Transcript The photoelectric effect

The photoelectric effect
To be a wave or a particle? That is
the question.
Wave Nature of Light
 How do you start a fire without a match or
lighter?
 Get a magnifying glass and focus the sun’s
rays on a piece of paper till the paper gets
hot!
 If the paper heats up, the light must have
supplied the energy!
 How did it do this?
Wave Nature of Light
 An alternating electric field produces a
magnetic field and an alternating magnetic
field produces an electric field
 That is where we get the term
“electromagnetic waves”!
 These waves must transport the energy!
Wave Nature of Light
 Why would we think that light is a wave?
 DIFFRACTION!!!
 Have you noticed the way light bends
around a door? This is diffraction.
 To see light diffraction, we have to have
some small obstacles
Wave Nature of Light
 See how the wavelets at the edges let the
light bend around corners!!!
 As the slit gets smaller, the bending is more
obvious.
The Wave Nature of Light
 The second big argument for light being a
wave is the phenomenon of interference
Young’s Double Slit
Young’s Double Slit
Young’s Double Slit
Quick overview:
 X-ray machine hits target with electrons and
EM radiation flies out
 In the photoelectric effect, you hit target
with EM radiation and electrons fly out!
 The electrons ejected from the target are
called “photoelectrons”
The setup
 An adjustable voltage is
applied. Voltage can be
forward or reverse
biased (which slows
down the electrons)
 Photoelectrons return to
cathode through an
ammeter which records
the current
Concluding statements
 Einstein figured out the photoelectric effect in
1905 (the same year he developed the theory of
special relativity and explained Brownian
Motion). This is what he got the Nobel prize for.
 Think of the photoelectric equation in terms of
conservation of energy
light energy ejecting electron & KE
KE  eDV
hf = f + KE = f + eDV
So is light a particle?
Or is light a wave?