Chapter. 3 Wave & Particles I
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PHYS-3301 Aug. 27, 2020 Lecture 2 Chapter. 3 Wave & Particles I Outline : • Blackbody Radiation (Plank; 1900; 1918*) • The Photoelectric Effect (Einstein; 1905; 1921*) • The Production of X-Rays (Rontgen;1901; 1901*) • The Compton Effect (Compton; 1927; 1927*) • Pair Production (Anderson; 1932; 1936*) • Is It a Wave or a Particle? à Duality? EM-“Waves” behaving like “Particles” Historical Development Newton(1704): light as a stream of particles. Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19 th century,the wave nature of light was established (interference and diffraction, transverse nature of EM-waves). Physics of the 19 th century: mostly investigation of light waves Physics of the 20 th century: interaction of light with matter One of the challenges – understanding the “black body spectrum” of thermal radiation Black body: In physics, a black body is an idealized object that absorbs all incident E&M radiation. No E&M radiation passes through the black body and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. An approximate realization of a black body as a tiny hole in an insulated enclosure Historical Development Newton(1704): light as a stream of particles. Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19 th century,the wave nature of light was established (interference and diffraction, transverse nature of EM-waves). Physics of the 19 th century: mostly investigation of light waves Physics of the 20 th century: interaction of light with matter One of the challenges – understanding the “black body spectrum” of thermal radiation Black body: In physics, a black body is an idealized object that absorbs all incident E&M radiation No E&M radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. (see Fig.) This thermal radiation from a black body is termed black-body radiation. As the temperature decreases, the peak of the black-body radiation curve moves to lower intensities and longer wavelengths.
Transcript of Chapter. 3 Wave & Particles I
PHYS-3301
Aug. 27, 2020
Lecture 2
Chapter. 3Wave & Particles I
Outline:
• Blackbody Radiation (Plank; 1900; 1918*)• The Photoelectric Effect (Einstein; 1905; 1921*)• The Production of X-Rays (Rontgen;1901; 1901*)• The Compton Effect (Compton; 1927; 1927*)• Pair Production (Anderson; 1932; 1936*)• Is It a Wave or a Particle? à Duality?
EM-“Waves” behaving like “Particles”
Historical DevelopmentNewton(1704): light as a stream of particles.
Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19th century,thewave nature of light was established (interference and diffraction, transverse natureof EM-waves).
Physics of the 19th century: mostly investigation of light wavesPhysics of the 20th century: interaction of light with matter
One of the challenges – understanding the “black body spectrum” of thermal radiation
Black body:In physics, a black body is an idealized object that absorbs all incident E&M radiation. No E&M radiation passes through the black body and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold.
An approximate realization of a black body as a tiny hole in an insulated enclosure
Historical DevelopmentNewton(1704): light as a stream of particles.
Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19th century,thewave nature of light was established (interference and diffraction, transverse natureof EM-waves).
Physics of the 19th century: mostly investigation of light wavesPhysics of the 20th century: interaction of light with matter
One of the challenges – understanding the “black body spectrum” of thermal radiation
Black body:In physics, a black body is an idealized object that absorbs all incident E&M radiationNo E&M radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. (see Fig.)This thermal radiation from a black body is termed black-body radiation.
As the temperature decreases, the peak of the black-body radiation curve moves to lower intensities and longer wavelengths.
Black Body Radiation(Max Planck 1900)
(More in Appendix C)
Experiment shows that as frequency increases, the blackbody spectral energy density reaches a max. then fall off. But, classical theory predicts a divergence !!Do we need a new theory?
Historical Development
In 1900, Planck suggested a solution based a revolutionary new idea:Emission and absorption of E&M radiation by matter has quantum nature:i.e. the energy of a quantum of E&M radiation emitted or absorbed by a harmonic oscillator with the frequency f is given by the famous Planck’s formula
E h f= ,where h is the Planck’s constant 346.626 10h J s-» ´ ×
E w=! where 341.05 10 J s-» ´ ×!2 fw p=Also, in terms of the angular frequency 2
hp
=!
- at odds with the “classical” tradition, where energy was always associated with amplitude, not frequency
The Energy (E) in
the electromagnetic radiation
at a given frequency (f)
may take on values restricted to
E = nhfwhere:
n = an integer
h = a constant
(“Planck Constant”)
The Planck’s Black-Body Radiation Law:
346.626 10h J s-» ´ ×
Blackbody Radiation: A New Fundamental Constant
Plank’s spectral energy density is the critical link between temperature and E&M radiation.
Interestingly, although the assumption E = nhf might suggest EM radiation behaving as an integral number of particles of energy hf, he hesitated at the new frontier -others carried the revolution forward.
For the discovery, Plank was awarded the Nobel prize in 1918 !!
Experimental Fact:
E = nhf
BUT Why should the energy of an
Electromagnetic wave be
“Quantized”?
(n= integer)
No Explanation
until 1905
Albert Einstein
The Photoelectric Effect
A wave is a
Continuous
Phenomenon metal
The Photoelectric Effect
(Albert Einstein 1905)
Phenomenon observed long time before Einstein,
and something very strange was observed:
Albert Einstein postulated the existence of quanta of light -- photons -- which,when absorbed by an electron near the surface of a material, could give theelectron enough energy to escape from the material.
metal
The Photoelectric Effect(Albert Einstein 1905)
Even With Very strong light of low frequency
NO
electrons
ejected
Contradicting C
lassical
Wave P
hysics
The Photoelectric Effect(Albert Einstein 1905)
Even With Very-Very weak light intensity,
but of high enough frequency
Electrons
ejected
Planck’s Law
(E = nhf)
Albert Einsteinproposed:
Photoelectric Effect
(Threshold frequency)
The light is behaving as a collection of particles
called “photons” each of them having energy
E = hf
The Photoelectric Effect(Albert Einstein 1905)
Even With Very-Very weak light intensity,
but of high enough frequency
Electrons
ejectednhfE
hfE
beam
photon
=
=
What happens
is that
1 PHOTON ejects 1 ELECTRON
NO
Electrons
ejected
nhfE
hfE
beam
photon
=
=
There is no PHOTON capable of
ejecting an ELECTRON
Example (1): Very intensive light beam, low
frequency light
SMALL (below the threshold)
LARGE (n is large)
1 electron
ejected
nhfEE
hfE
photonbeam
photon
==
=
The PHOTON ejects 1 ELECTRON
Example (2): SINGLE PHOTON
Very weak light beam of high frequency
LARGE
(above the threshold)
Energy
Conservation:
hfE photon =
!
!"= hfKEmax
To free an electron from the metal, one has to
“pay” a certain amount of energy
the Work Function
Also known at that time:
nm380=!
VoltsU 2<
Repels electrons
?="?
?
min
max
=
=
f
!
• Determine the max. wavelength light that can eject electrons from this metal.
• Electrons are ejected from plate 1 with a certain max. KE.
• If none have enough KE to surmount the electrostatic PE difference (qV), no electrons will reach place 2.
• qV ((=1.6x10-19 C) (2V) = 3.2 x 10-19 J = 2 eV) is the max. that can be surmounted, so the max KE must be 2 eV.
• Using the equation,
• Determine the max. wavelength (l’) light that can eject electrons from this metal.
• The limit of ejecting electrons occurs when an incoming photon has only enough energy to free an electron from the metal, with none left for KE.
Problems1. The work function of tungsten surface is 5.4eV. When the surface is illuminated by lightof wavelength 175nm, the maximum photoelectron energy is 1.7eV. Find Planck’s constantfrom these data.
ecK hf W h Wl
= - = -
Problems1. The work function of tungsten surface is 5.4eV. When the surface is illuminated by lightof wavelength 175nm, the maximum photoelectron energy is 1.7eV. Find Planck’s constantfrom these data.
ecK hf W h Wl
= - = -( ) ( ) 7
158
15 19 34
1.7 5.4 1.75 104.1 10
3 10 /4.1 10 1.6 10 / 6.6 10
eK W eV eV mh eV s
c m seV s J eV J s
l --
- - -
+ + ´ ´= = = ´ ×
´= ´ × ´ ´ = ´ ×
2. The threshold wavelength for emission of electrons from a given metal surface is 380nm.(a) what will be the max kinetic energy of ejected electrons when lis changed to 240nm?(b) what is the maximum electron speed?
0
ch Wl
=1 1 0 1 0
1 1 1.9ec c cK h W h h hc eVl l l l l
æ ö= - = - = - =ç ÷
è ø(a)
2 52/ 2 8.2 10 /ee e
e
KK m v v m sm
= = = ´(b)
