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5.3
Physics and the Quantum
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Light
• The amplitude of a wave is the wave’s height
from zero to the crest.
• The wavelength, represented by  (the Greek
letter lambda), is the distance between the
crests.
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Light
• The frequency, represented by  (the Greek
letter nu), is the number of wave cycles to
pass a given point per unit of time.
• The SI unit of cycles per second is called a
hertz (Hz). Mathematically the unit that works
best for frequency is sec-1.
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Light
The wavelength and frequency of light
are inversely proportional to each other.
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Light
The product of the frequency and wavelength
always equals a constant (c), the speed of light.
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Light
According to the wave model, light consists of
electromagnetic waves.
• Electromagnetic radiation includes radio
waves, microwaves, infrared waves, visible
light, ultraviolet waves, X-rays, and gamma
rays.
• All electromagnetic waves travel in a vacuum
at a speed of 3.0  108 m/s.
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Light
Sunlight consists of light with a continuous range
of wavelengths and frequencies.
• When sunlight passes through a prism, the
different frequencies separate into a
spectrum of colors.
• In the visible spectrum, red light has the
longest wavelength and the lowest frequency.
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Light
The electromagnetic spectrum consists of
radiation over a broad band of wavelengths.
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SAMPLE PROBLEM 5.1
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SAMPLE PROBLEM
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Practice Problems for Sample Problem 5.1
Problem-Solving 5.15 Solve
Problem 15 with the help of an
interactive guided tutorial.
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Practice Problems
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Physics and the Quantum
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In the Rutherford model, there
was no limitation on the
distance from the nucleus to an
electron.
The model was a major
advance, but it did not explain
all properties of the atom
• Proton-electron attraction
• Why didn’t atom collapse?
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The Photoelectric Effect
•Early 1900’s
•Only certain colors of
light would cause the
Photoelectric Effect
•If wave this not the case
•Explained by Max Planck
•Planck studied emission
of light from hot objects
•Said light emitted in small
packets called quanta
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Max Planck
•Planck determined the Energy emitted
and carried in the quanta was related to
the frequency of light emitted.
•ν α E
•E=hν
•E – energy in Joules
•H – Planck’s Constant (6.626 x 10-34 J∙sec)
•ν – frequency (sec-1)
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Quantum Mechanics
In 1905, Albert Einstein successfully explained
the Photoelectric Effect by proposing that light
could be described as quanta of energy.
• The quanta behave as if they were particles.
• Light quanta are called photons.
• Therefore the frequency of the photons was
directly related to the Energy of the photons
by Planck’s equation.
• Different colors of light have different energy.
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Einstein’s Contribution
•Einstein said electromagnetic radiation has a Dual WaveParticle nature
•Light can act both like a wave and like a stream of particles
•Particles called photons.
•Photon – particle of electromagnetic radiation with zero
mass and carrying a quantum (packet) of energy
•Photoelectric effect – electromagnetic radiation only
absorbed in whole numbers of photons. The energy of the
photon is related to its frequency
•Eject an electron by hitting it with a photon with enough
energy to free electron
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Atomic Spectra
A prism separates light into the colors it contains.
When white light passes through a prism, it
produces a rainbow of colors aka a continuous
spectrum.
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Atomic Spectra
•According to the Rutherford Model, there
was no limitation on the location of electrons
relative to the nucleus.
•Therefore, the electron could exist at any
distance and therefore any energy
•Thus it was expected that when excited, an
atom would give off a continuous spectrum.
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Atomic Spectra
When atoms absorb energy, electrons
move into higher energy levels. These
electrons then lose energy by emitting
light when they return to lower energy
levels.
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Emission and Absorption of
Energy as Electrons Change
Energy Level
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Atomic Spectra
When light from a helium lamp passes through a
prism, discrete lines are produced. Obviously
this was unexpected. This led to the
development of the Bohr Model of the atom.
.
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An Explanation of Atomic Spectra
In the Bohr model, the lone electron in the hydrogen
atom can have only certain specific energies.
• When the electron has its lowest possible energy,
the atom is in its ground state.
• Excitation of the electron by absorbing energy
raises the atom from the ground state to an
excited state.
• A quantum of energy in the form of light is emitted
when the electron drops back to a lower energy
level.
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An Explanation of Atomic Spectra
The light emitted by an electron moving
from a higher to a lower energy level has
a frequency directly proportional to the
energy change of the electron.
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Hydrogen’s Emission Spectrum
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An Explanation of Atomic Spectra
The three groups of lines in the hydrogen
spectrum correspond to the transition of
electrons from higher energy levels to lower
energy levels.
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The Bohr Model
Unfortunately Bohr could only work out his
model, and apply it to Hydrogen.
Cannot explain spectra of multi-electron
atoms
Cannot explain chemical behavior of atoms
What is next????
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Quantum Mechanics
Classical mechanics adequately
describes the motions of bodies much
larger than atoms, while quantum
mechanics describes the motions of
subatomic particles and atoms as waves.
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Quantum Mechanics
• In 1924, De Broglie developed an
equation that predicts that all
moving objects have wavelike
behavior.
•Postulated electrons also have a
dual wave-particle nature
•Electrons act like waves confined
around the nucleus (standing
waves) and therefore could only
have certain frequencies
Tacoma Narrows
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De Broglie
•These correspond to
the energy of Bohr’s
orbits.
•Investigation showed
that electrons can be
diffracted and can
interfere with each other
•These are wave
properties
•Electrons have a dual
wave-particle nature.
Electron Motion
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Quantum Mechanics
The Heisenberg uncertainty principle states
that it is impossible to know exactly both the
velocity and the position of a particle at the same
time.
• This limitation is critical in dealing with small
particles such as electrons.
• This limitation does not matter for ordinarysized object such as cars or airplanes.
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Quantum Mechanics
The Heisenberg Uncertainty Principle
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5.3 Section Quiz.
Assess students’ understanding
of the concepts in Section 5.3.
Continue to:
-or-
Launch:
Section Quiz
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5.3 Section Quiz.
1. Calculate the frequency of a radar wave with
a wavelength of 125 mm.
a. 2.40 x 109 Hz
b. 2.40 x 1024 Hz
c. 2.40 x 106 Hz
d. 2.40 x 102 Hz
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5.3 Section Quiz.
2. The lines in the emission spectrum for an
element are caused by
a. the movement of electrons from lower to
higher energy levels.
b. the movement of electrons from higher to
lower energy levels.
c. the electron configuration in the ground
state.
d. the electron configuration of an atom.
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5.3 Section Quiz.
3. Spectral lines in a series become closer
together as n increases because the
a. energy levels have similar values.
b. energy levels become farther apart.
c. atom is approaching ground state.
d. electrons are being emitted at a slower
rate.
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