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what do I do to prepare for the midterm?
Phys 1020, Day 19
Questions?
Review for Midterm
Notes / Reminders:
Final project topics tonight
Help Room/ Labs for review
Midterm Thurs
1
Phys 1020 Class XIX - Review
Bring calculator, pencils, and note cards (two 3x5 cards)
About the Exam:
•
Format same as Exam 1: Part multiple choice, part essay
•
Exam focuses on material covered since Exam 1, but you should review
previous material since we have been building on physics from beginning of
semester.
•
best preparation- go over homework and solutions, also class notes
(particularly questions in class), and be sure you understand answers.
•
Note Cards: good way to study is to make list of most important ideas and
formulas needed.
•
Homeworks to focus on: Circuits (Diodes), TVs, Sunlight and
Color, Spectra (Lab), Lasers, Optics
2
Format of Exam
BASED ON YOUR FEEDBACK!
No clear consensus for change in any direction,
So test will be similar to the first exam in format,
Maybe a slight shift in the split between multiple choice and true/false
3
What questions do you have
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Cameras / Optics
Lasers / Discharge Lamps
Light & Color
TVs
Magnetic Fields
Diodes
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Topics
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What does a diode do (for electrical current)?
Where does the energy go from an electron moving through a NP junction in an
LED?
Light & color, how to create light, what colors we see means in terms of photons
Photons, E = h f = h c / l
Light scattering: why is sunset red and sky blue
Energy levels of individual atoms
Speed of light and index of refraction, c = lf (vacuum) and c / n = lf (material)
Magnetic fields and electron beams
TVs, electron beams, magnetic fields, phosphors and resolution
LEDs and bandgaps
Discharge lamps, how they function and where the colors come from
Conditions to make a laser and why laser light is special
Refraction of light in a medium (and car analogy)
5
The Sun
1. Get electrons hot (5500 C) with nuclear fusion (H + H -> He)
2. Shake electrons around with a range of frequencies
3. Give off Electromagnetic radiation (light!) at those
frequencies.
Exactly the same as type of radiation from incandescent light
bulb.
Called
“Black body” or “thermal” radiation.
++
+
+
Free electrons and protons
6
Note two different ways
to make light
• Atomic energy level shift (absorption /
emission)
(light from discharge lamps
or LEDs)
• Oscillations (of mobile electrons)
Light from Sun,
++
incandescent lightbulb
+
+
7
Electromagnetic Radiation Review:
Generated by accelerating charged particles…
oscillating electrons in a TV antenna or
oscillating electrons at the surface of the sun.
Wave of changing electric field
… electric field puts force on an electron (and magnetic field) traveling away from source.
Characterized by a wavelength and frequency.
Source
Periodic oscillations
in the electric field.
e-
Travels away
from source at
the speed of
light
(c = 3 x 108 m/s)
What do radio waves and uv light have in common?
a. Both are propagating waves of changing electric field.
b. Both travel at the speed of light through space
c. Both are generated by accelerating charged particles
d. All of the above
8
Electromagnetic Radiation Review:
Generated by accelerating charged particles…oscillating electrons in a TV
antenna or oscillating electrons at the surface of the sun.
Wave of changing electric field (and magnetic field) traveling away
from source. Characterized by a wavelength and frequency.
Source
Periodic oscillations
in the electric field.
e-
Travels away
from source at
the speed of
light
(c = 3 x 108 m/s)
What do radio waves and uv light have in common?
d. All of the above. These are characteristics common to all forms of electromagnetic
radiation (Radio Waves, Microwaves, Infrared (heat), Visible light, UV light, X-rays,
Gamma rays).
What is different between these forms of EM radiation?
Wavelength. Frequency.
Amount of energy per packet of radiation (photon).
9
Which of the following statements are true:
1. All EM (electromagnetic) radiation comes in small packets called photons.
2. Every blue photon has the same amount of energy as every other photon of
that exact same blue color.
3. It is possible for a red photon to have the same amount of energy as a blue
photon.
4. If I measure the energy of the red light emitted by a red LED, it is possible to
measure any amount energy 0 J and higher.
a. 1
b. 1 and 2
c. 1 and 3
d. 1, 2, and 4
e. 1, 3, and 4
10
Which of the following statements are true:
1. All EM (electromagnetic) radiation comes in small packets called photons.
True: A photon is a unit of light energy. Can have one photon, two photons, etc.
Cannot have 1.5 photons of red light.
2. Every blue photon has the same amount of energy as every other blue photon.
True: A photon of a specific color of light has a specific energy. All photons of
that exact color of light will have that same amount of energy… no more, no less.
3. It is possible for a red photon to have the same amount of energy as a blue
photon.
False: For each frequency of electromagnetic radiation (Radio waves,
Microwaves, visible, UV), the energy in a photon of EM radiation at that
frequency is different. This energy is given by
Energy of photon = Planck’s constant x frequency = 6.626x10^-34 x frequency
(E = h f )
Speed of light = frequency x wavelength (c = l f) [same as with sound but with v]
4. If I measure the energy of the red light emitted by a red LED, it is possible to
measure any amount energy from 0 Joules and higher.
False: Detect one photon of energy, two photons, three photons, 1,000,000
photons, but never 1,000,000.1 photons.
a. 1
b. 1 and 2
c. 1 and 3
d. 1, 2, and 4
e. 1, 3, and 4
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Calculating Energy / Photon etc
Note for one photon:
E=hf
Energy = (plank’s const) (freq)
f=c/l
because c
lf
E=hc/l
For many photons
E=Nhf
(N = number of photons)
12
Practice
At what frequency must an electron in the sun be
vibrating to produce UV light at 300 nm?
c = (frequency ) x (wavelength) = f l
f=c/l
= 3 x 108 m/s / 300 x 10-9 m = (1/ 100) x(1017) (1/sec) = 1015 Hz)
(on a calculator)
3 E8 / 300 E-9 = 1 E 15
How many of photons must be hitting you to deposit 1 Joule of
energy (or 1 Watt for 1 second)
E=Nhf
N = E / (h f) = (1 Joule) / [(6.6 x 10-34 J-sec) (1015 1/sec)]
= 1 J / 6.6 x 10-19 J
= 1.5 1018 (photons)
13
White paint has little particles of transparent material in it to
make paint look white. The main reason this works is
because:
a. They add texture to the surface so it is not smooth
b. They each reflect little bit of light because speed of light in
particles is slower than in base material of paint.
c. They each reflect all of the light that hits them, but oriented
in different directions so on average wall looks white.
d. Little particles have no function.
e. (a and b)
magnify
Answer is e.
a) Smooth would reflect light back up… or at angle incident
b) Light changes speed going into particles… a little bit of light reflects off
each particle, enough particle in paint to reflect all the light.
Any light not reflected is absorbed (dye absorbs light). Energy turns into heat!
14
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
GRID
Phosphor
Coating
+
+
Deflecting
Coils (A)
+
+
+
+
+
+
+
+
+
Deflecting
Coils (B)
+
15
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
Big V
GRID
ANODE, high voltage,
lots of excess positives
Phosphor
Accelerating Coating
Anode, lots of
excess
positives
+
+
+
+
+
+
+
+
+
+
+
CATHODE: heated a bunch,
boil off electrons … so now free in
vacuum tube.
+
Each electron hits screen creates a photon of light. Grid controls number of
electrons in beam. How and why?
a. Add a bunch of excess negative charges on grid. Electrons trying to
escape pushed back inside.
b. Add a current through grid to create magnetic field. Electrons trying to
escape pushed back inside.
c. Add a bunch of excess positive charges on grid. Electrons sucked up by
grid.
16
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
GRID
Phosphor
Accelerating Coating
Anode, lots of
excess
positives
+
ANODE, high voltage,
lots of excess positives
+
+
+
+
+
+
+
+
+
+
-- - -- - CATHODE: heated a bunch,
boil off electrons … so now free in
vacuum tube.
+
Answer is a. Add bunch of negatives to grid! Pushes electrons back
towards cathode … fewer into beam.
Control where beam hits screen with magnetic fields!
17
Forces on moving electrons by magnetic fields:
An electron is moving at speed V through a uniform magnetic field.
In which case is the force exerted by the magnetic field on the moving electron
greatest?
C)
A)
Greatest B)
V
e-
force
V
e-
Some force, part
of velocity is
perpendicular to
magnetic field
V
e-
No force on
electron, velocity
is NOT AT ALL
perpendicular to
magnetic field
D) The electrons in A, B, and C all experience the same size force
because the magnetic field is the same strength in all cases.
E) Constant magnetic fields cannot exert forces on charges.
Correct answer is A.
Force
Amount of force on Electron
electron depends on Velocity
how perpendicular
velocity is to
Magnetic field
magnetic field.
e-
V
18
TV’s…. Some questions on interaction of magnetic fields and
moving electrons
Can you create a steady magnetic field with a coil of
wire?
N
a. Sure, Spin coil or move coil up and down rapidly
b. Sure, Send steady current through coil
c. Sure, Send steadily changing current through coil
d. No, It is not possible
S
Answer is B. Steady current produces steady magnetic field. (Remember
creating electromagnet in lab.. Picking up washers.
S
Larger current, stronger magnetic field
Smaller current, weaker magnetic field
No current, no magnetic field
Reverse current, magnetic field in opposite direction …
N
19
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
+
+
Deflecting
Coils (A)
GRID
Phosphor
Coating
+
+
+
+
+
+
+
+
+
Deflecting
Coils (B)
Current through Coils A
(same current, same
direction, through both)
+
Vertical magnetic field.
Beam deflected horizontally
20
What will happen when I bring up magnet from end then
swing to front?
beam will
a. nothing at first, then deflect up or down more as swing to side
b. not move at all,
c. deflect horizontally towards or away from magnet
do exper.
a. Nothing at first then more and more up and down as magnetic field
becomes more and more perpendicular to electron velocity.
v
force
B (magnetic field direction)
21
Circuits: understand where electrons are flowing and why.
Components: Resistors, Transistors, Capacitors, Diodes
LED
Audio Amplifier
+9V Power Source
+
50 Ohm
Resistor
B
A
Gate
-
Electron
Flow
N
P
C D
S
N
Gate
Permanent
Magnet
E
Ground
Diode:
Joined P-Type
and N-type
semiconductor
22
Important things to keep in mind about circuits:
Electrons always want to flow from lower voltage
to higher voltage (towards excess positive charge).
If no voltage difference, no reason for electrons to flow
Rate of electron flow (current) from lower to higher
voltage depends on resistance between these two points
Electrons are attracted to excess positive charge
Electrons will repel each other (like charges repel)
If both of these then force of attraction to
positives will balance force of repelling of each
other, else electrons will move (flow).
23
Transistors control current flow by changing resistance:
Control current by varying amount of positive charge at Gate (A)
A
Gate
 
Gate
NO Charge at Gate
Current valve closed
Infinite Resistance
I=0
+++
A
I Lots of positive
charge at Gate
Current Valve Open
Small Resistance
I is big
Adjust positive charge on Gate to get current in between I = 0 and I = big
NOTE: No current or electron flow across here.
There is an insulator there.
24
Diode Review:
• Pass current in only one direction.
• Junction of P doped and N doped semiconductors
Pure Semiconductor N-type semiconductor
empty
empty
full
full
Mostly Silicon w/
tiny fraction Phosphorus
P-type semiconductor
empty
full
1 empty level
per boron
atom
Mostly Silicon w/
tiny fraction Boron
Si, 4 electrons go into
valence band, just fill
What is required for material to be a good conductor?
a. A small energy gap between the bands
b. A band that is completely full of electrons so a lot of electrons can flow
c. Empty energy levels very, very close in energy to the highest energy electrons
25
Diode Review:
• Pass current in only one direction.
• Junction of P doped and N doped semiconductors
Pure Semiconductor N-type semiconductor
empty
empty
full
full
Mostly Silicon w/
tiny fraction Phosphorus
P-type semiconductor
empty
full
1 empty level
per boron
atom
Mostly Silicon w/
tiny fraction Boron
Si, 4 electrons go into
valence band, just fill
What is required for material to be a good conductor?
Answer is c. Empty energy levels very, very close in energy to the highest energy
electrons. Both N-type and P-type semiconductors satisfy this requirement.
26
Diode Review:
• Electrons flow N to P only. One direction.
P-type
N-type
empty
empty
full
full
To
+Voltage
Electrons can flow from
N-type to P-type
In other direction, voltage can’t give electrons enough
energy to jump band gap so current does not flow.
Light emitting diode.
One photon with each electron that passes across P, N junction.
If current is steady, bigger energy gap between bands will mean:
a. brighter light , b. dimmer light c. different color light
d . Both a and c.
e. both b and c.
27
Note: diode makes light differently than lightbulb (no filament!)
Voltage, Current, and Resistance
Adding more positive charge to Transistor’s Gate is like lowering
resistance of R2. Effect is increase current and lower voltage at B.
+9 V
+
A
Gate
What voltages are possible at B?
a. any voltage
b. any voltage greater than 0 V
c. 0 to 20 V
d. 0 to 9 V
R1 =
50 Ohms e. 1 to 8 V
Answer is d. 0 to 9 V.
B
0 V if transistor is wide open… no
resistance from transistor and current
limited by R1.
R2
9 V if transistor resistance is infinite… no
current flow at all.
GROUND
(0 V)
28
Capacitors – two metal plates that store charge;
insulator in between plates.
No current or electron flow
across here.
+9V “Above Ground”
Adjustable
Power Supply
GROUND
(0 V)
29
Why do we care about photons?
Interaction of Electromagnetic Radiation and Matter:
Photon Energy is what counts!
Energy = h x f = Planck’s Constant x Frequency
Application: Interaction of EM radiation with molecules in our skin
cells. Many energy levels in molecules.
Radio waves (0)  really long wavelength, really low
energy photon, no levels that close in energy, no effect.
E
2
0
1
Microwave, infrared waves (1)  low energy photon,
electron hops up small level, turns into atom motion as
goes back down, heat.
Visible light (2)  Medium energy photon, electron
jumps to medium level, energy sometimes turns into
heat or chemistry or sometimes back out as light.
(Example: sunlight on plant, red and blue into chemical
changes and heat, “light absorbed”, green spit back out
as light in all directions “scattered”.)
30
What happens if energy is greater than visible light…
UV light?
Electrons in molecules in skin cells. Many energy levels.
Send in EM radiation of different wavelengths.
UV light (3)  Higher energy- jumps up to such high
level, electron has enough energy to fly out of
molecule, break it up.
Result is damage to these molecules!!
These molecules are DNA. Cause of SUNBURN
31
typical biomolecule in skin cell
H
c
H
H
c
H
H
c
H
H
c
H
UV!
before UV hits it
32
typical biomolecule in skin cell
after exposure to UV
H
c
H
H
c
H
H
c
H
H
c
H
before UV hits it
e
H
c
H
H
c
H
red and heat of sunburn is body working on
cleaning out and replacing dead cells.
33
UV light-- lots of energy, break apart molecules in
skin and damage it.
Skin cancer. Same thing but damage DNA molecule
without enough damage to kill cell.
“Maimed” DNA- turns cancerous- skin cancer.
Which would give the worst sunburn?
a. 1000 photons of green light (Total E =3.2 x 10^-24 J)
b. 2 photons of UV light (Total E = 8 x 10^-27 J)
c. 100,000 photons of radio waves
34
UV light-- lots of energy, break apart molecules in
skin and damage it.
Which would give the worst sunburn?
a. 1000 photons of green light (E =3.2 x 10^-24 J)
b. 2 photons of UV light (E = 8 x 10^-27 J)
c. 100,000 photons of radio waves
b. - radio waves not even absorbed by skin,
green light lots more energy, but goes into light and heat. UV
has enough energy to go to much higher levels, break up
molecules!
Green much more energy so makes skin warmer, but not break
up molecules as sunburn.
Think about getting hit by 1000 nerf balls compared to 2 bullets!
35
Review of atom discharge lamps-- neon signs.
Energy levels in
Energy levels
isolated atom.
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.
36
atoms lazy- always want to go back to lowest energy state.
Have to get rid of energy, send it off as light.
2
Excited atom
1
3
Fast electron or right
color light hits atom
Atom back to
low energy
e
or
e
e
Light
emitted
e
Ground state
(lowest possible)
37
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)
Everything to know about interaction of light and atoms. 3 easy steps.
photon
atom
1. absorption of light
2. Spontaneous emission of light.
Electron jumps down from upper
level, gives off light. Randomly
in any direction.
electron in higher
energy level
e
3. Stimulated emission. Incoming
photon de-excites atom which
gives off second photon (same
color, direction, phase)
38
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)
39
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.
40
If you shine lots and lots of light
in (pumping)
c. Expect 50/50 mix
- just as easy to excite (absorb)
- as to de-excite (spontaneous emission)
41
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,
2
slower the rate from 3 to 1. So these two
factors why
3 easier to get lasers at longer
also can kick up by
wavelengths than at shorter wavelengths.
bashing with electron
1
“pumping process to produce population inversion”
42
so now see how to get population inversion, will give
amplification of red light. If enough atoms in upper, will lase.
PhET Simulation
But much easier if not let light all escape. Reuse.
mirror to reflect the light.
43
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.
See PhET laser (HW).
If only pump a little bit what happens?
44
PhET Simulation
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.
45
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/l must match difference in energy levels for
electron.
e
atom 1
e
atom 2
46
What have we learned about lasers and light:
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.
47
Comparing a microwave photon, a radiowave photon, and a photon of
red visible light. Rank them from MOST energy to LEAST energy.
A) Radiowave then microwave then red visible light.
B) Microwave then red visible light then radiowave.
C) Red visible light then radiowave then microwave
D) Red visible light then microwave then radiowave
E) It depends on how fast each photon is moving.
Correct Answer is D. Visible light has the highest energy (smallest wavelength),
then microwaves, then radiowaves.
Red light has a lower frequency than blue light, so you know that, in
empty space,
A) blue light travels faster than red light.
B) red light has a shorter wavelength than blue light.
C) blue light has a shorter wavelength than red light.
D) red light travels faster than blue light.
Correct Answer is C. The lower (smaller) the frequency, the longer (larger) the
wavelength.
Velocity = wavelength * frequency.
Velocity = (distance per cycle) * (cycles per second)
The leaves of nearly all plants look green, what colors are the leaves
absorbing most?
A) blue
B) green
C) red
D) b and c
E) a and c
Correct Answer is E. If leaves look green then they must be reflecting green to
your eye and absorbing blue and red.