Electromagnetic Radiation and Spectra

Connection between EM Radiation and Electrical/Magnetic Forces

• how do you hear a radio broadcast?

  • voice makes sounds, which travel out compression waves

  • waves move magnet in microphone ---> change electric current

    

  • current is moving electrons down a wire ---> electromagnetic wave travels out from antenna at radio station

  • wave detected by your radio's antenna ---> converted to electric current

    

  • electric current moved small magnet in loudspeaker ---> compression waves (sound!)

• wave or particle (photon)?

  • light waves can add, subtract, and pass through each other

  • photoelectric effect (light comes in certain packets of energy, two low energy photons don't equal one high energy photon)

    

What is a spectrum?

• plot of INTENSITY vs. WAVELENGTH for the light coming from an object

  

• different types of spectra
  • let's begin with a thermal (or blackbody or continuous) spectrum

    

    • change in temperature causes change in wavelength of peak of emission and in total amount of light (remember Planck Spectra and Wien's Law?)

      Blackbody spectra: interactive demo

      Blackbody spectra: The Game

    • anything we can see with our eyes (without shining light on it) must be a few thousand degrees hot (in other words, all stars are several thousands of degrees hot on the surface)

    • BUT why does the Sun not have exactly a blackbody spectrum? what are those dark lines?

      

      To see any part of the solar spectrum, go here.

    • Sun has absorption lines
      • continuum of photons with different wavelengths flow from high to low tempertures toward the surface

      • absorption of photon causes atom to enter excited state

      • excited atom subsequently either de-excites collisionally (true absorption): lower temperature gas in upper layers leads to more radiative excitations followed by collisional de-excitations than collisional excitations followed by radiative de-excitations

      • or de-excites radiatively, by the emission of a photon in a different direction (resonant scattering): also leads to appearance of dark line, because star has surface from which photons escape (boundary) and there is greater statistical tendancy for photons travelling in our direction to be scattered away than those originally travelling away from us to be scattered back into the line-of-sight

  • Kirchhoff's Laws: continuous (or thermal or blackbody), absorption, emission spectra (demo)

    

    

    

    

• different elements have different lines in their spectra (see examples here)

Spectroscopy of the Sun and the Discovery of Helium

The Sun is 300,000 times closer than the next nearest star!

Galileo and the sunspots (today's sunspots). Rotation 25 days at poles, 33 days on equator.

Helium in the laboratory...

Which one is helium?

Composition of the Sun

Element Name		Percentage of Total Mass		atomic number (number of protons)
Hydrogen		76.4		1
Helium		21.8		2
Oxygen		0.8		8
Carbon		0.4		6
Neon		0.2		10
Iron		0.1		26