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Light and Matter
Chapter 2
enLIGHTened
• Objectives
– Models of light
• light is a wave
• light is a particle
– Absorption and radiation of light by atoms
– Measuring light to know
• composition of stars
• temperature of stars
• motion of stars
Models of light
light is a wave
light is a particle
So which one is right?
•They are both right...and they are both wrong.
•That’s called wave-particle duality
•In some experiments, the wave model works best.
•In other experiments, the particle model works best.
•Thus, we use both.
Light is a wave
• propagating wave of oscillating electric and magnetic
fields
• described by wavelength, , and frequency, f.
– f = v where v is the speed of the wave.
– In a vacuum, v = c = 3.00 x 108 m/s.
– large wavelength corresponds to small frequency and
small wavelength corresponds to large frequency.
• synonyms for “light” are
– electromagnetic wave
– electromagnetic radiation
– radiation
• visible light is light that our eyes are sensitive to; however,
that is not the only type of electromagnetic radiation
Wavelength and Frequency
Light comes in many
wavelengths
• When white light passes through a glass
prism (or a diffraction grating), it
separates into colors.
• These colors have different wavelengths.
• This group of wavelengths is the visible
part of the electromagnetic spectrum.
• When you “see” the entire spectrum with
no thin dark bands, it is a continuous
spectrum.
Electromagnetic spectrum
Wavelength and frequency
Practice
• What is the wavelength of white light?
• Which color of light has a longer
wavelength purple or red?
• Suppose that a certain medical treatment
requires exposing certain tissues to high
frequency radiation. Would that radiation
likely be gamma rays or radio waves?
Light is a particle
• Albert Einstein proposed that light consisted of
photons.
• A photon is a “particle” or “packet” of energy.
• A photon has an energy of E=hf where h is called
Planck’s constant and f is frequency.
• High frequency (low wavelength) photons have
high energy; low frequency (high wavelength)
photons have low energy.
Absorption and emission
How is light absorbed and emitted by
atoms in interstellar gases or stars?
Bohr model
(I hope it’s not bohring)
• The Bohr model is a
planetary model, where
the electron orbits the
nucleus like a planet orbits
the Sun.
• An electron is only
allowed in DISCRETE
orbits (n=1, n=2, n=3,
etc.)
• The higher the orbit, the
higher the energy of the
electron.
Modern view of hydrogen
Now, we know that the electron has
discrete energy levels, but it does not
orbit the nucleus at fixed distances
from the nucleus. In fact, it may be
found anywhere in certain allowed
regions called orbitals. Each orbital
corresponds to a certain energy of the
electron.
Absorption, emission, and energy
photon
Absorption
photon
Emission
• When an atom absorbs a
photon, it gains energy.
• When an atom loses energy, it
emits a photon.
• An atom can only absorb
photons or emit photons of just
the right energy.
• Those “right energies”
correspond to the
DIFFERENCES in energy
between the allowed energy
levels.
Hydrogen
• only certain energies are
allowed
energy level
energy
n=5
n=4
n=3
-0.544 eV
-0.850 eV
-1.51 eV
n=2
-3.40 eV
n=1
-13.6 eV
• the change in the energy
between two levels
corresponds to a certain
color photon absorbed or
emitted by the atom
• the lowest energy level
is the ground state
• higher energy levels are
called excited states
Absorption
• If light of a continuous spectrum is incident on a gas of
hydrogen atoms, then electrons will absorb some of the
light.
• As a result, bands of the spectrum are missing; these are
called absorption lines.
• By the way, these same atoms emit the same colors in an
emission spectrum!
Emission
• If excited hydrogen atoms fall to lower energy states,
photons will be emitted.
• The emitted photons will be detected as light of certain
bands of frequencies (i.e. colors).
• The collection of bands (or lines) forms an emission
spectrum.
What’s so EXCITING?
• Sure, electrons get excited when they change
energy levels, by why do we get so excited?
• Each element absorbs and emits a different set of
spectra.
• By measuring the spectral lines, we can know
what element a gas is made of.
• Now, we have way of determining what elements
stars like the Sun are composed of
• Here are spectra for the most abundant elements
that compose the Sun.
Clouds of gas (nebulae) emit light, some
by absorption and some by emission
emission
nebula
Practice
• See the “Spectrum”
handout
energy level
energy
n=5
n=4
n=3
-0.544 eV
-0.850 eV
-1.51 eV
n=2
-3.40 eV
n=1
-13.6 eV
•If an atom is in the ground
state (n=1) and is excited to
n=3, what energy photon was
absorbed? What part of the
spectrum does this
correspond to?
• If a hydrogen atom is in the
state n=4, to what level must
it “fall” in order to emit a
blue photon?
Practice
• If an atom absorbs a photon, does the atom’s
energy increase, decrease, or remain constant?
• Suppose that a gas of 4 hydrogen atoms has an
atom in each of the 4 lowest energy levels. How
many distinct photons can be emitted by this gas?
• Suppose that a particular gas will only emit a red
photon and a yellow photon. What colors will it
absorb if visible light is incident on the gas with
many of its atoms in the ground state?
Using light to know
the temperature of stars
Blackbody radiation
• A perfect absorber of
light is a blackbody.
• A blackbody is also a
perfect emitter.
• The emission
spectrum of a
blackbody is
continuous and
depends on
temperature.
T ~ 4000 K
Blackbody curves
Temperature and brightness
As T increases,
the wavelength
for peak
brightness
decreases (i.e.
shifts toward the
violet and
ultraviolet
wavelenghs).
As T increases,
the brightness
increases.
Using light to know
the motion of stars, planets, and other
objects in the sky
Doppler shift
• As a star approaches you, the frequencies of
the absorption lines increase (so the
wavelengths decrease). They are
blueshifted.
• As a star recedes away from you, the
frequencies of the absorption lines decrease
(so the wavelengths increase). They are
redshifted.
Blueshift
Redshift
Light tells us
• what something is made of
– by analyzing emission and absorption spectral
lines
• what temperature it is
– blackbody curve
• how fast it is moving toward us or away
from us
– doppler shift of spectral lines