Astronomy 100

Lectures

Table of Contents

Astro 100

Lecture 8
Light: The Electromagnetic Spectrum

Outline

What is Light Made of?
The Electromagnetic Spectrum

Terms to Know

spectrum
wavelength (

)
frequency (

)
amplitude
photon

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 very slightly 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

E=h

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?