Lecture 14: Understanding Electromagnetic Radiation

Properties of Electromagnetic Radiation (see a moving wave here)

• wavelength or frequency

• velocity (all electromagnetic waves travel at the same velocity, the speed of light!)

• intensity

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 Plank 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 (absorption/emission demo)

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

    

    

    

    

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

Making Observations by Collecting and Analyzing Light

A photon's journey...

• emitted by a source (such as a star)

• perhaps absorbed by interstellar material along the way, perhaps re-emitted...

• crashes through our atmosphere, perhaps absorbed (sunsets are red), perhaps scattered (the daytime sky is blue)

• falls onto our telescope mirror (bounces on several mirrors)

• is seen by our eye or photographed or digitally imaged

Key Points of Detecting Radiation

• atmosphere limits light that reaches group by absorbing and scattering it, and also and blurs the image

  

• telescopes collect light

  

  Optics are used to reflect, refract, or disperse light. Click here for applet to demonstrate reflection and refraction.

  Click here for simulation of refraction on waves.

  Ray tracing demonstration (lenses or mirrors) here.

  Click here for applet demonstrating the use of two lenses to form an image.

• at present, large optical telescopes can improve the resolution to at best ~ 0.5 arcseconds

• instruments (computers and detectors) enable photometry and spectroscopy, instruments are large and sophisticated on modern telescopes...

  

  and running them and the telescope is a computer-intensive activity...

  

• other ground based observing facilities in the infrared and radio wavelength regimes...

• space-based observatories remove the constraints of the atmosphere (blurring and blocking of certain wavelengths of light)

  Mediocre image from the ground	Hubble Space Telescope image

  (the orientation on the sky is different between the two images)