LECTURE 19
PHOTOELECTRIC EFFECT
Instructor: Kazumi Tolich
Lecture 19
2
Reading chapter 30.2
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Waves and particles
Photoelectric effect
Quiz: 1
3
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Which of the following is/are characteristics of classical waves?
Choose all that apply.
A.
B.
C.
D.
Traveling in straight lines between collisions
Exhibiting interference
Exhibiting diffraction
Exchanging energy with other objects in collisions that occur at specific
points in space and in time.
Quiz: 19-1 answer
4
A.
Traveling in straight lines between collisions
B.
Exhibiting interference
C.
Exhibiting diffraction
D.
Exchanging energy with other objects in collisions that occur at specific points in space and in time.
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Choices A. and D. are characteristics of classical particles.
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Light exhibits interference and diffraction, and Ampere’s law and Faraday’s law yielded a wave
equation for electromagnetic waves with a propagation speed of 𝑐, which matched the measured
speed of light.
So it seemed like there were enough pieces of evidence that light is a wave….. until in the early
20th century, Einstein showed that the photoelectric effect is due to particle nature of light.
Photoelectric effect and apparatus
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In photoelectric effect, electrons are emitted from matter after the absorption of
energy from electromagnetic radiation.
Photoelectric effect apparatus:
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Light of a single frequency illuminates the metal plate (emitter) to emit electrons.
Negatively charged electrons are attracted to positively charged collector.
The resulting electric current can be measured with the ammeter.
Quiz: 2
6
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Which of the following is expected if wave theory of
light is correct in describing the result as you increase
the intensity of the incident light in a photoelectric
effect apparatus?
A.
B.
The maximum kinetic energy of the ejected electron
will increase.
The number of electrons ejected will increase.
Quiz: 19-2 answer
7
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The maximum kinetic energy of the ejected electron will increase.
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if wave theory of light is correct
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1.
As the intensity of the incident light is increased, the energy incident on the metal plate is increased, and the energy
absorbed by the individual electrons would increase.
2.
The ejected electrons would have greater kinetic energy, and the maximum kinetic energy of electrons emitted would
increase.
But these were not the results observed.
1.
The maximum kinetic energy of the emitted electron increases as the frequency of the incident light increases.
2.
The number of electrons emitted is proportional to the intensity of the incident light.
Einstein explained the photoelectric effect experimental result using light quanta, photons.
1.
A photon with a certain energy collides with an electron in the metal, transferring its energy to the electron. The maximum
energy that can be transferred is related to the energy carried by the photon.
2.
The intensity of the incident light is proportional to the rate of number of photons hitting the metal. One emitted electron
received its energy from one photon in their collision.
Quiz: 3
8
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Which has more energy? A photon of:
A.
B.
C.
D.
E.
red light
yellow light
green light
blue light
all have the same energy
400 nm
500 nm
600 nm
700 nm
Quiz: 19-3 answer
9
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Blue light
The photon with the highest frequency has the most energy.
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A higher frequency corresponds to a shorter wavelength: 𝑓 = .
%
400 nm
500 nm
600 nm
700 nm
Photons
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The energy 𝐸 of each photon is given by
𝐸 = ℎ𝑓
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where 𝑓 is the frequency of the light, and ℎ is called Planck’s constant and measured to be:
ℎ = 6.626 × 10-34 J·∙s = 4.136 × 10-15 eV·∙s.
The maximum kinetic energy of ejected electrons is given by:
𝐾)*+ = 𝐸 − 𝑊.
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where 𝑊. is called work function, the minimum energy necessary to remove an electron from
a metal surface. 𝑊. is a characteristic of the particular metal.
Experimental confirmation
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Millikan’s experimental data
agrees with Einstein’s
photoelectric equation.
If photons have 𝑓 < 𝑓. (cutoff
frequency), they do not have
enough energy to eject an
electron from a particular metal.
𝑊. and 𝑓. are related by
𝑊.
𝑓. =
ℎ
𝐾)*+ = ℎ𝑓 − 𝑊.
The slope is ℎ.
𝑓.
Quiz: 4
12
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If the frequency of light causing photoemission of electrons is doubled,
the maximum kinetic energy of the ejected electrons
A.
B.
C.
doubles.
increases by less than a factor of two.
increases by more than a factor of two.
Quiz: 19-4 answer
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If the frequency of light causing photoemission of electrons is doubled, the
kinetic energy of the ejected electrons increases by more than a factor of
two.
𝐾)*+ = 𝐸 − 𝑊. = ℎ𝑓 − 𝑊.
𝐾)*+, 234 = ℎ 2𝑓 − 𝑊. = 2 ℎ𝑓 − 𝑊. + 𝑊. = 2𝐾)*+, 789:92*; + 𝑊.
Quiz: 5
14
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A metal surface with a work function of 𝑊. =
is struck with blue light and electrons are
==. 2)
released. If the blue light is replaced by red light of the same intensity, what is the result?
Choose all that apply.
A.
B.
Emitted electrons are more energetic.
Emitted electrons are less energetic.
D.
More electrons are emitted in a given time interval.
Fewer electrons are emitted in a given time interval.
E.
No electrons are emitted.
C.
400 nm
500 nm
600 nm
700 nm
Quiz: 19-5 answer
15
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No electrons are emitted.
The cutoff frequency is 𝑓. =
the wavelength of 𝜆. =
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,
==. 2)
which corresponds to
= 550 nm (yellow light).
Red light has a wavelength of about 700 nm with a smaller photon energy.
400 nm
500 nm
600 nm
700 nm
Example: 1
16
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A 1.0 mW light beam from a
helium-neon laser (λ = 633 nm)
shines on a screen. How many
photons strike the screen each
second?
Example: 2
17
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When a surface is illuminated with
electromagnetic radiation of
wavelength 780 nm, the maximum
kinetic energy of the emitted electrons
is 0.37 eV. What is the maximum
kinetic energy if the surface is
illuminated using radiation of
wavelength 410 nm?
Quiz: 6
18
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A photocell is illuminated with light with a frequency above the cutoff
frequency. Which of the following does the magnitude of the current
produced depend on? Choose all that apply.
A.
B.
C.
D.
wavelength of the light
intensity of the light
frequency of the light
None of the above
Quiz: 19-6 answer
19
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intensity of the light
Each photon can knock out only one electron. So to increase the current, we
would have to knock out more electrons, which means we need more
photons, which means we need a greater intensity!
Changing the frequency or wavelength will change the energy of each
electron, but we are interested in the number of electrons in this case.
PMT
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A photomultiplier tube (PMT) can detect single photons
and is widely used in experimental nuclear and particle
physics.
It amplifies a single photoelectron and creates an
electrical pulse.
SuperKamiokande detector