Transcript 2.8-2.9 - BYU Physics and Astronomy

7:Photons
Plan: 1.Solving HW: Important formulae: E *
wavelength = h*c = 1240 nm*eV
Also 1 eV = 1.602 x 10-19 J.
We changed the momentum on problem 4 to 474 eV/c
Extra credit and labs.
2. Photoelectric Effect: Encyclopedia of Video Demos and
http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm &
www.colorado.edu/physics/phet/simulations/photoelectric/photoelectri
c.jnlp
3. Compton Scattering: 2-7
4. Pair Production 2-8
5. Discussion of the writing assignment and collection of
the survey.
6. The exam 1: pass it out and discussion
Did you complete at least
70% of Chapter 2?
A. Yes B. No
The photon was originally called a “light quantum” (das
Lichtquant) by Albert Einstein.[5] The modern name “photon”
derives from the Greek word for light, φῶς, (transliterated
phôs), and was coined in 1926 by the physical chemist Gilbert
N. Lewis, who published a speculative theory[9] in which
photons were “uncreatable and indestructible”. Although
Lewis' theory was never accepted—being contradicted by
many experiments—his new name, photon, was adopted
immediately by most physicists. Isaac Asimov credits Arthur
Compton with defining quanta of light as photons in
1927.[10][11] http://en.wikipedia.org/wiki/Photon
In the photoelectric effect, no
electrons are emitted if the
frequency of the light is ______
than the cutoff frequency.
A. greater
B. less
Photoelectric Effect
• Sodium Metal
• Blue light  electrons
Red light  no electrons
Blue light: Kmax ≈ 0.5 eV
http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod2/pelec.gif
Photon—quantum of light
•Work function- minimum energy needed to
eject an electron
•Sodium Ø = 2.4 eV
Energy of
photon E = hf
blue light, hf =
2.9 eV
red light, hf =
1.9 eV
• Kmax = hf - Ø
http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/pelec5.gif
Extra Credit Activity (2 points) SIM:
Photoelectric Effects
Run the photoelectric effect simulation at
http://phet.colorado.edu/new/simulations/sims.php
?sim=Photoelectric_Effect
Perform the simulation with several sets of
parameters. Write a paragraph about how the
simulation supports the discussion of the
photoelectric effect from the book and any
insights you gained from performing the
simulation. You may include figures or a table if
you'd like.
7. (1 point)
• A silver ball is suspended by a string in a
vacuum chamber and ultraviolet light of
wavelength 200 nm is directed at it. What
electrical potential will the ball acquire as a
result?
On line “experiment”
• www.colorado.edu/physics/phet/simulation
s/photoelectric/photoelectric.jnlp
• Millikan and students provided the data
that gave Einstein the Nobel. But it took m
time to get happy with light as particle.
• http://focus.aps.org/story/v3/st23
X Rays
•
•
•
•
•
Discovered in 1895 by Roentgen
Generated when electrons hit target
Materials transparent to them
Not deflected by magnetic field
Roentgen saw no interference and little
diffraction!
• Bragg diffraction – explained in 1912
• X rays are EM waves with wavelengths from
0.01 to 0.10 nm
X-Ray Tube
+
Tungsten
Target
Anode
-
Filament
Cathode
Bragg Diffraction
θin=θout
θ
d
Bragg Diffraction
2d sin(θ) = m λ
θ
d
Bragg Diffraction
What do X-Rays Have to Do with
Quantum Mechanics?
Compton Effect
• Compton noticed that x-rays were “softer”
after they had passed through a
substance.
• Compton showed that this could be
explained by treating scattering as
“collision” between a photon of light and a
particle of matter
In the Compton effect, the
scattered photons have _____
energy than the incident
photons.
A. more
B. less
Compton Scattering of a “Photon”
Before
After
ee-
Are you pondering what
I’m pondering?
Yes, but how will we ever get that
many rubber bands?
No you buffoon! Why do you
need x-rays to see this?
Compton Shift
Consider the Compton effect. Which of the
electrons shown would you expect to be
knocked out with more energy? (pp)
A.
B.
A
B
Consider the Compton effect. Which
of the scattered photons shown has
the longest wavelength? (pp)
Assume the incident photon have
same wavelength.
A.
B.
Pair production
• Suppose I have a steep, really long hill
and get going 0.99c and hit a bump, Can I
make energy into mass?
• What happens to that new mass as it goes
down the hill can it make new mass, too?
Review if needed
If we apply a constant force
on an object, the acceleration
of the object, as it
approaches the speed of
light, will _______. (pp)
A. decrease
B. increase
C. remain constant