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CHAPTER

5

R I V U X G
The Ring Nebula is a shell of glowing gases surrounding a dying star. The spectrum of the emitted light reveals which gases are present.
(Hubble Heritage Team, AURA/STScI/NASA))

The Nature of Light

LEARNING GOALS

By reading the sections of this chapter, you will learn

5–1 How we measure the speed of light
5–2 How we know that light is an electromagnetic wave
5–3 How an object’s temperature is related to the radiation it emits
5–4 The relationship between an object’s temperature and the amount of energy it emits
5–5 The evidence that light has both particle and wave aspects
5–6 How astronomers can detect an object’s chemical composition by studying the light it emits
5–7 The quantum rules that govern the structure of an atom
5–8 The relationship between atomic structure and the light emitted by objects
5–9 How an object’s motion affects the light we receive from that object

In the early 1800s, the French philosopher Auguste Comte argued that because the stars are so far away, humanity would never know their nature and composition. But the means to learn about the stars was already there for anyone to see—starlight. Just a few years after Comte’s bold pronouncement, scientists began analyzing starlight to learn the very things that he had deemed unknowable.

We now know that atoms of each chemical element emit and absorb light at a unique set of wavelengths characteristic of that element alone. The red light in the accompanying image of a gas cloud in space is of a wavelength emitted by nitrogen and no other element; the particular green light in this image is unique to oxygen, and the particular blue light is unique to helium. The light from nearby planets, distant stars, and remote galaxies also has characteristic “fingerprints” that reveal the chemical composition of these celestial objects.

In this chapter we learn about the basic properties of light. Light has a dual nature: It has the properties of both waves and particles. The light emitted by an object depends upon the object’s temperature; we can use this to determine the surface temperatures of stars. By studying the structure of atoms, we will learn why each element emits and absorbs light only at specific wavelengths and will see how astronomers determine what the atmospheres of planets and stars are made of. The motion of a light source also affects wavelengths, permitting us to deduce how fast stars and other objects are approaching or receding. These are but a few of the reasons why understanding light is a prerequisite to understanding the universe.

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