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Electromagnetic Spectrum

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Title: Electromagnetic Spectrum


1
Electromagnetic Spectrum
Energy low ? medium ?
high
2
Electromagnetic Radiation Quick Facts
  • There are different types of EM radiation,
    visible light is just one of them
  • EM waves can travel in vacuum, no medium needed
  • The speed of EM radiation c is the same for all
    types and very high (? light travels to the moon
    in 1 sec.)
  • The higher the frequency, the smaller the
    wavelength (?? f c)
  • The higher the frequency, the higher the energy
    of EM radiation (E h f, where h is a constant)

3
Visible Light
  • Color of light determined by its wavelength
  • White light is a mixture of all colors
  • Can separate individual colors with a prism

4
Three Things Light Tells Us
  • Temperature
  • from black body spectrum
  • Chemical composition
  • from spectral lines
  • Radial velocity
  • from Doppler shift

5
Temperature Scales
Fahrenheit Centigrade Kelvin
Absolute zero ?459 ºF ?273 ºC 0 K
Ice melts 32 ºF 0 ºC 273 K
Human body temperature 98.6 ºF 37 ºC 310 K
Water boils 212 ºF 100 ºC 373 K
6
Black Body Spectrum
  • Objects emit radiation of all frequencies, but
    with different intensities

Ipeak
Higher Temp.
Ipeak
Ipeak
Lower Temp.
fpeakltfpeak ltfpeak
7
Cool, invisible galactic gas (60 K, fpeak in
low radio frequencies)
Dim, young star (600K, fpeak in infrared)
The Suns surface (6000K, fpeak in visible)
Hot stars in Omega Centauri (60,000K, fpeak in
ultraviolet)
The higher the temperature of an object, the
higher its Ipeak and fpeak
8
Wiens Law
  • The peak of the intensity curve will move with
    temperature, this is Wiens law
  • Temperature wavelength constant
  • 0.0029 Km
  • So the higher the temperature T, the smaller
    the wavelength, i.e. the higher the energy of the
    electromagnetic wave

9
Example
  • Peak wavelength of the Sun is 500nm, so
  • T (0.0029 Km)/(5 x 10-7 m) 5800 K
  • Instructor temperature roughly 100 F 37C 310
    K, so
  • wavelength (0.0029Km)/310 K
  • 9.35 10-6 m
  • 9350 nm ? infrared radiation

10
Measuring Temperatures
  • Find maximal intensity
  • ? Temperature (Wiens law)

Identify spectral lines of ionized elements ?
Temperature
11
Color of a radiating blackbody as a function of
temperature
  • Think of heating an iron bar in the fire red
    glowing to white to bluish glowing

12
Kirchhoffs Laws Dark Lines
  • Cool gas absorbs light at specific frequencies
  • ? the negative fingerprints of the elements

13
Kirchhoffs Laws Bright lines
  • Heated Gas emits light at specific frequencies
  • ? the positive fingerprints of the elements

14
Kirchhoffs Laws
  1. A luminous solid or liquid (or a sufficiently
    dense gas) emits light of all wavelengths the
    black body spectrum
  2. Light of a low density hot gas consists of a
    series of discrete bright emission lines the
    positive fingerprints of its chemical elements!
  3. A cool, thin gas absorbs certain wavelengths from
    a continuous spectrum
    ? dark absorption (
    Fraunhofer) lines in continuous spectrum
    negative fingerprints of its chemical elements,
    precisely at the same wavelengths as emission
    lines.

15
Spectral Lines Fingerprints of the Elements
  • Can use this to identify elements on distant
    objects!
  • Different elements yield different emission
    spectra

16
Spectral Lines
  • Origin of discrete spectral lines atomic
    structure of matter
  • Atoms are made up of electrons and nuclei
  • Nuclei themselves are made up of protons and
    neutrons
  • Electrons orbit the nuclei, as planets orbit the
    sun
  • Only certain orbits allowed ?Quantum jumps!

17
  • The energy of the electron depends on orbit
  • When an electron jumps from one orbital to
    another, it emits (emission line) or absorbs
    (absorption line) a photon of a certain energy
  • The frequency of emitted or absorbed photon is
    related to its energy
  • E h f
  • (h is called Plancks constant, f is
    frequency)
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