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What is light?
What is Light?
The photoelectric effect
Is there any “evidence” that light is
a form of energy?
Energy
• Light travels in a
Straight Line
• Can be carried as a waves (continuous)
• Can be carries by baseballs (discrete)
• Which is more like light?
• Light Reflects
• Consider how light behaves.
• Light Refracts
Clearly, Light Can Be
Described as a Wave
• Light Diffracts
• Light Interferes
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Waves Seem to Work
By the 1850s the wave model of light was the
generally accepted one.
Light as a Wave Also Explains
Doppler Shift
We are Familiar With the Shift in
Sound Waves
Doppler Shift in Light Waves
(a) Sound of a train moving
towards us is higher pitched
(b) Sound of a train moving away
from us is lower pitched
If source approaches, light appears bluer.
If source recedes, light appears redder.
But…
Waves Cannot Explain ALL of
Lights Behavior
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When Don’t Waves Work?
Electron transitions in the Bohr model of the atom
and their emission of light (spectral lines)
provides an example of when light should be
viewed as a “packet” of energy called a photon.
Waves also cannot explain photographic film
exposures.
There are two further pieces of evidence
of this packet-like nature of light:
• blackbody radiation
• photoelectric effect ← ← ←
The Photoelectric Effect
Photoelectric Effect – the
emission of electrons from a
metal surface upon exposure to
light. (How solar cells work.)
This effect cannot be
explained using a wave
model of light.
Why is this surprising?
How can electrons get
free of their material?
What is the energy used
for?
If light behaves like other
wave phenomena, what
effect would changing the
color and brightness of
the light be “expected” to
have on the ejected
electrons (sometimes
called “photoelectrons”)?
Do you understand and
agree with these
predictions?
When the experiments
are carried out, the
results DO NOT agree
with the predictions. In
fact, they are nearly the
opposite.
Light as a
particle
What should a scientist
do when the results of an
experiment are not as
expected?
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Unexpected results can
cause a researcher to
tweak an existing model
or scrap it altogether in
favor of a completely
different idea.
Summary of particle
observations
Light Packets!
Photons
Photons
Photons
Perhaps light is not
behaving as a wave at
all. Instead it behaves in
this experiment as small
independent “bits” of
discrete energy amounts
called photons.
Photons
Each electron can
absorb one and
only one photon.
This is medium brightness blue light.
More electrons can
absorb a photon
and escape.
Since the energy of
the photons
remains the same,
the KE of the estays the same.
• The next slides will review the previous
drawings. Test yourself to see if you got it.
• Don’t got it? Keep reviewing until you do.
The photons are
energetic enough
that the electrons
can escape and
have some KE.
If the photon is
energetic enough,
the electron can
escape with KE of
motion.
Bright light is light
that has MORE
photons.
Let’s see if you got it
How would the
picture look
different if the
blue light were
brighter?
How will the behavior of the electrons
be different with brighter light?
What if the blue
light is exchanged
for higher
frequency purple
light (or UV)?
How will the
picture look
different?
How will the behavior of the electrons
be different with higher frequency
light?
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Higher frequency
light has more
energetic (larger)
photons.
Each electron
absorbs a larger
packet of energy
and therefore has
more KE (travels
faster) as it flies
away.
What if the blue
light is exchanged
for lower
frequency red
light? How will
the picture look
different?
Electrons absorbing purple light (or
UV) fly off with great energy
(speed).
Why doesn’t red light work, no
matter how bright it is?
Lower frequency
light has less
energetic (smaller)
photons.
Each electron
absorbs a smaller
packet of energy.
It cannot even
escape.
How will the behavior of the electrons
be different with lower frequency
light?
Electrons absorbing red light
NEVER fly off no matter how bright
the light is.
Photons Make Sense!
• Red light is used in photographic darkrooms because
it is not energetic enough to break the halogen-silver
bond in black and white films
• Ultraviolet light causes sunburn but visible light does
not because UV photons are more energetic
• Our eyes detect color because photons of different
energies trigger different chemical reactions in retina
cells
• Red photons have very little energy.
• Each electron only absorbs one photon.
• Red photons do not provide enough
energy to even escape the material, let
alone have speed.
• No matter how bright the red light (how
many red photons there are), no electrons
escape.
Light has a “dual nature”.
• In some experiments it behaves like a
wave.
• In other experiments, it behaves like a
photon.
• It never acts like both at the same time.
• It always acts like one or the other.
• What is light, wave or particle?
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End of Photoelectric Effect
• Assign: Worksheet 27W
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