Transcript Quantum Mechanics

Quantum Physics
“If quantum mechanics hasn't profoundly
shocked you, you haven't understood it yet. “
-Niels Bohr
Quantum mechanics
a branch of physics that provides a
mathematical description of the dual particlelike and wave-like behavior and interaction of
matter and energy at the subatomic level.
Quanta- The smallest unit.
Example: A photon is the smallest unit of light.
First, some history on the dual nature of light…
Isaac Newton’s concept of light:
• Light is composed
of particles, that
carry an electric
and magnetic field.
• These fields ‘blend’
to form the colors
we see but obey
the same laws as
other masses.
Thomas Young 1801
The double slit
experiment:
Light interferes
to produce
fringe patterns.
Interference effects support the
concept that light traveled as a wave.
Max Planck-1900
• proposed quantum theoryenergy is always emitted or absorbed
in discrete (specific) units called quanta
Planck’s Law: Energy of A Light Quantum
E = hƒ or E = hv
Energy
frequency
(Greek letter nu)
law states that the energy of
each quantum is equal to the
frequency of the radiation
multiplied by the universal
constant h.
Planck’s Law links the particle and wave
properties of a single beam of light.
Each photon of light
carries a specific
amount of Energy
Ex. Egreen=hfgreen
E = 6.63 x 10-23 ergs/Hz
(6.0 x 1014 Hz)
= 4.0 x 10 -12 ergs
When visible light is
considered to be a wave,
its color is determined by
the frequency but when
it is considered as a
particle, the amount of
energy of the photon
determines its color.
Is Light a Wave or a Particle?
The photoelectric effect shows that the amount of
energy each photon contains is determined by its
frequency and therefore, wavelength.
What is the Photoelectric Effect?
• The photoelectric effect
refers to the emission
(ejection) of electrons
from the surface of a
metal in response to
incident light.
• Einstein showed that Light
consisted of individual
quanta, called photons,
that interacted with the
electrons in the metal like
discrete particles, rather
than as continuous waves.
•
The Photoelectric Effect
Photoelectric Effect
video
• While the Intensity (amount) of the light increases
the number of electrons that are ejected, electrons
can only acquire the energy necessary to escape if
the light is of sufficiently high frequency.
• Light of frequencies lower than this threshold has no
effect regardless of the intensity of the light source.
The frequency of the light determine the
energy of a single ejected electron.
Which wavelength of light would eject an
electron of the greatest energy? The least?
violet
Red
As a result of the photoelectric effect, a
photon transfers kinetic energy to the
electron.
Photons with
greater energy
(frequency),
eject electrons
with greater
velocities
(kinetic energy).
The emission of electrons produces
current and therefore, a potential
difference. Higher frequency light
emits electrons of greater voltage.
In the visible
spectrum (ROYGBIV),
which light would
cause an electron of
the greatest voltage?
Violet
Two Important Applications of the
Photoelectric Effect
1) Solar cells: Convert sunlight to electrical current
Solar Cell Video
2) light-sensitive diodes: One electrode in a photocell
consists of a metal that will emit electrons when
exposed to light while the other electrode attracts the
electrons released, thus producing current.
Examples: remote controls, smoke detectors and
cameras
– Flash on a camera –
• When light comes into the light
meter, it strikes a metal object that releases electrons and
creates a current. This automatically opens and closes the
lens to adjust for high and low lighting conditions.
The Quantization
of Energy
• Quantum mechanics
predicts that there
are only certain
energy levels in an
atom the electrons
can occupy.
• The energy of the
electron is quantized.
Every element has it’s own unique energy
levels occupied by it’s electrons.
As an electron makes a “jump” (called a
quantum jump) from one energy level to
another it absorbs or emits energy in a very
specific wavelength.
Outside orbital’s have higher energy, so if an electron jumps
from a lower orbit to a higher one then energy is absorbed.
When an electron falls back to a lower energy level
(negative electron is attracted to the positive protons in
the nucleus), then energy is released.
1. The frequency of the
photon emitted during the
drop in energy is proportional
to the length of the arrow,
i.e., to the change in energy.
2. Therefore, a jump from
n=5 to n=1 emits the highest
frequency of photon. And a
jump from n=1 to n=5, would
require the greatest
frequency of light absorption.
Which of the following has the shortest wavelength:
UV, visible light, or IR rays?
Answer: UV, note that is has the largest drop from N=6 to N=1
When an
electron jumps
from a higher
energy level to
its ground
state, a photon
with the same
energy as the
orbital change
is released.
Every element can be identified by the
unique set of absorption and emission
spectral lines they produce when their
electrons are excited.
Absorption Lines
Absorption &
eMISSION
Emission Lines
The line spectra of light emitted from a gas-discharge
tube are used to identify elements since their atoms
and molecules exist in certain specific energy states.
The discharge tube above is filled with a gas. When a high enough voltage
is applied across the tube, the gas becomes a plasma and acts like a
conductor, allowing a current to flow through the circuit. The current
excites the atoms of the ionized gas, which fall back to their ground state,
emitting photons to carry off the excess energy.
Spectral
lines can be
used to
determine
the
composition
of matter,
such as
distant stars.
Neon verses Incandescent Light
• Neon tubes convert a gas
into plasma (ionized gas)
to produce light.
Different elements are used
to produce a blend of photons
and thus different color light.
• An incandescent bulb uses heat
caused by an electrical current.
The wire, or filament, gets so hot
that it glows and gives off light.
CFL: Compact Fluorescents Lamp
A fluorescent lamp is a glass tube
filled with argon gas and a tough
of mercury. When electrical
current is passed through the gas
the atoms of the gas pick up
energy and radiate it in the form
of ultra-violet light (and some
heat).
The UV light then strikes the inside
of the tube, which is coated with
a phosphor. The phosphor glows,
giving off the light we see.
Mercury is a toxin
so the bulbs
should NOT be
thrown out;
rather, they need
to be disposed of
at a collection
center.
LED
video
Light-emitting diodes (LED) produces
light when electrons move around
within its semiconductor structure.
The different colors are produced by the
different semiconductor materials
used.
Which should you use?