Transcript electromagnetic radiation & light

Light
and the
Quantum Mechanical Model
of the Atom
Light
• The study of light led to the
development of the quantum
mechanical model.
• Light is a kind of electromagnetic
radiation.
• Electromagnetic radiation includes
many kinds of waves
• All move at 3.00x108 m/s or
3.00x1010 cm/s (abbreviated: c)
Parts of a wave
Crest
Wavelength
Amplitude
Origin
Trough
Parts of Wave
•
•
•
•
Origin - the base line of the energy.
Crest - high point on a wave
Trough - Low point on a wave
Amplitude - distance from origin to
crest
• Wavelength - distance from crest to
crest
• Wavelength - is abbreviated
letter lambda)
l (Greek
Frequency
• The number of waves that pass
a given point per second.
• Units are cycles/sec or hertz
(Hz)
• Abbreviated
nu)
n (the Greek letter
c = ln
Frequency and Wavelength
• Are inversely related
• As one goes up the other goes
down.
• Different frequencies of light
are different colors of light.
• There is a wide variety of
frequencies
• The whole range is called a
spectrum
Electromagnetic Spectrum
High
Low
energy
energy
Radio Micro Infrared
Ultra- XGamma
waves waves
violet Rays Rays
Low
High
Frequency
Frequency
Long
Short
Wavelength
Wavelength
Visible Light
Energy and Frequency
•
•
•
•
•
E = (h)(n)
E is the energy of the photon
n is the frequency
h is Planck’s constant
h = 6.626 x 10 -34 Joules*sec.
The Math You WILL need
to Know for this Chapter
Only 2 equations:
• c = ln
• E = hn
• c is always
3.00 x 108 m/s
• h is always
6.626 x 10-34 J•s
Examples
• What is the frequency of red light with
a wavelength of 4.2 x 10-5 cm?
• What is the wavelength of The River
105.9, which broadcasts at a
frequency of 640 kHz?
• What is the energy of a photon of
each of the above?
Atomic Spectra
How light & color tells us about
atoms
White Light
• It is made up of all the
colors of the visible
spectrum.
• Passing it through a
prism separates it.
• This is called a
continuous spectrum
• because you see all the
colors of the visible
spectrum blurred together
(ROYGBIV)
If the light is not white
• By heating or electrifying a
sample of atoms scientists
can get it to give off colors.
• Passing this light through a
prism does something
different
• You get colors of light that
relate to specific frequencies
and therefore specific
energies
• This is called …
Atomic Emission Spectrum
or Line Spectrum
• Each element gives off
a unique set of colors.
• This info can be used
to help identify atoms
in substances
• Ex: stars
http://jersey.uoregon.edu/elements/Elements.html
An explanation of
Atomic Spectra
How it all begins…
• In the atom all
electrons start in the
lowest energy level
they are normally
found in.
• This is called the
ground state.
Hydrogen atom
• Let’s take a closer look at a hydrogen atom
Changing the energy
• Adding energy can move the electron up
to higher energy levels
• The more energy added, the more energy
levels the e- moves
• As the electron falls back to its original
ground state it gives the absorbed energy
back as light we can see
• The further the e- fall, the more energy,
and the higher the frequency of light.
Max Planck
• 1900 – from Germany
• Tried to explain why metals changed
color the way they did when they are
heated
• They only show certain colors, not all
the colors in the visible spectrum.
WHY?
• The energy is changed in a specific
amount called a quantum
Light as a Particle
• Energy is quantized.
• These smallest pieces of light
are called photons.
So…
What is light then??
• Light is a wave  we can measure its
wave length and determine its frequency
and it behaves as a wave
• Light is also a particle  it comes in
chunks called photons
• Albert Einstein came up with this idea
Photoelectric Effect
• Metals eject electrons when light of a specific
frequency (called a threshold frequency)
shines on them.
• The light has to have enough energy and if
the light is the wrong frequency then it won’t
work at all (no matter how bright the light)
• Uses of this: solar cells, camera flash, &
automatic doors
• FYI: Explaining this effect is how Einstein won his
Nobel Prize
More obvious with the
very small
• To measure where a electron is,
we use light.
• But the light moves the electron
• And hitting the electron changes
the frequency of the light.
Matter is also a Wave
• BUT…
• This does not apply to objects
bigger than an atom b/c their
wavelengths are too small
• Ex: A baseball has a wavelength of
about 10-32 m when moving 30 m/s
• An electron at the same speed
has a wavelength of 10-3 cm
• This length is big enough to
measure.
• We called these “matter waves”
The physics of the very
small
• Quantum mechanics explains
how the very small behaves.
• Quantum mechanics is based on
probability because
Heisenberg Uncertainty
Principle
• It is impossible to know exactly
the speed and position of a
particle.
• The better we know one, the
less we know the other.
• The act of measuring changes
the properties.
Before
Photon
Moving
Electron
After
Photon
changes
wavelength
Electron
changes velocity