated back down. When really lot of light just go to balance with equal up and down. NO POPULATION INVERSION!! To get population inversion, need at least one more energy level involved. to have most of the atoms piled up in state 3 (pop. inversion with lower) you would need a. rate out of 3 slower than rate up to 2. b. rate from 2 to 3 faster than 3 to 1, c. rate out of 3 faster than 2 to 3 but slower than 1 to 2, d. a and b 2 3 also can kick up by bashing with electron d. rate out of 3 slower than into it. 1 “pumping process to produce population inversion”

21. To get population inversion, need at least 3 energy levels involved. Need rate into 3 faster than rate out. What color light could come out on 3 to 1 transition? a. green, b. blue, c. red , d. a and b., e. a and c c. red. Has to be lower energy than the green needed for 1 to 2. Smaller the separation between 3 and 1, slower the rate from 3 to 1. So these two factors why easier to get lasers at longer wavelengths than at shorter wavelengths. 2 3 also can kick up by bashing with electron 1 “pumping process to produce population inversion”

22. so now see how to get population inversion, will give amplification of red light. If enough atoms in upper, will lase. p2k animation But much easier if not let light all escape. Reuse. mirror to reflect the light.

23. gas laser like Helium Neon. Just like neon sign with with helium and neon mixture in it and mirrors on end. Diode laser- same basic idea, but light produced like in light emitting diode at P-N diode junction. Mirrors on it. p2k laser. Show working. If only pump a little bit what happens?

24. p2k laser. If pump just a little tiny bit a. never will produce laser light, b. will take very long time but will give laser light, c. will give laser light as quickly but will just produce very little light. a. spontaneous emission of light will keep too few atoms in excited level to have population inversion. Never will amplify.

25. two excited atoms in a row skip? what will come out on the right? Think before you pick… be ready to explain. a. 1 photon, b. 2 photons, c. 3 photons, d. 4 photons, e. 8

26. what will come out on the right? Think before you pick… a. 1 photon, b. 2 photons, c. 3 photons, d. 4 photons, e. 8 c. 3 Double at first atom, then both hit second but atom only has enough energy to give off one more photon. Remember, photon energy E = hc/ must match difference in energy levels for electron. e e atom 1 atom 2