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Quantum Mechanical Ideas
Photons and their energy
When electromagnetic
waves are exhibiting
their “particle-like”
nature, we call those
little mass-less
bundles of energy
PHOTONS.
There are photons of
visible light, photons
of UV, photons of
microwaves, photons
of IR, etc.
New Units of measurement
Sometimes the wavelengths of photons are
measured in meters, sometimes in
nanometers and sometimes in Angstroms,
where
one Angstrom = 1 x 10-10 meters
Also, the ENERGY of electrons is often
given in “electron-Volts”, eV, instead of
Joules, where
one eV = 1.6 x 10-19 Joules
a very tiny amount of energy!
All electromagnetic photons carry
energy as they travel along at “the
speed of light”.
The energy of a photon, in eV, is given
by
E = hf
where
f is the frequency of the photon,
measured in Hertz
h is a constant called Plank’s constant.
h = 4.14 x 10-15 eV·s
The different frequency of electromagnetic
waves (photons) determines if they are
visible light, radio wave, microwaves, etc.
higher frequency = more energy!
Which photon has more energyan X-ray photon or a microwave photon?
The different frequencies of visible light
correspond to different colors of light.
Blue light has a higher frequency than
yellow light. Which color of light has the
highest energy?
How can you produce different colors of light?
The amazing colors
produced in
fireworks are a
result of the
different types of
atoms that are
used to make the
fireworks.
What makes
one atom
different from
another?
Each atom has its own
unique number of
protons, neutrons,
and electrons.
Each electron in every
element is in an
“orbital” about the
nucleus and has a
unique energy.
That unique energy
determines the
amazing colors seen
in fireworks!
• A Quantum is a discreet unit of a physical quantity.
• For example: our money is measured in a quantum of
one cent. You can have 1 cent, 2 cents, 8 cents, etc., but
you can’t have 1.24 cents or 19.68 cents! You must
jump from 1 to 2 to 3 to 4, etc.
• Electric charge, which ultimately comes from either a
proton or an electron, is QUANTIZED.
• There is no such thing as a half of an electron or a fifth
of a proton, so everything that has electrical charge
must have some multiple of the charge of an electron or
proton- 5 electrons, 8 protons, etc. That’s why electric
charge is QUANTIZED.
The electrons, in their orbitals
about the nucleus, have
QUANTIZED levels of
energy that are determined
by which orbital they are in.
The orbitals are numbered
with “n” numbers, the
“principle quantum
number”:
n = 1, n = 2, n = 3, etc. where
the orbital closest to the
nucleus is n = 1.
The “n-number” for each
atom’s electrons determine
that electron’s energy.
The larger the “n”, the larger
the energy.
What does the energy of
an electron in its orbital
have to do with the
colors of fireworks???
When an electron absorbs energy from an
external source in any form (heat,
electricity, a collision, etc.), it jumps to a
higher orbital- called an “excited state”.
Energy
When the electron falls back down to its original
orbital, called its “rest state” or “ground state”,
it must give up that extra energy. The energy
is emitted in the form of a photon!
energy
photon
Some of those emitted photons are visible light of
different colors- some photons are not visible to
us, like UV or IR or microwaves or X-rays
If an atom is continually
absorbing energy, all kinds of
transitions between higher
and lower orbital levels are
possible, resulting in many
different types of emitted
photons of many different
colors.
Metal
Color
Strontium Red
Copper
Blue
Barium
Green
Sodium
Yellow/Orange
Calcium
Orange
Gold
Iron
What elements are used in
fireworks to produce different
colors of light?
Atomic Spectra
• Since the electrons’ energy are unique for each
element, each element produces a unique spectra
of colors when supplied energy.
• We may see with our eyes only many overlaping
colors of light.
• To see all the distinct colors in the atom’s spectra
requires a “diffraction grating”.
Spectra for Neon
Each element produces a unique spectra of colored lines
when viewed through a diffraction grating
Argon
Nitrogen
Mercury
Helium
Because each element
produces a unique
emission spectra,
scientists use “spectral
analysis” to determine
the composition of
unknown substances.
The spectra is like a
fingerprint- absolutely
unique for each element!
Argon
Astronomers use
“spectral analysis”
to determine the
composition of
stars as well.
However… the line
spectra is shifted
toward the red end
of the spectra. This
is called “red shift”
and is an example
of the Doppler shift
due to the stars
moving away from
us.
The “red shift” is one
of the primary
evidences of an
expanding universe!
Using a Spectrometer to determine the
identity of a elemental gas
1. The gas will not glow until it is energized. Energy can be provided in the
form of heat or by applying a high voltage.
2. If you look at the glowing tube with just a diffraction grating, the emission
spectrum lines of color are visible.
Using a Spectrometer to determine the
identity of a elemental gas
1. If you look at the glowing tube through a
“spectrometer”, which contains a diffraction
grating, you can actually precisely measure
wavelengths of the spectral lines.
2. Since each element emits only certain
wavelengths, the gas can be identified.
Hydrogen
The emission spectrum of
Hydrogen is the most studied
spectrum because it is also
the simplest.
Hydrogen has only ONE
electron.
But that ONE electron can be
energized to many different
orbitals, “excited states” and
will emit photons as it returns
to its “ground state”.
Light behaves like a wave AND like
a particle
The first clear demonstration of
the particle-like behavior
of light was in
The Photoelectric
Effect
Albert Einstein won the Nobel
Prize in Physics for his study
of the Photoelectric Effect.
The Photoelectric Effect
The ejection of electrons from certain metals
when light falls upon them
Shining light on a metal can
liberate electrons from its surface.
The light has to have enough
energy (high enough frequency)
for this effect to occur.
The energy of the
“photoelectrons” liberated from
the surface depends on the
frequency (the energy) of that
incident light- NOT its intensity!
Increasing the intensity of the
light increases the number of
photoelectrons emitted, but not
the energy of each photoelectron.
When will Photoelectrons
be produced?
PHet simulation
If no electrons are ejected, you must…
…increase the frequency of the light
If only a few electrons are ejected and you want
more, your must…..
…increase the intensity of the light
If you want to increase the kinetic energy of the
electrons, you must…
…increase the frequency of the light.
Practical applications of the
photoelectric effect
Garage door automatic openers
Auto light meters in
camera flashes
Or Burglar alarm systems
Solar cells
to produce
electricity