1. Homework 13 available now. It’s the last one!
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2. This week: fluorescent lights.
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3. Next week is review for final. Please email topics
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Pick up little plastic diffraction grating from back of class!
Hold the grating only by its edges...oil from your hands ruins grating.
Recall: Incandescent lights
vub
westreic
If you’re on either list, let me know!
Where does most of the electrical energy end up?
c. as infrared light
At temperatures light bulb filament can stand, most of shaking is
at infrared light frequencies. Less than 10% is visible light!
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Power supply provides energy to push electrons around circuit.
In the filament electrons bump into atoms and heat up the filament.
Shaking the electrons produces light of many wavelengths.
Where does most of the electrical energy end up?
a. as visible light
b. making wires and filament hot
c. as infrared light
d. as ultraviolet light
Different atoms emit different colors of light
120 Volts voltage difference or more with long tube
How to do better - atomic discharge lamps (neon signs,
yellow and green streetlights) and florescent lamps.
Direct conversion of electrical (electron) energy into visible light.
Very efficient!
I. how atoms work, how electron energy into light.
II. problems with single colors/wavelengths, not white. How eye
perceives color. Requirements for “white” light.
III. How florescent bulb produces “white” light.
Different atoms emit different colors of light
120 Volts voltage difference or more with long tube
Moving electrons
Colliding with atoms
In atomic discharge lamps, lots of electrons are given a bunch of
energy (voltage). Then they bash into atoms. (“discharge tube”)
Hold grating only by edges...oil from hands ruins grating.
Hold close to eye... See rainbow from lights.
Turn so rainbow is horizontal.
Moving electrons
Colliding with atoms
Each atom has a unique set of electron energy levels.
Look at discharge lamps with diffraction gratings.
Mercury, Sodium, neon
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Nucleus
What is it about atoms, anyway?
Where does the light come from?
Outermost electron – where can it go?
Atomic “core”: nucleus and most electrons
Nucleus
Electron
++
++
-
When electron moves to location further from the nucleus,
a. energy of electron decreases because energy is released as positive
and negative charges are separated, and there is a decrease in
electrostatic potential energy of electron since it is now further away
b. energy of electron increases because it takes energy input to separate
positive and negative charges, and there is an increase in the
electrostatic potential energy of the electron.
c. energy of electron increases because it takes energy input to separate
positive and negative charges, and there is a decrease in the
electrostatic potential energy of the electron.
Electron
++
++
What is it about atoms, anyway?
Where does the light come from?
-
This higher orbit
has a higher energy.
When electron moves to location further from the nucleus,
Answer is b. energy of electron increases because it takes energy input
to separate positive and negative charges, and there is an increase in
the electrostatic potential energy of the electron.
This low orbit
has a low
energy.
Only certain orbits are allowed.
When the electron “jumps” from one orbit to a lower
one, it releases a certain amount of energy.
What is it about atoms, anyway?
Where does the light come from?
There is no orbit
with any energy in
between!
Only certain orbits are allowed.
The energy released comes out as light.
We call it a photon.
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Atoms are basically lazy- they always want to go back to lowest
energy state.
2
Example: the wavelength of red light is about 650 nm.
How much energy is in a single photon of this light?
Excited atom
1
3
Atom back to
low energy
Fast electron hits atom
a)
b)
c)
d)
e
e
Light emitted
Answer b: hc/ = (6.626 x 10-34 J s)(3.0 X 108 m/s) / (650 X 10-9 m)
= 3.1 X 10-19 J
Very little energy in a single photon!
energy in light emitted depend on wavelength
(“photon energy”)
E = h = hc/, Planck’s constant h = 6.626 x 10-34 J s
Neon lamp emits a strong red line. Sodium emits a strong yellow line. What
accounts for this difference?
a.
The electrons in the discharge hit the neon atoms with more speed than
the electrons hit the sodium atoms
b.
The electrons in the discharge hit the neon atoms with more speed than
the electrons hit the sodium atoms
c.
The energy spacing between the electronic energy levels in the neon atom
are smaller than in the sodium atom.
d.
The energy spacing between the electronic energy levels in the neon atom
is larger than in the sodium atom.
Answer is c. When the energy spacing is smaller, the emitted photon has less
energy. Red photons (longer wavelength/ lower frequency light) have less
energy than yellow photons.
When an emission line (color) appears brighter that means:
a.
Multiple photons are emitted for each electron transition
b.
More electron transitions are occurring each second
c.
Each photon has more energy and thus appears brighter
d.
a and b
Answer is b. Each electron transition produces only
e.
b and c
one photon with energy equal to energy difference
between electron energy levels. More electron
transitions, more photons, brighter line.
An atom can emit a photon by going from level 3 to level 1, or
from level 2 to level one. One of these photons is blue and one
is red. Which is which?
650 J
3.1 X 10-19 J
4.3 X 10-31 J
1.98 X 10-25 J
energy in light emitted depend on wavelength
(“photon energy”)
E = h = hc/, Plank’s constant h = 6.626 x 10-34 J s
Rank the following photons from highest to lowest
energy:
a)
b)
c)
d)
microwave, red light, ultraviolet
red light, microwave, ultraviolet
ultraviolet, red light, microwave
red light, ultraviolet, microwave
Ans. c: ultraviolet wavelength is < 400 nm
red wave length ~ 650 nm
microwave ~ 12 cm
energy
levels
Radiation damage: when exposed to light,
atoms can absorb photons, too.
atom
nucleus (protons+ and neutrons)
electron cloud
a) A is blue, B is red
b) B is blue, A is red
energy
Ans. A. More energy is emitted
in A, so this photon has a higher
frequency, smaller wavelength.
3
2
A
B
when as light
E = h = hc/
h = 6.626 x 10-34 J s
excited electron
cloud
1
3
Photon energy and your safety
typical biomolecule in cell (including part of gene)
typical biomolecule in cell (including part of gene)
microwave
after exposure to UV
before UV
H
c
H
H
c
H
H
c
H
H
c
H
H
c
H
H
c
H
c
H
e
H
c
H
H
H
H
c
H
c
H
Microwave photon has too little
energy to break up molecule.
Instead, it shakes it around (heats up).
Only causes damage if it gets really hotthen burns.
sunburn is body replacing dead cells. Cancer is result
of damaged molecules in genes.
1) Free electrons
speeding down
tube bash atom
A
2) Electrons in atom
jump to higher energy
(Excited Atom)
3) Electrons in atom jump back to
lower energy state, gives off light.
A
Energy of photon of light = Plank’s constant x Speed of Light
wavelength
E = hc/, where h = 6.6 x 10-34 J s and c= 3 x 108 m/s
red- 650 nm, green- 530 nm, blue- 460 nm
VERY efficient way to convert electrical energy into light, but each
atom has its own set of colors (wavelengths) it produces.
Look at my blue shirt when illuminated only by the yellow
light of sodium lamp. It will look
a. green, b. dim blue, c. dim yellow, d. bright yellow,
e. dark like my pants.
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