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Announcements

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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 ... – PowerPoint PPT presentation

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Title: Announcements


1
Announcements
  • Exam Tuesday night, 730 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 dont. 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 gt Nlower, more out than in. (atoms change)
Nupper lt 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
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