Class 14-light and lasers - PDF Free Download

Class 14-light and lasers Today - light & glass - light & 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 ideas are used to make laser. lasers- 0.001 W laser pointers 10,000 W metal cutters (infrared) (5 W in lab, hurts!) all kinds of colors, times--continuous down to 0.000 000 000 000 001 sec long pulses. 1

Q: Why is the sky blue? Light scatters off atoms in the atmosphere. Blue light scatters more than red, SUN Scattering process depends upon wavelength (blue better) closer to matching energy levels of nitrogen and oxygen. (Not quite same as book, but right.) Blue scatters Red goes through excited level of nitrogen (O close) Earth surface Energy of light E = hc/λ, Red bigger wavelength, smaller energy photons. Blue closer to match, more absorption and spitting out 2 = scattering.

sunset light, gone through so much air all the blue is scattered out. Only red left. 3

Absorption, Scattering, Reflection Sky blue and sunset red because of how light scatters off atoms and dust in the air. Dyes different colors because of which colors absorbed, the rest scatter off. Other ways light can interact with matter: reflection and refraction why snow is white, water and ice shiny, stars twinkle, glass lenses bend light. 4

Speed of light through space = c = 3 x 10 8 m/s Light travels slower in all other materials (air, glass, diamonds, water) Speed of light in a material = c/(index of refraction) = c/n Speed of light in space > speed in air > speed in water > speed in glass What happens when light suddenly slows down when hits glass? Air, velocity =.9999c (n = 1.0001) Some reflected Glass n glass ~ 1.5 Slower velocity Velocity of light = wavelength x frequency (frequency fixed wavelength changes, smaller in glass) Some Refracted (heading in different Direction). 5

Light travels more slowly in glass than in air. Car Model of bending. Amount of Bend depends on: 1. Speed of light in material. Index of Refraction (n). For Glass, n=1.5 (light travels 50% slower in glass than air) 2. Angle at which light hits surface. Amount of light reflected: 1. Depends on angle of light hitting interface 2. Depends on difference in speed of light.. Greater change, more reflection AIR, N=1 GLASS, N=1.5 AIR, N=1 Small Reflection Light Beam 6

A car changes directions at a concrete/sand interface because the sand slows it down. Concrete (air) Sand (glass) v v 7

Diffraction gratings bend longer wavelengths more. What about prisms? Prism demo from last time Speed of light in glass: A) Is slower for red photons than for blue photons B) Is slower for blue photons than for red photons C) Is the same for blue photons and for red photons 8

What happens when light exits glass. Which path does it follow: a. b. c. Small reflection a b AIR, N=1 GLASS, N=1.5 c AIR, N=1 ans. a. Like car. Front right tire in air first, goes faster, turns car more toward top. 10

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!! 11

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 12

magnify 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! White paint works same way. Scatters all colors of light equally, absorbs none. 13 Grey absorbs all a little.

Why do stars and distant lights twinkle? Bending of light by air. reflected light slows down in transparent material- index of refraction, n air, v =.9999c (n = 1.0001) material, v = c/n refracted. Amount of bend depends on angle of surface and the difference in indices of refraction (speed of light in air vs. material) index of refraction and hence amount of bending from air always small but depends on pressure and temperature. 14

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. 15

What would make Murray the MOST WORRIED about his eyesight a. Shining a laser into his eye because it is a more dangerous color b. Shining a laser into his eye because it has more power in the beam c. Shining a laser into his eye because power is concentrated to a much smaller spot and could hurt his retina d. Shining a flashlight into his eye because it would contract the black of his eye (pupil) e. Nothing worries Murray because he is Mister cool 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. 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. On to how laser works. 16

Review of atom discharge lamps-- neon signs. Energy levels metal, bulb filament, or not stuck in atom (like sun). If hot, jump between all diff. levels. Wiggle around, all colors. Energy levels in isolated atom. 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 17

atoms lazy- always want to go back to lowest energy state. Have to get rid of energy, send it off as light. 1 Fast electron or right color light hits atom 2 Excited atom 3 Atom back to low energy e or e Light emitted e e Ground state (lowest possible) 18

Glow-in-the-dark toys (phosphorescence) Pumping ON e e e e e e e e e e OFF Lifetime of an excited state: the average time electrons spend in the excited state (somewhere between < 10-9 s and > 1000 s) 19

look at neon lamp with diffraction gratings. (much more stuff like this in lab this 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. 20

Review: compare flor. light, neon lamp and laser light from pointer. If you look at light from laser pointer with a diffraction grating, 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) 21

Small detour: Spectra of materials what it is good for Every material (atom, molecule) has a characteristic emission spectrum ( fingerprint ). Thus by looking at the spectrum of an unknown material you can figure out its composition (cool!). This is how we knew the rings of Saturn are mostly composed of water ice before we actually sent there a satellite to look closely. Astronomers learn about the Universe by observing light from distant astronomical objects, like stars or galaxies. Light contains information, and since it is much easier to observe a star than it is to travel to one, there is clearly a benefit to being able to understand what the light is telling us! Fig. A false color image of the water content in Saturn s rings. (Taken in UV and not visible). More blue (turquoise) means more water, red means less water. Note the fine 22 structure of the rings.

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) or? photon, little piece of wave, we often draw as little ball because less work. Everything to know about interaction of light and atoms. 3 easy steps. photon 1. absorption of light atom electron in higher energy level e 2. Spontaneous emission of light. Electron jumps down from 23 upper level, gives off light. Randomly in any direction.

3. Stimulated emission of light. First realized by A. Einstein Photon hits atom already in higher energy level. What happens? a. gets absorbed by the atom, electron goes 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. Second identical photon comes out. Cloning photons. Amplifying amount of light x 2. P2k applet. If send in light faster, what happens? 24

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. 25

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 26 and it does not matter how many are in the lower.

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 N upper -N lower. N upper > N lower, more out than in. (atoms change) N upper < N lower, fewer out than in. (and atoms change) 27

so get amplification if more in upper level than lower this condition very unnatural known as population inversion how to get population inversion? 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. 28

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. 2 also can kick up by bashing with electron Smaller the separation between 3 and 1, slower the rate from 3 to 1. So these two factors why 3 easier to get lasers at longer wavelengths than at shorter wavelengths. 1 pumping process to produce population inversion 29

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. 30

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? 31

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. 32

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/λ must match difference in energy levels for electron. e e atom 1 atom 2 33

What have we learned today: 1) Lasers (pump up to population inversion, put mirrors around it, stimulated emission will take care of the rest) 2) For operation, lasers need at least 3 energy levels (ground state and 2 excited states). It helps if the middle level has a long lifetime ( metastable) 3) How glow in the dark toys work 4) Lots of cool demonstrations. Looked at emission spectra. Disassembled a working laser. 34

skip? two excited atoms in a row 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 35

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. 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. 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 36

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. 2 also can kick up by bashing with electron (probably skip this and the next slide) 3 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 1 d. rate out of 3 slower than into it. pumping process to produce population inversion 37