Transcript L11: Photons (Ch.38 S.1-2)

Photoelectric effect, photons
Physics 123
7/24/2016
Lecture X
1
Concepts
•
•
•
•
•
Photoelectric effect
Work function and stopping potential
ElectronVolt (eV)
Photons
Blackbody radiation
7/24/2016
Lecture X
2
Photoelectric effect
•
•
•
•
Light shines on metal surface P
Electrons are emitted from P
Current flows between P and C
Measure kinetic energy of emitted
electrons KEe by applying a
stopping potential V0:
KEe=eV0
• No surprises so far, because light
as a wave carries energy
7/24/2016
Lecture X
3
Electronvolt
Energy  5000eV
7/24/2016
• Energy that one electron
gains when being accelerated
over 1V potential difference
is called electronvolt eV:
• 1eV=1.6x10-19C 1V= 1.6x10-19J
• Yet another unit to measure
energy,
• Commonly used in atomic
and particle physics.
Lecture X
4
Photoelectric effect
• Inside metal electrons are
sitting in potential “wells”
• Need supply some minimum
energy W0 to get them out
• W0 is called work function –
different for different
materials
• Light provides this energy EL
EL=Ne(W0+KEe)
• Ne- number of emitted
electrons
7/24/2016
Lecture X
light
e-
W0
e-
5
Photoelectric effect
EL=Ne(W0+KEe)
• Here is a surprise:
• Increase light intensity EL– expect
– More electrons Ne - True
– With higher kinetic energy KEe – False
• Do not expect KE to depend on light
wavelength l , but it does
– More over if l>l0 – no electrons come
out no matter how intensive the light
source is!!!
7/24/2016
Lecture X
6
Photoelectric effect
• Explanation of photoeffect A.Einstein
• Postulate: light is transmitted in tiny
particles (!!) – photons (g)
• Each photon carries energy
proportional to its frequency:
Eg=hf=hc/l
e-34
.
• Planck’s constant h=6.626 x 10 J s
• hc=1243 eV.nm
• One electron absorbs only one photon
• If Eg<W0 – no electrons are emitted:
• Eg<W0 f<f0 l>l0
7/24/2016
Lecture X
Eg
Eg
Eg
W0
ef0=W0/h
l0hc/W0
7
Photoelectric effect
Eg=hf=hc/l
• Photon energy (Eg) is spent to get
electron out (W0) and on
electron’s kinetic energy (KEe):
Eg= W0+ KEe
e• If stopping potential is applied
electron’s kinetic energy KEe is
converted into potential energy
of the electron eV0:
Eg= W0+ eV0
7/24/2016
Lecture X
Eg
Eg
Eg
KEe
W0
e-
8
Photons – particles of light
• Photon= particle of light = quantum of light =
gamma(g)-quant
• Intensity of electromagnetic wave – sum of
energies carried by quanta of light – photons:
I=NEg
• Each photon carries energy proportional to the
frequency of the EM wave:
Eg=hf=hc/l
7/24/2016
Lecture X
9
Problem 38-15
• I=0 when l>l0570nm
• Work function W0-?
• What stopping potential (V0) must be applied if
light of l=400 nm is used?
7/24/2016
Lecture X
10
Photon absorption and emission
• Photons can be absorbed by matter – converted into other
forms of energy – e.g. kinetic, potential, thermal. At this
point photon ceases to exist
Final energy=Initial energy+Eg
• Photons can be emitted thus reducing the energy of the
remaining system. Photon was NOT hiding inside matter
waiting to be released, it is created by converting other
forms of energy into light (EM-wave)
Final energy+Eg=Initial energy
• Energy is conserved in either case
7/24/2016
Lecture X
11
More particle properties of light
• Sun tan –
– chemical reaction with threshold
energy W0
– visible light l=700-400 nm does not
have enough energy to start this
reaction
– ultraviolet – l<400 nm - more
energetic light – does have enough
energy
Eg
Eg
Eg
KEe
e-
W0
e-
• Photographic film exposure
– Why red light is safe?
7/24/2016
Lecture X
12
Blackbody radiation
• Another problem was elegantly solved by introducing
“light particles”
• Classical explanation of Blackbody (no reflection
radiation diverged at low wavelength
• Max Planck suggested replacing integral over
frequencies with summing a series (photons!)
peak lpT=2.9x10-3 mK
• Planck did not seek deep meaning behind this seemingly
mathematical trick
• Fundamental explanation was suggested by Einstein in
1905 – energy is quantized
7/24/2016
Lecture X
13