Transcript AtomLightEmissQuantum

The Atom, Light
Emission, and the
Quantum
Models
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Impossible to know what atoms “look like”
since they’re so small.
Instead, use models (conceptual, graphical
and or mathematical representation)
Classic model of the atom
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Planetary model. Very outdated, and not an
accurate representation of the atom.
Still useful for understanding certain
processes, like light emission
Light Quanta
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Is light a particle or wave?
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Wave behaviors of light include reflection,
refraction, diffraction, and interference
Light also displays particle behavior (example
later)
Light Quanta
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Light occurs in elemental quantized
packets of energy, called photons. A
quantum is a unit--the smallest amount of
something
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Think-The mass of a gold ring is the mass of a
single gold atom multiplied by the number of
atoms in the ring.
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The energy of the emitted photon is related to the
frequency, given by the equation E = hf (proposed
by Max Planck)
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h = Planck’s constant (6.6 x 10-34 Js)
 Constant in nature of Energy to frequency
 Sets a limit on the smallness of things
 Gives the smallest amount of energy that can be converted to
light with frequency f
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So light is emitted from an atom as a stream of photons,
each photon with a frequency and energy of hf
Photoelectric Effect
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Einstein found support for quantum theory of
light in the “Photoelectric Effect”
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The ejection of electrons from certain metals when
light falls upon them. (these metals are
“photosensitive”)
High frequency light, even when dim, can eject
electrons from a photosensitive surface
Low frequency light, even if very bright, may not
be able to eject electrons from the surface
Photoelectric Effect
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Einstein explained by thinking of light in
terms of photons, instead of continuous
waves
The number of photons in a beam controls
the brightness. The frequency of the light
controls the energy of individual photons
Photoelectric Effect
Waves as particles
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So what is light? A particle? A wave? A
particle that waves as it goes by?
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Light shows properties of both
This is known as Wave-Particle Duality
Particles as Waves
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Recall Thomas Young’s double slit
experiment that showed an interference
pattern
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Shoot beams of photons (dim light)
An interference pattern forms photon by
photon on the screen
Each single photon has wave and particle
properties. Different aspects show at
different times
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A photon behaves as a particle when it is being
emitted or absorbed by detectors, and behaves
as a wave in traveling to a source to a detector
Particles as waves
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So if light can show wave and particle
properties, what about other forms of
matter?
Turns out that all bits of matter have wave
properties
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Explained by Louis de Broglie
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Wavelength = h/momentum
Check your understanding
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Does a 0.5 kg baseball moving 10 m/s
have a wavelength?
Electron waves
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You usually think of electrons as negatively
charged particles
In “Bohring Spectra”, you saw that
electrons have specific energy levels.
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This is best understood by considering the wave
properties of an electron
Orbital radius of electron
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Whole number integers of de Broglie wavelengths
are needed for an orbital radius to be possible.
Will the above radius work?
How about this one?
De Broglie Wavelengths
Black Body Radiation
Section
A27.1
Particle Model of Waves
Radiation from Incandescent Bodies
When the dimmer control is
used to increase the voltage
to the bulb, the temperature
of the glowing filament
increases.
As a result, the color
changes from deep red to
orange to yellow and finally,
to white.
Section
A27.1
Particle Model of Waves
Radiation from Incandescent Bodies
This color change occurs
because the highertemperature filament emits
higher-frequency radiation.
The higher-frequency
radiation comes from the
higher-frequency end of the
visible spectrum (the violet
end) and results in the
filament appearing to be
whiter.
Section
A27.1
Particle Model of Waves
Radiation from Incandescent Bodies
What would you expect to see if you viewed the glowing
filament through a diffraction grating?
When viewed in this way, all of the colors of the rainbow would
be visible.
The bulb also emits infrared radiation that you would not see.
A plot of the intensity of the light emitted from a hot body over a
range of frequencies is known as an emission spectrum.
Section
A27.1
Particle Model of Waves
Radiation from Incandescent Bodies
Emission spectra of the incandescent body at temperatures of
4000 K, 5800 K, and 8000 K are shown in the figure. Note that
at each temperature, there is a frequency at which the
maximum amount of energy is emitted.
The Bohr Model of the Atom
Quantized Energy
As shown in the figure below, the quantization of energy in
atoms can be likened to a flight of stairs with decreasing-height
steps.
To go up the stairs, you must move from one step to the next—it
is impossible to stop at a midpoint between steps.
The Bohr Model of the Atom
Energy of an Atom
The change in energy of the
atom equals the energy of
the emitted photon.
The Bohr Model of the Atom
Energy and Electron Transitions
Some of hydrogen’s
energy levels and the
possible energy level
transitions that it can
undergo are shown in
the figure at right.
Note that an excited
hydrogen atom can
emit electromagnetic
energy in the
infrared, visible, or
ultraviolet range
depending on the
transition that occurs.
The Bohr Model of the Atom
Energy and Electron Transitions
Ultraviolet light is
emitted when the atom
drops into its ground
state from any excited
state.
The four visible lines in
the hydrogen spectrum
are produced when the
atom drops from the n
= 3 or higher energy
state into the n = 2
energy state.
Light Emission Spectra
Atoms and Light Emission