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Unit 4
Atomic Physics and Spectra
The Electromagnetic Spectrum
Waves
• type of disturbance that can propagate or travel.
wavelle lnlgth ()
crest
amplitude (A)
velocity (v)trough
Wavelength () is a distance, so its units are m, cm, or mm, etc.
Period (T): time between crest (or trough) passages
Frequency (n): rate of passage of crests (or troughs
c= n
(units: Hertz or cycles/sec)
Equilibrium position
• Shorter wavelength (l) = higher frequency (n) and therefore higher energy
Waves bend when they pass through material of different densities.
swimming pool
air
water
prismairair
glass
Refraction
All radiation (including visible light) travels as Electromagnetic waves.
Examples of objects with magnetic fields:Magnetthe Earththe Sun
Examples of objects with electric fields:Electrical appliancesLightningProtons, electrons
Electromagnetic Radiation
How we get information about the cosmos
e.g. Visible Light
nc =
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
The Electromagnetic Spectrum
When you bend light, bending angle depends on wavelength, or color.
Refraction of light
(Prism demo)
Types of Spectra
1. "Continuous" spectrum luminous solid, liquid, or dense gas, emits
light of all wavelengths, produces a continuous spectrum
2. "Emission" spectrum low-density, hot gas emits light whose
spectrum consists of a series of bright emission lines that are characteristic of the composition of the gas.
1. "Absorption” Spectrum cool, thin gas absorbs certain wavelengths
from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.
Pattern of lines is a fingerprint of the element
For a given element, emission and absorption lines occur at the same wavelengths.
Sodium emission and absorption spectra
The Particle Nature of Light
Light interacts with matter as individual packets of energy, called photons.
photon energy is proportional to frequency:
example: ultraviolet photons are more dangerous than visible photons.
The Nature of Atoms
The Bohr model of the Hydrogen atom:
_
+
proton
electron
"ground state"
_
+
"excited state"
When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system
Other elements
Helium Carbon
neutron proton
Each element has its own allowed energy levels yielding a unique spectral fingerprint.
Atmospheric blocking
Radio Waves
• Emitted by TV, radio, stars, galaxies
• Radio wave image of CO gases in our Milky Way galaxy
Microwaves
• Penetrate clouds, fog communication, radar
Infrared• Thermal (heat)
IR image of Milky Way
Visible
• Detection with eyes
Ultraviolet
• Higher energy than visible light
UV image of the Sun
X ray radiation
X ray image of the Sun
Gamma-rays
• Shortest wavelength, therefore highest E• Produced by supernovae, pulsars, black holes, …• Gamma ray bursts (source ???)
• http://science.hq.nasa.gov/kids/imagers/ems/gamma.html
• https://www.e-education.psu.edu/astro801/content/l3_p4.html
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