Properties of Waves and Light
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What Do We Easily See about Light?

  • What kinds are there? We can see different colors, and there are other types that we can't see.
  • How does it affect matter? Light can come out of matter, or be absorbed, reflected, transmitted; It carries heat-energy.
  • How does it move? Apparently instantaneously, in reality at 300,000 km/sec! It travels in straight lines, except...

A Few Notions about Waves

  • Waves: Period, wavelength, amplitude, frequency, velocity or speed; Related by velocity = (wavelength) (frequency).
  • Examples: Waves in water, strings (transverse); Sound (longitudinal); light.
  • How can we tell if something is a wave? Interference and diffraction (like when looking through fogged glasses).

  What Are Light and Radiation?

  • Electromagnetic waves: Made of changing electric and magnetic fields.
  • Different types: Colors depend on wavelength; but visible light is only a small part of the entire spectrum.
  • Different effects: Which ones make it through the atmosphere? Which ones can our bodies detect?
  • Why are we interested in detecting waves of different wavelengths? Some objects don't emit much visible light; Visible light cannot traverse certain regions; and different kinds of radiation provide different kinds of information.

Different Types, Based on Wavelength

* A star indicates a type of radiation that can traverse the Earth's atmosphere relatively easily.

Type and Wavelength

Examples; Common Sources

 Sources and Uses in Astronomy
Radio * (l from mm's to 1000's of m) Radio, TV; Antennas Radar studies of planets; Interstellar gas clouds; Active galaxies and galactic structure
Microwaves (l from mm's to cm's) Cell phones; Ovens Cosmic background radiation
Infrared * (l from microns to mm) Communications, remote controls; Warm objects Interstellar dust and star forming regions; Cool stars
Visible * (l from 400 to 700 nm) Ordinary light; Hot objects Solar system planets; Stars; Galaxies
Ultraviolet (l from 10–9 to 10–7 m) Solar radiation; Atoms Hot stars; Interstellar medium
X-rays (l from 10–11 to 10–8 m) Medical applications; Atoms Stellar atmospheres; Neutron stars and Black holes; Galaxy clusters; Active galactic nuclei
Gamma rays
(l less than 10–11 m)
Medical applications; Nuclear reactions Neutron stars, cosmic ray collisions; Active galactic nuclei; Gamma Ray Bursts.

  What Can We Learn from Analyzing Light?

  • Distance effect: Inverse square law for brightness. Conclusion: Compare apparent brightness with actual one to get the distance!
  • Velocity effect: Doppler shift of wavelengths or frequencies from radial motion. Conclusion: Check the wavelength to get the velocity along the line of sight!
  • Can we get more information? Some sources emit only at a few specific wavelengths, others at all wavelengths. We need to know which ones, and what affects the amount of light we get at each wavelength.

Examples of Uses of Different Types of Radiation

  • Infrared: Asteroids (the amount of infrared radiation tells us how much sunlight they absorb, thus their albedo, and helps us find their size even if we cannot resolve it with a telescope).

So, light is a wave, but we will see that this is not the end of the story...

page by luca bombelli <bombelli at>, modified 10 sep 2012