PHOTOELECTRIC EFFECT
13.1.1-13.1.4
Reading Recommendation: Pages 910-918
PHOTOELECTRIC EFFECT
A phenomenon in which a metal is struck by
incoming light, which causes electrons from the
metal to be emitted from the metal’s surface.
 Examples where this can be observed:

 Light
shining on a negatively-charged electroscope
causes it to be discharged
 Light shining on a neutral electroscope causes it to
become charged (positively)
PHOTONS…AND WHAT EINSTEIN THOUGHT OF THEM

What is a photon?
It is a “packet” of light
 “Quantized” light
 Light acting as if it were a particle, not a wave, that is
carrying its energy.


Wait…Quantized? What kind of word is that?
It means that there is a distinct, discrete amount of
energy being carried by a “particle” of light, dependent
on the qualities of the light.
 Waves carry energy continuously, and in varying
amounts/intensities depending on the position within
the wave at which you look…continuous ≠ discrete

EINSTEIN’S LOGIC…
Radiant light is emitted from objects that have
a distinct amount of thermal energy (i.e. those
objects that are at a measurable temperature
emit radiant energy, the magnitude of which is
dependent on the temperature)
 If the kinetic energy of the atoms is discrete
 And if the kinetic energy determines the
temperature…
 Shouldn’t the radiant energy being emitted also
be discrete?

AND WHAT IF IT’S NOT?
IF the radiant energy being emitted from the hotter
object is NOT discrete, then there is no way to show
that energy is conserved during the thermal energy
transfer from hot object to cooler
object/surroundings
 However, it has been shown time and again that
energy IS conserved.
 So Einstein is probably right. 


(and earned the 1921 Nobel Prize in Physics as a result)
DEMOS OF THIS EFFECT:

Demo 1:
https://www.youtube.com/watch?v=WO38qVDGgqw
Demo 2: Phosphorescent strip (done in person
in class )
 Watch this video describing the photoelectric
effect


Demo 3 (watch at home):
https://www.youtube.com/watch?v=kcSYV8bJox8
ELECTRONVOLT

Reminder:
 An
electronvolt (eV) is a measure of the amount of
energy gained by an electron as it travels through a
potential difference of exactly 1 V
1 eV = 1.6 x 10-19 J
 We
will be using electronvolts as our energy unit
throughout this topic!
BLACK BODY RADIATION
Objects with
measurable
temperatures will
radiate heat in the
form of radiant
energy
 Depending on the
temperature,
different
wavelengths of
light will be emitted

WIEN DISPLACEMENT LAW

Relates the wavelength of light with the
temperature of the Black Body:
max  T  2.9 10 m  K
3

For example…determine the surface
temperature of an object that predominantly
emits in the visible spectrum:
STEFAN-BOLTZMANN LAW

Relates the total energy being emitted by the
blackbody to its temperature:
CLASSICAL THEORY RESULT:

Combining the Wien law and the StefanBoltzmann law, it was shown, classically, that
1
I 4


What happens as the wavelength gets shorter?
Ultraviolet Catastrophe!
MAX PLANCK

Assumptions made to explain why the
ultraviolet catastrophe doesn’t actually happen
 Thermal
Oscillators (the atoms emitting the
radiation) have a specific, discrete amount of
energy
 Unlike blackbody radiation, the energy was not
emitted in a broad, continuous spectrum
 Energy was emitted in packets of energy, equivalent
to the amount of energy contained by the atoms.
PLANCK’S HYPOTHESIS
Frequency of the
emitted radiation is
proportional to the
energy the radiation
carries away from the
thermal oscillator.
 Planck’s constant (an
experimental value!)
= 6.63 x 10-34 J·s
= 4.14 x 10-15 eV·s

QUANTUM OF ENERGY

The term given to the quantity hf

Planck’s hypothesis is used to explain quite a
few phenomena not otherwise following a
classical explanation.
QUANTUM OF LIGHT

Light carries a specific (discrete) amount of
energy:
𝐸 = ℎ𝑓
Planck’s hypothesis connects the wave nature
of light with the particle nature of photons (i.e.
wave-particle duality)
 Helps explain the photoelectric effect, which
cannot be explained classically.

#1

Calculate the energy of a photon of ultraviolet light,
wavelength 3.00 x 10-7 m. Express your answer in
both Joules and in electronvolts.
𝒉𝒄
𝑬 = 𝒉𝒇 =
𝝀
(𝟔. 𝟔𝟑 × 𝟏𝟎−𝟑𝟒 )(𝟑. 𝟎𝟎 × 𝟏𝟎𝟖 )
𝑬=
(𝟑. 𝟎 × 𝟏𝟎−𝟕 )
𝑬 = 𝟔. 𝟔𝟑 × 𝟏𝟎−𝟏𝟗 𝑱
𝟔. 𝟔𝟑 × 𝟏𝟎−𝟏𝟗 𝑱
𝟏 𝒆𝑽
×
= 𝟒. 𝟏𝟒 𝒆𝑽
−𝟏𝟗
𝟏. 𝟔𝟎 × 𝟏𝟎 𝑱
SIMULATION LAB
Extra Review:
Watch this video after finishing your lab and
after reviewing your notes:
 https://www.youtube.com/watch?v=ubkNGwu_
66s

PHOTOELECTRIC EFFECT

Work Function (φ) the minimum amount of
energy that must be absorbed by an electron in
order to be emitted from a metal

Depends on the metal!
Threshold frequency (f0)the lowest frequency
that will allow electrons to be emitted from a
metal’s surface.
ℎ𝑓 = 𝐸𝑘 + 𝜑
E = kinetic energy of emitted electron
 At the threshold frequency, the kinetic energy of
the emitted electron = 0 eV

STOPPING VOLTAGE (STOPPING POTENTIAL)
The amount of potential needed to just stop the
flow of electrons through the vacuum tube. The
resulting current becomes 0 A
 The stopping voltage is equivalent to the
amount of kinetic energy of the emitted
electrons

𝑉𝑠 = 𝐸𝑘
GRAPHING PHOTOELECTRIC EFFECT