CHAPTER 6 | LIGHT AND TELESCOPES 99
Radiation: Information
from SpaceJust as a book on baking bread might begin with a discussion
of fl our, this chapter on telescopes begins with a discussion of
light—not just visible light, but the entire range of radiation
from the sky.Light as a Wave and a Particle
When you admire the colors of a rainbow, you are seeing light
behave as a wave. But when you use a digital camera to take a
picture of the same rainbow, the light hitting the camera’s detec-
tor acts like a particle. Light is peculiar in that it is both wave and
particle, and how it acts depends on how you observe it.
Light is a form of electromagnetic radiation and carries
energy through space as electric and magnetic waves. We use the
word light to refer to electromagnetic radiation that we can see,
but visible light is only a small part of a larger range that includes
X-rays and radio waves. Electromagnetic radiation travels through
space at 300,000 km/s (186,000 mi/s). Th is is commonly
referred to as the speed of light, c, but it is in fact the speed of all
electromagnetic radiation.
Some people fl inch at the word radiation, but that refl ects a
Common Misconception. Radiation refers to anything
that radiates from a source. High-energy particles emitted from
radioactive atoms are called radiation, and you have learned to be
a little bit concerned when you see this word. But light, like all
electromagnetic radiation, spreads outward from a source, so you
can correctly refer to light as a form of radiation.
Electromagnetic radiation can act as a wave phenomenon—
that is, it is associated with a periodically repeating disturbance,
a wave. You are familiar with waves in water: If you disturb a
pool of water, waves spread across the surface. Imagine that you
use a meter stick to measure the distance between the successive
peaks of a wave. Th is distance is the wavelength, usually repre-
sented by the Greek letter lambda ().
Wavelength is related to frequency, the number of waves
that pass a stationary point in 1 second (■ Figure 6-2). Wavelength
and frequency are related, and you can calculate the wavelength
by dividing the speed of light by the frequency.
^ _cf^
When you tune in your favorite FM station at, say, 89.5 on the dial,
which means 89.5 MHz (million cycles per second), you are adjust-
ing your radio to detect radio waves with a wavelength of 3.35 m.
Note that the higher the frequency, the shorter the wavelength.
Sound is another example of a wave, in that case a mechanical
disturbance that travels through air from source to ear. Sound
requires a medium; so, on the moon, where there is no air, there
can be no sound. In contrast, light is made up of electric and mag-
netic fi elds that can travel through empty space. Unlike sound,Th e strongest thing that’s given us to see with’s 6-1
A telescope. Someone in every town
Seems to me owes it to the town to keep one.
—ROBERT FROST, “THE STAR-SPLITTER”S
tarlight is going to waste. Every night it falls on
trees, oceans, and parking lots, and it is all wasted. To
an astronomer, nothing is so precious as starlight. It
is the only strong link to the universe, so the astronomer’s quest
is to gather as much of it as possible and extract from it the
secrets of the heavens.
Th e telescope is the symbol of the astronomer because it
gathers and concentrates light for analysis. Most of the interest-
ing objects in the sky are faint, so astronomers are driven to build
huge telescopes to gather the maximum amount of light
(■ Figure 6-1). Some telescopes collect radio waves or X-rays, and
some go into space, but they all gather information about our
universe.
In the quotation that opens this chapter, Robert Frost sug-
gests that someone in every town should own a telescope.
Astronomy is more than technology and scientifi c analysis. It
helps tell us what we are, and every town should have a telescope
to keep us looking upward.
■ Figure 6-1
Astronomical telescopes are often very large to gather large amounts of
starlight. The Southern Gemini telescope stands over 19 m (60 ft) high
when pointed straight up, and its main mirror, shown at lower left, is 8.1 m
(26.5 ft) in diameter—larger than some classrooms. The sides of the tele-
scope dome open to allow quick equalization of inside and outside tempera-
tures at sunset. (Gemini Observatory/Aura)