4: Light and Matter
- Page ID
- 55004
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- 4.1: The Behavior of Light
- James Clerk Maxwell showed that whenever charged particles change their motion, as they do in every atom and molecule, they give off waves of energy. Light is one form of this electromagnetic radiation. The wavelength of light determines the color of visible radiation. Wavelength (λ) is related to frequency (f) and the speed of light (c) by the equation c = λf. Electromagnetic radiation sometimes behaves like waves, but at other times, it behaves as if it were a particle- called a photon.
- 4.2: The Electromagnetic Spectrum
- The electromagnetic spectrum consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared, and radio radiation. Many of these wavelengths cannot penetrate the layers of Earth’s atmosphere and must be observed from space, whereas others—such as visible light, FM radio and TV—can penetrate to Earth’s surface. The emission of electromagnetic radiation is intimately connected to the temperature of the source.
- 4.3: Spectroscopy in Astronomy
- A spectrometer is a device that forms a spectrum, often utilizing the phenomenon of dispersion. The light from an astronomical source can consist of a continuous spectrum, an emission (bright line) spectrum, or an absorption (dark line) spectrum. Because each element leaves its spectral signature in the pattern of lines we observe, spectral analyses reveal the composition of the Sun and stars.
- 4.4: The Structure of the Atom
- Atoms consist of a nucleus containing one or more positively charged protons. All atoms except hydrogen can also contain one or more neutrons in the nucleus. Negatively charged electrons orbit the nucleus. The number of protons defines an element (hydrogen has one proton, helium has two, and so on) of the atom. Nuclei with the same number of protons but different numbers of neutrons are different isotopes of the same element.
- 4.5: Formation of Spectral Lines
- When electrons move from a higher energy level to a lower one, photons are emitted, and an emission line can be seen in the spectrum. Absorption lines are seen when electrons absorb photons and move to higher energy levels. Since each atom has its own characteristic set of energy levels, each is associated with a unique pattern of spectral lines. This allows astronomers to determine what elements are present in the stars and in the clouds of gas and dust among the stars.
- 4.6: The Doppler Effect
- If an atom is moving toward us when an electron changes orbits and produces a spectral line, we see that line shifted slightly toward the blue of its normal wavelength in a spectrum. If the atom is moving away, we see the line shifted toward the red. This shift is known as the Doppler effect and can be used to measure the radial velocities of distant objects.
Chapter 4: Light and Matter — Introduction
Almost everything we know about the universe beyond Earth comes from light. Astronomers cannot travel to stars and galaxies to collect samples or make measurements directly. Instead, they study the radiation that reaches us from these distant objects.
Light carries much more information than we can see with our eyes. By examining different wavelengths of electromagnetic radiation and the patterns produced when light interacts with matter, astronomers can determine properties such as an object's temperature, composition, and motion. To understand how this works, we also need to look at the structure of atoms and the ways that atoms absorb and emit energy.
In this chapter, we will explore the behavior of light, the electromagnetic spectrum, and the formation of spectra. These ideas provide some of the most important tools astronomers use to investigate objects throughout the universe and will form the foundation for our study of stars in the next chapter.