Spectroscopy
• spectroscopy: breaking up light into its
component colors to study how atoms and
light interact
• dispersion: spreading out of white light
• spectrum: the spectra of colors produced
by sending white light through a prism
• spectroscope: instrument used to see
spectra (ex. in class)
Types of Spectra
• continuous spectrum: a spectrum with no breaks. A
continuum of unmixed shades of color.
• spectral (emission) line: a well defined, thin line of one
specific color
• emission-line spectrum: thin lines of specific colors
against a dark background
• absorption line: a dark line on a continuous spectrum
marking a lack of intensity of a specific color (not
necessarily a complete absence of that color)
• absorption-line (dark-line) spectrum: dark lines of
missing color against a continuous background (ex: solar
spectrum)
More on spectra
• polarized light: all light propagating
(waving) in the same direction. A
spectroscope polarizes light prior to its
dispersion. This ensures that all spectral
lines are vertical and parallel.
• uniform speed of light: light travels at
same speed through the same medium.
Light travels fastest through a vacuum.
Kirchoff’s Rules
• 1. A hot, opaque solid liquid, or highly compressed gas
emits a continuous spectrum.
Example: filament of a incandescent light bulb
• 2. emission-line spectrum => A hot, transparent gas
produces a spectrum of bright lines (emission lines). The
number and colors of these lines depend on which
elements are present in the gas.
Example: a neon sign
• 3. absorption-line spectrum => If a continuous spectrum
(from a hot opaque solid, liquid, or gas) passes through
a gas at a lower temperature, the cooler gas causes the
appearance of dark lines (absorption lines). Their colors
and numbers depend on the elements in the cool gas.
(99% of stars have this spectrum)
Example: sunlight
• Show me
Kirchoff’s Rules
Spectral Summary
• every element /isotope displays a unique
arrangement of lines in its spectra
• emission and absorption lines for the
same element are in identical positions
Balmer Series
• The set of hydrogen absorption or emission lines that lie
in the visible part of the spectrum, the first of which is the
H-α line (in the red visible region)
• This very orderly pattern of spectral lines led scientists to
look for a cause in the internal structure of the atom
Particle nature of light
• quanta: “chunks” of light energy (photons)
emitted by radiating matter
• photon: a quanta (piece) of light
• photoelectric effect:
Ephoton = hf = hc / λ
•
•
•
•
•
(since f = c / λ)
E = energy of the photon
f = frequency of the photon
h= Planck's constant (6.63 x 10-34 J·s)
λ: wavelength of photon
c : speed of light (3 x 108 m / s in a vacuum)
Bohr’s atomic model
• explains and predicts why absorption and
emission of photons take place.
• Emission and absorption of light is due to
transitions between electron energy levels
• energy levels: electrons have a large number of
levels with specific energies (stair step example)
• ground state: the 1st (lowest) energy level.
When the electron occupies this level, the atom
is at its minimum energy.
• the higher the total energy of the atom, the
closer the orbits (or electron energy levels) are
to each other.
Excitation & De-excitation
Model of atomic energy levels
• excitation: moving an electron to a higher level (yields
an absorption line)
– 2 methods of excitation: collision with another atom or absorption
of a specific photon with sufficient energy.
• de-excitation: the electron descends to a lower level
(yields an emission line)
– The electron loses energy and this exact energy is given off by
the emission of a photon.
• electrons only move to exact energy levels (don't take
half-steps)
• Ionization: if an atom gains enough energy, the electron
flies away from (escapes) the nucleus. It is no longer
bound to the atom.
• Ionization energy: the amount of energy needed to ionize
an electron.
Balmer series explained
• Emission or
absorption lines
arising from
transitions between
the 2nd level and all
others (except the 1st)
• Photons at visible
wavelengths
Hydrogen-specific series
•
•
•
5
4
5
4
3
3
2
2
1
1
Lyman series
•
Emission or absorption
•
lines arising from transitions
between the 1st level and all
others.
Photons at ultraviolet
wavelengths
•
Balmer series
Emission or absorption
lines arising from
transitions between the
2nd level and all others
(except the first).
Photons at visible
wavelengths
•
•
•
Paschen series
Emission or absorption
lines arising from
transitions between the 3rd
level and all others (except
the first and second).
Photons at infrared
wavelengths
Balmer Thermometer
• Balmer Lines: spectral lines of H caused by bound-bound
transitions to and from the 2nd energy level
–
–
–
–
from 3->2: red line
from 4->2: blue line
from 5->2: violet line
from 6->2: violet line
• If a star is too cool; few electrons are excited to or above
the 2nd energy level (3000K; 6000° F)
• If a star is too hot; most electrons are excited above the
2nd level
• "In between" stars have the strongest Balmer Lines
(Spectral Class: A)
Spectral Classification
•
•
•
-Original Method:
– depended on Balmer line strength only;
– no understanding of relationship to temperature;
– classified from strongest (A) to weakest (Z) Balmer lines
But, weak Balmer lines can be from hot and cool stars...
Modern Stellar Spectra Sequence: reorganized classes by temperature:
(hot) O
b
l
u
e
/
w
h
i
t
e
•
Also a color scale!
B
b
l
u
e
/
w
h
i
t
e
A
w
h
i
t
e
/
b
l
u
e
F
w
h
i
t
e
/
y
e
l
l
o
w
G
y
e
l
l
o
w
K
o
r
a
n
g
e
M
r
e
d
(cool)
Spectral sub-classes
• Spectral subclasses. (0-9) differ between
intensities of specific absorption lines
• ex: (hotter) G0, G1, G2... K0, K1 (cooler)
• Strengths of Balmer lines suggest
differences in stellar spectra & reflect
differences in temperatures
• (the hotter the) Temperature --> (the more)
Collisions ---> (the more) Ionization