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.

Look at perfect giant block of ice (frozen lake) where beam of sunlight is hitting it. It will look what color?a. blue b. whitec. blackd. yellowe. red

c. black!! If perfect block, light onlyreflects straight up or goes on through.No light scatters or reflects into eyeball.If no light comes from object, it looksblack!!

Why snow and white paint are white.

Same thing but add some more ice on top of it in form ofsnow. Look at where beam of sunlight is hitting it. It will look what color?a. blue b. whitec. blackd. yellowe. red

Same thing but add some more ice on top of it in form ofsnow. Look at where beam of sunlight is hitting it. It will look what color?a. blue b. whitec. blackd. yellowe. 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. Scattersall colors of light equally, absorbs none.Grey absorbs all a little.

magnify

white where reflect, black if not

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

light slows down in transparentmaterial- “index of refraction, n”

Why do stars and distant lights twinkle? skip, but in notes.Bending of light by air.

index of refraction and hence amount of bendingfrom air always small but depends on pressureand temperature.

patch of colder air, blowingthrough, bends light to go into eyefor a moment, then blows away.Amount of light increases then drops.

twinkling just cause air bends lightaround and different temp blobs of air movingaround bending light a little bit so amount reachingeye increases and decreases.

must be long ways away, so goes throughlarge air blob to bend enough.

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.

Pick up little plastic diffraction grating !

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, …

reading quiz.

1. “stimulated emission of radiation” refers toa. a type of decay of radioactive nuclei, b. the emission of aduplicate 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 isa. 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” mostlya. amplifies light, b. absorbs light, c. calls back ghosts of deadlasers, d. reflects light, e. none of the above

ans. a

Review of atom discharge lamps-- neon signs.

In discharge lamps, lots of electrons given bunch of energy(voltage). Bash into atoms. (“discharge tube”)

120 V or more with long tube

Energy levelsmetal, bulb filament,or not stuck in atom(like sun). If hot,jump between alldiff. levels. Wigglearound, all colors.

Energy levels inisolated atom.

kick up,only certain wavelengths whencome down.

Excited atom

Light emitted

Fast electron or rightcolor light hits atom

e

Atom back tolow energy

1

2

3

e

atoms lazy- always want to go back to lowest energy state.Have to get rid of energy, send it off as light.

e

e

or

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.

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

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 colorc. will not show up at all when you look at it throughdiffraction grating.d. will be a single bright color but too intense to look at withoutdiscomfort.

b. It is light of all exactly the same color, so no spread withgrating.

Focus flashlight beam and laser beam with lens.a. both will focus to same size spot. b. laser will focus to muchsmaller spot with much more power in it. c. flashlight will focusto 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 intensebecause spread out over much larger region. All laser light the sameso focuses into same spot, (or very nearly)

light from lasers are much more likely to damage the retina of the eye than lightfrom a light bulb becausea. laser is at a more dangerous color, b. has lots more power in the beamc. light is concentrated to a much smaller spot on the retina, d. lightfrom bulb is turning off and on 60 times per second so light is not asintense.

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 withoutdamaging stuff in front, send down tiny fiber running downartery 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 thesame 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.

Laser-- Light Amplification by Stimulated Emission of Radiationrepeated 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)

1. absorption of light

Everything to know about interaction of light and atoms. 3 easy steps.

electron in higher energy level

2. Spontaneous emission of light. Electron jumps down fromupper level, gives off light. Randomly in any direction.

photon atom

photon, little piece of wave,we often draw as little ball because less work.or ?

e

P2k applet. If send in light faster, what happens?

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 righton through, atom does not notice, c. original photon continuesand second identical one comes out of atom. d. original photonscatters off in random direction.

c. second identical photon comes out.Cloning photons. Amplifying amount of light x 2.

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.

I would expect that a. more photons will come out right hand end of tube, b. lesscome out right, c. same number as go in, d. none will come out.

glass tube full of atoms, discharge lamp

I would expect that b. less come out right, 2 atoms inside getexcited. 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, and3,2,1 or 0 photons make it through. Depends on number ofatoms and how close they are together. But most of the time will be lessphotons coming out because more lower level atoms than upper.

glass tube full of atoms, discharge lamp

To amplify number of photons going through the atoms I needa. more atoms in lower energy level, b. half in lower, half upper,c. more in upper energy level, d. a sufficient number in upper leveland it does not matter how many are in the lower.

To amplify number of photons going through the atoms I needc. more in upper energy level than in lower. Lower eats them up, upperclones 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)

so get amplification if more in upper level than lowerthis condition very unnatural known as “population inversion”

how to get population inversion?

try getting by shining lots of light on atoms. If turn light wayup will get a. population inversion with all atoms excited to upper level, b. no atoms in upper, c. half the atoms in upper.

skip?

half the atoms in upper. Once in lower level absorb, so startabsorbing, going to upper level. But then ones in upper levelstart getting stimulated back down. When really lot of lightjust go to balance with equal up and down.

NO POPULATION INVERSION!!

To get population inversion, need at least one more energylevel involved.

1

2

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 butslower than 1 to 2,d. a and b

also can kick up bybashing with electron

“pumping process to produce population inversion”

d. rate out of 3 slower than into it.

(probably skip this and the next slide)

To get population inversion, need at least 3 energy levels involved. Need rate into 3 faster than rate out.

1

2

3also can kick up bybashing with electron

“pumping process to produce population inversion”

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 greenneeded 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 longerwavelengths than at shorter wavelengths.

so now see how to get population inversion, will giveamplification of red light. If enough atoms in upper, will lase.

But much easier if not let light all escape. Reuse. mirror to reflect the light.

p2k animation

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 inlight emitting diode at P-N diode junction. Mirrors on it.p2k laser. Show working. If only pump a little bit what happens?

p2k laser.If pump just a little tiny bita. never will produce laser light, b. will take very long timebut will give laser light, c. will give laser light as quickly butwill just produce very little light.

a. spontaneous emission of light will keep too few atomsin excited level to have population inversion. Never willamplify.

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

two excited atoms in a rowskip?

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 hasenough energy to give off one more photon. Remember, photonenergy E = hc/ must match difference in energy levels for electron.

e e

atom 1 atom 2