Download Chapter 4: Spectroscopy

Lecture II
Light spectra
The Birth of the Quantum
• Max Planck
– The energy contained in radiation is related to
the frequency of the radiation by the
relationship
E  nhf
• n is a positive integer called the quantum number
• f is the frequency of the oscillation
– A discreet packet of energy, later to become
known as “a photon”
Implications of Planck’s Law
• The energy levels of
the molecules must be
discreet
• Only transitions by an
amount E=hf are
allowed
• The implication is that
light is discreet or
quantised
energy
n
4hf
3hf
2hf
1hf
0
4
3
2
1
0
These quantum levels are
known as number states
Spectroscope
Three Types of Spectra
Spectral Analysis of the Elements
Studying the light emitted by an object
in order to know something about that object!
Continuous Spectrum: a collection all possible
wavelengths/ frequencies of light
Emission Spectra
Pattern of bright spectral lines
produced by an element.
Absorption Spectra
Pattern of dark spectral lines
where light within a number of
narrow frequency ranges has been
removed.
Hydrogen
Helium
Argon
Neon
Krypton
Wavelength
Bright Line Emission Spectra
Kirchoff’s Laws
• 1st law: A luminous solid or
liquid, or a sufficiently dense
gas, emits light of all
wavelengths and produces a
continuous spectrum of
radiation.
• 2nd law: A low-density hot
gas emits light whose
spectrum consists of a series
of bright emission lines which
are characteristic of the
chemical composition of the
gas.
• 3rd law: A cool thin gas
absorbs certain wavelengths
from a continuous spectrum,
leaving dark absorption lines
in their place superimposed
on the continuous spectrum.
Spectra and Background
Type of spectrum seen depends on the temperature of the
thin gas relative to the background temperature.
TOP: thin gas cooler than background, absorption lines seen.
BOTTOM: thin gas hotter than background, emission lines seen.
Studying the Stars:
Analyzing the light from a star can tell us:
1. The composition of the star.
2. The relative motion & rotation of the star.
3. The star’s temperature.
Shows limited Range of Light Energies Reaching Earth’s Surface