Astronomy 100


Lectures Table of Contents Astro 100

Astronomical Tools
Light: The Electromagnetic Spectrum


  1. What is Light Made of?
  2. The Electromagnetic Spectrum

Terms to Know

wavelength ( )
frequency ( )

1. What is Light Made of?

Light is a form of energy . It can be transformed into other forms, such as kinetic or potential energy, or even into matter (remember Einstein's famous equation E=mc2!), and back again.

Light travelling in a vacuum always travels at speed c=3.0 x 1010 cm/sec. It slows down when it encounters other media, such as glass, air, or water.

Pretty much all the information we get from astronomical objects comes in the form of light; we can look, but we can't touch. That's why studying light is so important to astronomy!

2. The Electromagnetic Spectrum

"Light" = all forms of electromagnetic radiation (not just the kind you can see with your naked eyes). From lowest to highest energy, here is the whole EM spectrum, or range of "colors":
  • radio
  • microwave
  • infra-red
  • optical=visible (ROY G BIV)
  • ultra-violet
  • X-ray
  • Gamma ray

EM radiation can be described in two very different ways:
1. Waves radiating out from a source (like pond waves from a stone)
2. Particles of energy = photons (like raindrops)

Which description is "correct"? They both are! It depends on which physical scenario is being described.

Both waves and photons are described by the simple equation:

(wavelength x frequency = speed of light). , the wavelength, is the distance from one wave peak to the next. , the frequency ("wiggle speed"), is the number of peaks passing by you per second. When gets bigger, gets smaller, and vice versa.

The energy of a single photon is given by


where h=Planck's constant. You can also write E=hc/ .

High energy Low energy
high low
short long

Which photons have more energy, blue or red?

Temperature and Black Body Radiation


  1. Black Body Radiation
  2. The Inverse Square Law of Light

Terms to Know

black body radiation
Wien's Law
Stefan-Boltzmann law
absolute zero
inverse square law (of light)

1. Black Body Radiation and Temperature

Everything in the Universe emits radiation all the time. This includes stars, planets, and people. Hot things emit lots of high energy ("blue") radiation, while cooler things emit little radiation at all, most of it low energy ("red"). This is expressed in two laws:
  • Wien's Law tells you about the color
    peak = 2.9 / T, with T in K (degrees Kelvin) and in mm
  • Stefan-Boltzmann Law tells you about the total amount of radiation
    E T4

For the Sun, peak = 2.9/6000 = 4.8 x 10-4 mm, or 480 nm, corresponding to yellow-green. This is almost exactly the same wavelength to which the human eye is most sensitive. Why?

This thermal radiation from matter is called "black body radiation." It is caused by electrons in the matter emitting photons as they vibrate -- faster when hot, slower when cool. At absolute zero temperature, atoms stop vibrating completely, and only then are totally dark.

Black body spectra are smooth, or continuous, curves; such spectra are therefore known as continuum spectra.

A great deal of the light we collect from distant stars, galaxies, and quasars is black body radiation.

2. The Inverse Square Law of Light

As you get farther and farther from luminous objects (stars, planets, city lights), they appear to get fainter very quickly. To be precise, the intensity I, or amount of light received per second per cm2, of an object at distance r is I 1/r2. This is called the inverse square law of light , somewhat analogous to the inverse square law of gravity.

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100