Electrons,
Energy,
and
Light Waves
When electrons are in
the lowest possible
energy levels, they are
in their ground state
Electrons can absorb
a quantum of energy
and move to a higher
energy level.
This is called the
excited state.
Electrons can absorb
a quantum of energy
and move to a higher
energy level.
This is called the
excited state.
A quantum is a discrete amount of energy.
It has to be the exact amount needed to send
an electron to the next higher energy level.
The excited state
is unstable,
and electrons
quickly return
to the ground state.
When electrons return to the ground state,
they release some of the quantum of energy they
absorbed. It is released as a photon of light.
The excited state
is unstable,
and electrons
quickly return
to the ground state.
When electrons return to the ground state,
they release some of the quantum of energy they
absorbed. It is released as a photon of light.
Electrons can absorb a
larger quantum
of energy and move to
an even higher energy
level and a higher
excited state.
Electrons can absorb a
larger quantum
of energy and move to
an even higher energy
level and a higher
excited state.
The excited state
is unstable,
and electrons
quickly return
to the ground state.
When electrons return to the ground state,
they release some of the quantum of energy they
absorbed. It is released as a photon of light.
The excited state
is unstable,
and electrons
quickly return
to the ground state.
This electron absorbed a greater amount
of energy, so it releases a photon of light
with higher energy. It will be a different color than
the first example because it has more energy.
Atomic Spectra
(spectra is plural of spectrum)
• Bohr’s model finally
explained the colors
emitted by hydrogen
glowing in a gas
discharge tube
• Electrons can only
make certain jumps,
so the color of light
emitted from the jump is always the same
All elements have unique
atomic spectra
(these are emission spectra)
Kr
Ne
N
Properties of Light
•
•
•
•
Light travels as a wave
Light travels at “the speed of light”
Light is electromagnetic radiation (EMR)
Visible light is a very small part
of the EMR spectrum (ROYGBIV)
increasing energy
• Of visible light,
– Violet light has the most energy
– Red light has the least energy
Visible light is only a portion
of the EMR spectrum
Speed of Light, c
3.0010 m
c
s
8
'c' for 'constant' or the Latin celeritas for “swiftness”
The speed of all light (EMR) is constant,
and does not change!
Light is made
of photons.
A photon is
a tiny “packet”
of light energy.
:
Artistic conception of a photon by Jan-Henrik Andersen, professor
of Industrial Design at University of Michigan, in collaboration with Fermilab physicists
A photon is both a particle
and a wave…
Speed of light equals
wavelength times frequency
c  λν
λ  wavelength
ν  frequency
Units: m
Units: 1/s or Hz
Energy of a photon equals
the Planck constant
times frequency
Units: J
Ephoton  hν
thePlanckconstant:
h  6.62610 J  s
-34
How is energy related to wavelength?
Light and Electrons
• Electrons can absorb varying sized quanta
and go to varying excited states
• Each transition from excited state
to ground state has a different amount
of energy and a different wavelength of light
• Not all photons are in the visible light range
• Electrons can also act like waves
Electrons in specific energy
levels have specific wavelengths
1
2
4
3
Ex.: Level 4 contains
4 complete wavelengths
Practice #1
What is the wavelength
of electromagnetic radiation
having a frequency of 5.00 x 1012 Hz?
6.00 x 10-5 m
What type of radiation is this?
infrared
Practice #2
What is the frequency
of electromagnetic radiation
having a wavelength of 3.33 x 10-8 m?
9.01 x 1015 Hz
Practice #3
What is the energy of a photon
of green light with a frequency
of 5.80 x1014 1/s?
3.84 x 10-19 J
Recall:
e- can transition between levels
Calculate energy of a transition
• Calculate the energy of the electron in
both the initial (“before”) and final
(“after”) locations
- 2.1810 J
En 
2
n
-18
E  Enfinal  Eninitial
Practice #4
• Calculate the energy associated with an
electron transitioning from the second to
the fourth energy level
• Is that energy being absorbed emitted?
• What is the corresponding wavelength
of light?
The wavelength
of a body with mass
• DeBroglie equation:
h

mv
• v is velocity (not frequency, nu)
Practice #5
• What is the wavelength in nanometers
of an alpha particle (mass=6.64x10-27
kg) traveling at 3.0 x 107 m/s?
Practice #6
• What is the wavelength in nanometers
of a 1000 kg automobile traveling at 65
mi/hr?