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CH 26-1
Explaining a Continuous Spectrum
(called a blackbody spectrum)
First Evidence of Quantum Behavior:
Blackbody Radiation
Hot objects glow (toaster coils, light bulbs, the sun).
As the temperature increases the color shifts from
Red to Blue.
The classical physics prediction was completely
wrong! (It said that an infinite amount of energy
should be radiated by an object at finite
temperature.)
Wien’s Law
• As the temperature of
an object increases, it’s
peak intensity occurs at
shorter wavelengths.
lpT = 2.90×10-3 m·K
Temp. in Kelvin
Peak wavelength
Planck’s Quantum Hypothesis
• First evidence of “quantum” behavior.
Max Planck found he could explain these curves if he
assumed that electromagnetic energy was radiated in
discrete chunks, rather than continuously.
The “quantum” of electromagnetic energy is called the
photon.
Energy carried by a single photon is
E = hf = hc/l
Planck’s constant: h = 6.626 X 10-34 J·s
Wien’s Law (Example)
The peak wavelength of light from Sun is at about 502
nm.
What is the temperature of the Sun (specifically the
photosphere of the Sun)?
Poll
Bulb A’s filament is reddish yellow. Bulb B’s filament is
white. Which bulb’s filament has a greater temperature?
1. Bulb A
2. Bulb B
3. Neither; they have the same temperature.
Photoelectric Effect
• Light shining on a
metal can “knock”
electrons out of
atoms.
• Light must provide
energy to overcome
electrical attraction of
electron to nucleus
– This amount of energy
is called the work
function (W0) of the
metal.
Light on Metal: Wave Predictions
• Intensity of light is related to strength of electric field
in the electromagnetic wave.
– Increasing intensity means the light wave can apply a
stronger force to the electron.
– Increasing intensity means the kinetic energy of the ejected
electrons will increase.
• Frequency of light is related to how fast the
electric/magnetic fields oscillate.
– Increasing the frequency does not change the force applied
to electrons and so it will not change the kinetic energy of
the ejected electrons.
Light on Metal: Particle Predictions
Ejected
Metal
Electron
Photon (g)
Eg = hf
W0
work function
e
KEemax= hf – W0
Intensity is related to the number of photons.
-Increasing intensity will not change the kinetic energy
of the ejected electrons.
Frequency is related to the energy of a photon.
-Increasing frequency will increase the kinetic energy
of the ejected electrons.
Max Kinetic Energy of ejected electrons
Photoelectric Effect (Example)
A metal whose work function is 1.5 eV is illuminated
with 600 nm wavelength light. Will any electrons be
ejected from the metal? If so, what is the maximum
possible kinetic energy of the ejected electrons?
• Wave
Is Light a Wave or a Particle?
– Electric and Magnetic fields act like waves
– Superposition, Interference and Diffraction
• Particle
– Photons
– Collision with electrons in photo-electric effect
Both Particle and Wave !
Are Electrons Particles or Waves?
• Particles, definitely particles.
• You can “see them”.
• You can “bounce” things off them.
• How would we know if electron was a wave?
Look for interference!
Young’s Double Slit w/ electron
d
Source of
monoenergetic
electrons
L
2 slitsseparated
by d
Screen a distance
L from slits
Electrons are Waves?
• Electrons produce
interference
pattern just like
light waves.
– Need electrons to
go through both
slits.
– What if we send 1
electron at a time?
– Does a single
electron go through
both slits?
Electrons are Particles and Waves!
• Depending on the experiment electron
can behave like
– wave (interference)
– particle (localized mass and charge)
De Broglie Waves
So far only photons have wavelength, but De Broglie
postulated that it holds for any object with momentum- an
electron, a nucleus, an atom, a baseball,…...
Explains why we can see
interference and diffraction
for material particles like
electrons!!
Matter Waves (Example)
Compare the wavelength of an electron with that of a
two ton truck if each are moving at 30 m/s.
me = 9.11×10−31 kg
2 tons = 909 kg