The Electromagnetic Spectrum
Visible light—the light that human eyes can see—comes in a variety of colors. The color of
visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm
to700nm, correspondingtothecolorsvioletthroughred. ButwhataboutEMradiationwith
wavelengths shorter than 400 nm or longer than 700 nm? Such radiation exists all around
you—you just can’t see it! Visible light is part of a largerelectromagnetic spectrum,as
Figure23.3illustrates.
Figure 23.3: Visible light is part of a larger electromagnetic spectrum. The EM spectrum
ranges from gamma rays with very short wavelengths, to radio waves with very long wave-
lengths. ( 19 )
What does the electromagnetic spectrum have to do with astronomy? Every star, including
our Sun, emits light at a wide range of wavelengths, all across the visible spectrum, and even
outside the visible spectrum. Astronomers can learn a lot from studying the details of the
spectrum of light from a star.
Some very hot stars emit light primarily atultravioletwavelengths, while some very cool
stars emit mostly in theinfrared. There are extremely hot objects that emitX-raysand
evengamma rays. Light from some of the faintest, most distant objects is in the form of
radio waves. In fact, a lot of the objects most interesting to astronomers today can’t even
be seen with the naked eye. Astronomers use telescopes to detect the faint light from distant
objects and to see objects at wavelengths all across the electromagnetic spectrum.
Types of Telescopes
Optical Telescopes
Humans have been making and using lenses for magnification for thousands and thousands of
years. However, the first true telescopes were made in Europe in the late 16thcentury. These