Lecture 21 Professor Stephen Thornton April 13,...
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PHYS 202 Lecture 21 Professor Stephen Thornton April 13, 2005
Transcript of Lecture 21 Professor Stephen Thornton April 13,...
PHYS 202
Lecture 21Professor Stephen Thornton
April 13, 2005
Reading QuizAll but one of the following results originated from and is explained by quantum physics:
1) Photoelectrons are emitted from a metal instantaneously when light shines on it.
2) Light travels at speed c in all inertial frames of reference.
3) Photons have momentum given by h/λ.4) The Compton effect has both an
incident and exit photon.
Answer: 2Light traveling at speed c in all inertial frames is one of Einstein’s postulates of the special theory of relativity. We use the fact in quantum physics, but we can’t say that quantum physics originated the idea.
Exam III Wednesday, April 20Chapters 25-28
20 questions, bring single sheet of paper with anything written on it.
Last LectureRelativistic distortion.Discuss moving stick and manhole.Relativistic momentum, mass, energy.Einstein mass-energy relation.AntimatterPETGeneral relativity – bending of lightSpacetime warp, black holesWorked problems from last year’s Exam 3
TodayBlackbody radiationMax Planck and his hypothesisPhotoelectric effectPhotonsPhoton momentumCompton effectWork problems from last year’s Exam 3
An Ideal Blackbody
An ideal blackbody absorbs all the light that is incident on it.
An ideal blackbody is also a perfect emitter.
Blackbody Radiation
Heat up blackbody. Measure intensity emitted as a function of frequency.
Results don’t depend on material – only temperature.
Colors depend on frequency. Treat stars as blackbodies.
Couldn’t understand shapes.
peak10 -1 -1
Wien's displacement law***
5.88 10 s Kf Tσσ
=
= ×
From Lecture 19:But there were a couple of small problems…
Electromagnetic medium: what do light waves travel through?
Blackbody radiation: spectrum of electromagnetic radiation being emitted from a container.
Max Planck’s Quantum Hypothesis
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0,1,2,3,... ***
6.63 10 J snE nhf n
h −
= =
= × ⋅
The electromagnetic radiation energy must be quantized in units of hf.
h is called Planck’s constant.
The Photoelectric Effect
Light has electric field so it is no surprise that light can cause electrons to be emitted.
Photoelectric effect data
Constant light intensity3 different materials
Constant light frequency
A
C
B
D
Photoelectric effect summary1. Electrons are emitted only when f > f0, where
f0 depends on the metal. C2. For constant f, current is proportional to
intensity of light source. A3. Kmax of electrons is independent of light
intensity, but varies linearly with f . B,C, D4. There is less than 10-9 s delay between
arrival of radiation and emission of electron.Classical interpretation: only 2 can be explained.
Einstein proposed in 1905 that particle-like photons carry energy E = hf. There is a minimum energy required, W, to emit electrons in an atom, and no electrons will be emitted if hf < W. When hf > W, the excess energy is carried off by the electrons as kinetic energy.
212
mv hf W= −
W is the work function and depends on type of metal.It takes one photon to liberate an electron.Robert Millikan did experiment in 1916.
The Photon Model of Light(artist’s imagination only)
Photoelectric effect – energy equations
2max
20 max
20 max 0
0
Energy before (photon) = Energy after (electron) (electron)
1 ***2
1retarding (stopping) potential = 2
1 ***2
where depends on emitter material.
hf W K
hf W mv
eV mv
eV mv hf W hf hf
W hf
= +
= +
=
= = − = −
=
The Photoelectric Effect
hf
WKmaxeV0
Nobel Prizes in PhysicsPhillip Lenard, 1905 – experimental study of electrons.
Albert Einstein, 1921 – explanation of photoelectric effect.
Robert Millikan, 1923 – precise study of photoelectric effect and charge of the electron.
Applications of photoelectric effect
Photocells: open doors, camera exposure, photocopy machine density, automated rear view mirrors, automatic light control (streetlights, headlights), security purposes, flush toilets.
Part of solar cell panels for power
Conceptual Quiz:Both the classical and quantum interpretations can explain the following photoelectric effect result:
1) Photoelectrons are emitted instantaneously.2) The stopping potential of the
photoelectrons depends only on light f.3) The maximum kinetic energy of the
electrons is independent of light intensity.4) For constant light f, the electron current
is proportional to light intensity.
Answer: 4The classical interpretation cannot explain the other three answers, whereas the quantum interpretation explains all of them.
Conceptual Quiz:Light of a given wavelength is used to illuminate a metal surface, but no photoelectrons are emitted. In order to cause electrons to be ejected you should
1) use light of a longer wavelength.2) use light of a shorter wavelength.3) increase the intensity of the original
wavelength.4) decrease the intensity of the original
wavelength.
Answer: 2
You need a photon with higher energy, which means a photon with higher frequency, but lower wavelength.
Light as a photon or particle
The photoelectric effect indicates light behaves as a particle called a photon.
0 0 for a photonWe showed earlier that light has
momentum . Now we have
***
m
Epc
E hf hpc c λ
=
=
= = =
Optical Tweezers
Laser beams can trap and move DNA, bacteria, viruses, etc.
The Compton Effect
Energy equations – Compton effect
incident exit
'ehf hf K
hf hf K−= +
= +
Momentum equations – Compton effectdirection:
cos cos'
direction:
0 sin sin'
e
e
xh h p
yh p
θ φλ λ
θ φλ
−
= +
−
= −
,f λ
', 'f λ
Compton effect result
12
' (1 cos ) ***
2.43 10 m is called
the Compton wavelength of an electron.
e
e
hm c
hm c
λ λ λ θ
−
∆ = − = −
= ×
Compton effect data
Conceptual quiz:What scattering angle in the Compton effect produces the minimum change in frequency?
1) 900
2) 00
3) 450
4) 2700
5) 1800
Answer: 2
At 00 the wavelength change will be zero.
' (1 cos )e
hm c
λ λ λ θ∆ = − = −
Conceptual quiz:In a Compton scattering experiment, the scattered electron moves along the same direction as the incident x-ray photon. In what direction does the scattered photon move?
1) 00
2) 450
3) 900
4) 1800
5) Cannot tell from information given.
Answer: 4Look at equation for momentum along x- and y-directions. If φ = 0, then θ = 00 or 1800 to conserve momentum along y-direction. We must have λ < λ’to conserve energy. The scattered photon must move backwards (1800) to conserve momentum and energy.
Work Problems 30-430-2530-46
