The Solar System

CHAPTER 6 | LIGHT AND TELESCOPES 101

of Figure 6-3. Gamma rays, X-rays, and some radio waves are

absorbed high in Earth’s atmosphere, and a layer of ozone (O 3 ) at

altitudes of about 15–30 km absorbs ultraviolet radiation. Water

vapor in the lower atmosphere absorbs the longer- wavelength

infrared radiation. Only visible light, some shorter-wavelength

infrared, and some radio waves reach Earth’s surface through two

wavelength regions called atmospheric windows. Obviously, if

you wish to study the sky from Earth’s surface, you must look out

through one of these windows.

Optical Telescopes

Earth has two atmospheric windows, so there are two main

types of ground-based telescopes—optical telescopes and radio

telescopes. You can start with optical telescopes, which gather

light and focus it into sharp images. Th is requires sophisticated

optical and mechanical designs, and it leads astronomers to build

gigantic telescopes on the tops of high mountains.

Two Kinds of Optical Telescopes

Optical telescopes can focus light into an image by using either

a lens or a mirror, as shown in ■ Figure 6-4. In a refracting

telescope, the primary (or objective) lens bends (refracts) the

light as it passes through the glass and brings it to a focus to form

a small inverted image. In a refl ecting telescope, the primary

6-2

Vi

us

al

Visible light

Short wavelengths Long wavelengths

Wavelength (meters)

4 × 10 –^7

(400 nm)

10 –^1210 –^1010 –^810 –^410 –^21102104

5 × 10 –^7

(500 nm)

6 × 10 –^7

(600 nm)

7 × 10 –^7

(700 nm)

meters

Gamma

ray

Micro-

wave

Ultra-

X-ray violet Infrared

UHF VHF FM AM

Transparent

Visual

window

Opaque

Wavelength

Transparency ofEarth’s atmosphere

Radio

window

■ Figure 6-3

The spectrum of visible light, extending from red to violet, is only part of the electromagnetic spectrum. Most radiation is absorbed in Earth’s
atmosphere, and only radiation in the visual window and the radio window can reach Earth’s surface.

SCIENTIFIC ARGUMENT

What would you see if your eyes were sensitive only to X-rays?

Sometimes the best way to understand a concept is to build a

scientifi c argument and change one factor so you must imagine

a totally new situation. In this case, you might at fi rst expect to

be able to see through walls, but remember that your eyes detect

only light that already exists. There are almost no X-rays bouncing

around at Earth’s surface, so if you had X-ray eyes, you would be

in the dark and would be unable to see anything. Even when you

looked up at the sky, you would see nothing, because Earth’s atmo-

sphere is not transparent to X-rays. If Superman can see through

walls, it is not because his eyes can detect X-rays.

But now imagine a slightly different situation and modify your

argument. Would you be in the dark if your eyes were sensitive

only to radio wavelengths?