Chapter 4 (4.1, 4.4)
peaks, wavelength, speed of light, electromagnetic radiation, outward
Imagine waves in water: If you disturb a pool of water, waves spread across the surface. Now imagine placing a ruler parallel to the travel direction of the wave. The distance between __________ is the ___________. The changing electric and magnetic fields of electromagnetic waves travel through space at about 300,000 km per second (186,000 mi per second). That is commonly referred to as the _______ __ __________, but it is the speed of all __________________ ______________. It may seem odd to use the word radiation when talking about light, but radiation really refers to anything that spreads __________ from a source. Light radiates from a source, so you can correctly refer to light as a form of radiation.
instruments, techniques, light,
RADIATION: INFORMATION FROM SPACE Topic 4-2 Astronomers no longer study the sky by mapping constellations or charting the phases of the Moon. Modern astronomers analyze light using sophisticated _______________ (Figure 4-1) and _____________ to investigate the compositions, motions, internal processes, and evolution of celestial objects. To understand this, you need to learn about the nature of _________.
false-color images
________-________ _____________- A representation of graphical data with added or enhanced color to reveal detail.
Radio waves, microwave, ultraviolet, X-rays, gamma-rays
_________ __________ have even longer wavelengths than IR radiation. The radio radiation used for AM radio transmissions has wavelengths of a few hundred meters, while FM, television, and also military, governmental, and amateur radio transmissions have wavelengths from a few tens of centimeters to a few tens of meters. _____________ transmissions, used for radar and long-distance telephone communications, have wavelengths from about 1 mm to a few centimeters. Look at the electromagnetic spectrum in Figure 4-2 and notice that electromagnetic waves with wavelengths shorter than violet light are called ____________ (UV). Electromagnetic waves with wavelengths shorter than UV light are called _______________, and the shortest are _______________
array detectors
__________ _____________- Device for collecting and recording electromagnetic radiation using multiple individual detectors arrayed on the surface of a chip; for example, a CCD electronic camera.
charge-coupled devices (CCDs)
__________-___________ ______________- An electronic device consisting of a large array of light-sensitive elements used to record very faint images.
grating
___________- A piece of material in which numerous microscopic parallel lines are scribed. Light encountering a grating is dispersed to form a spectrum. (A prism bends light by an angle that depends on the wavelength of the light. Short wavelengths bend most and long wavelengths least. Thus, white light passing through a prism is spread into a spectrum.)
photon
____________- A quantum of electromagnetic energy that carries an amount of energy that depends inversely on its wavelength. ( is a quantum of light) packet of light A particle of electromagnetic radiation with no mass that carries a quantum of energy a discrete packet of electromagnetic radiation In physics, a photon is a bundle of electromagnetic energy. It is the basic unit that makes up all light. The photon is sometimes referred to as a "quantum" of electromagnetic energy. Quantum-a discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents. photon- little packets that make up light (waves and particles)
digitized
_____________- Converted to numerical data that can be read directly into a computer memory for later analysis.
photographic plate
______________ __________- The first image-recording device used with telescopes; it records the brightness of objects, but with only moderate precision.
spectrograph
_______________- A device that separates light by wavelengths to produce a spectrum.
X-rays
_______________- Electromagnetic waves with wavelengths shorter than ultraviolet light.
ultraviolet (UV)
_______________- The portion of the electromagnetic spectrum with wavelengths shorter than violet light, between visible light and X-rays.
gamma-rays
_______________- The shortest-wavelength electromagnetic waves.
Ångstrom (Å)
_________________- A unit of distance commonly used to measure the wavelength of light. 1Å=10^-10 m.
infrared (IR)
_________________- The portion of the electromagnetic spectrum with wavelengths longer than red light, ranging from 700 nm to about 1 mm, between visible light and radio waves.
wavelength
__________________- The distance between successive peaks or troughs of a wave, usually represented by a lowercase Greek lambda, λ.
spectrum
__________________-A range of electromagnetic radiation spread into its component wavelengths (colors); for example, a rainbow; also, representation of a spectrum as a graph showing intensity of radiation as a function of wavelength or frequency. ( A spectrum is electromagnetic radiation spread out into different wavelengths. )
atmospheric windows
___________________ ____________- Wavelength region in which our atmosphere is transparent—at visual, radio, and some infrared wavelengths.
electromagnetic radiation
____________________ _______________- Changing electric and magnetic fields that travel through space and transfer energy from one place to another; examples are light and radio waves. is the movement of energy - through space or a medium - composed of both electric and magnetic waves.
nanometer (nm)
____________________- A unit of distance equaling one-billionth of a meter (10-9m), commonly used to measure the wavelength of light.
photometers
_____________________- Sensitive astronomical instrument that measures the brightness of individual objects very precisely
ecliptic
the path the Sun follows as it appears to circle around the celestial sphere once each year. It crosses the celestial equator at a 23 1/2 degree angle, because that is the tilt of Earth's axis.
wave, particle, wavelike, particlelike, sound, medium, electric, magnetic waves, vacuum, electromagnetic radiation, visible light, repeating
4-1a Light as Waves and Particles Have you ever noticed the colors in a soap bubble? If so, then you have seen one effect of light behaving as a _______. When that same light enters a digital camera, the light meter on a camera, it behaves as a __________. How light behaves depends on how you treat it—light has both __________ and ___________ properties. Sound is another type of wave that you have already experienced. _________ waves are air pressure disturbances that travel through the air from source to ear. Sound requires a solid, liquid, or gas __________ to carry it; so, for example, in space outside a spacecraft, there can be no sound. In contrast, light is composed of a combination of _____________ and ____________ __________ that can travel through empty space. Unlike sound, light waves do not require a medium and thus can travel through a ______________. Because light is made up of both electric and magnetic fields, it is referred to as ___________________ ____________. _______________ __________ is only one form of electromagnetic radiation. Electromagnetic radiation is a wave phenomenon, that is, it is associated with a periodically ____________ disturbance (a wave) that carries energy.
electromagnetic spectrum, increasing, red, violet, electromagnetic spectrum, atmosphere
4-1b The Electromagnetic Spectrum The _________________ _____________ is, simply, the types of electromagnetic radiation arranged in order of ___________ wavelength. Rainbows are spectra (plural of spectrum) of visible light. The colors of visible light have different wavelengths: _______ has the longest wavelength and ____________ the shortest, as shown in the visible spectrum at the top of Figure 4-2. ((a) The spectrum of visible light, extending from red to violet, is only part of the __________________ ______________. (b) Most forms of light (electromagnetic radiation) are absorbed in Earth's __________________. Light can reach Earth's surface only through the visual and radio "windows.") radio window- is the range of frequencies of electromagnetic radiation that the earth's atmosphere lets through.
spread, wavelength, spectrum, prism, white light, mixture, colors, bent, most, least, spectrum, wider, lenses, focus, grating, recorded, multiobject,
4-4b Spectrographs To analyze light in detail, you need to ________ the light out according to _____________-into a _____________, a task performed by a spectrograph. You can understand how this works by reproducing an experiment performed by Isaac Newton in 1666. Boring a hole in his window shutter, Newton admitted a thin beam of sunlight into his darkened bedroom. When he placed a ________ in the path of the beam, the sunlight spread into a beautiful spectrum on the far wall. From this, and similar experiments, Newton concluded that ________ ________ was made of a ________ of all the ________. Newton didn't think in terms of wavelength, but you can use that modern concept to see that the light passing through the prism is ______ at an angle that depends on the wavelength (see Figure 4-13). Violet (short-wavelength) light bends the _______, and red (long-wavelength) light the ________. Thus, the white light entering the prism is spread into what is called a ________. A typical prism spectrograph contains more than one prism to spread the spectrum _________, plus ________ to guide the light into the prisms and to ________ the light onto a photographic plate. Most modern spectrographs use a grating in place of a prism. A _________ is a piece of glass with thousands of microscopic parallel lines scribed onto its surface. Different wavelengths of light reflect from the grating at slightly different angles, so white light is spread into a spectrum and can be _________, often by a CCD camera. Recording the spectrum of a faint star or galaxy can require a long time exposure, so astronomers have developed _____________ spectrographs that can record the spectra of as many as 100 objects simultaneously. Multiobject spectrographs automated by computers have made large surveys of many thousands of stars and galaxies possible.
modern, light,
4-CI CHAPTER INTRODUCTION Topic 4-1 In earlier chapters of this book you looked at the sky the way the first astronomers did, with the unaided eye. In the previous chapter, you got a glimpse through Galileo's telescope that revealed amazing things about the Moon, Jupiter, and Venus. Now you can consider the telescopes, instruments, and techniques of __________ astronomers. Telescopes gather and focus __________, so you need to study what light is and how it behaves on your way to understanding how telescopes work. You will learn about telescopes that capture invisible types of light such as radio waves and X-rays, enabling astronomers to put together a more complete view of celestial objects. (See CO 4.) This chapter will help you answer these five important questions: What is light? How do telescopes work? What are the powers and limitations of telescopes? What kind of instruments do astronomers use to record and analyze light gathered by telescopes? Why must some telescopes be located in space?
quantum, photon
A __________ is a unit in which waves are absorbed or emitted A ________ is a quantum of electromagnetic energy- or the basic unit of light.
false-color images, intensity, radio waves, contour maps
Astronomers also manipulate images to produce _________-_________ _________ (also known as representational-color images) in which the colors represent different levels of ___________ and are not related to the true colors of the object. For example, because humans can't see __________ _________, astronomers must convert them into something perceptible. One way is to measure the strength of the radio signal at various places in the sky and draw a map in which contours mark areas of uniform radio intensity. Compare such a map to Figure 4-12a, a seating diagram for a baseball stadium in which the contours mark areas in which the seats have the same price. ___________ ________ are very common in radio astronomy and are often reproduced using false colors (Figure 4-12b). ((a) A contour map of a baseball stadium shows regions of similar admission prices. The most expensive seats are those behind home plate. (b) A false-color radio map of Tycho's supernova remnant, the expanding shell of gas produced by the explosion of a star in 1572. The radio contour map has been color-coded to show intensity)
nature, opaque, absorbed, long, visible light, short, atmospheric windows
Astronomers are interested in electromagnetic radiation because it carries almost all available clues to the _______ of planets, stars, and other celestial objects. Earth's atmosphere is __________ (nontransparent) to most electromagnetic radiation, as shown in Figure 4-2. Gamma-rays, X-rays, and some radio waves are ___________ high in Earth's atmosphere, and a layer of ozone (O3) at an altitude of about 30 km (20 mi) absorbs almost all UV radiation. Water vapor in the lower atmosphere absorbs ________-wavelength IR radiation. Only ________ ________, some ________-wavelength infrared radiation, and some radio waves reach Earth's surface through what are called __________________ ___________. To study the sky from Earth's surface, you must look out through one of these "windows" in the electromagnetic spectrum.
matter, spectrum
Because astronomers understand how light interacts with _________, a _________ carries a tremendous amount of information, as you will see in more detail in Sections 5-4 and 5-5 of Chapter 5. A spectrograph is the astronomer's most powerful instrument. Astronomers are likely to remark, "We don't know anything about an object until we get a spectrum," and that is only a slight exaggeration.
infrared radiation, 700nm, 1mm, skin, invisible
Beyond the red end of the visible range lies ___________ (IR) _________, with wavelengths ranging from ______ ___ to about ____ ___. Your eyes are not sensitive to this radiation, but your _______ can sense some of it as heat. A heat lamp is nothing more than a bulb that gives off large amounts of infrared radiation. English astronomer William Herschel discovered infrared radiation, and thus, discovered that there is such a thing as _______________ light (Figure 4-3). (Depiction of Sir William Herschel in 1800 discovering that sunlight contains radiation detectable by thermometers but not by human eyes. He named that invisible light "infrared," meaning "below red.")
analyze, instruments
Chapter 4-4 ASTRONOMICAL INSTRUMENTS AND TECHNIQUES Just looking through a telescope doesn't tell you much. To learn about planets, stars, and galaxies, you must be able to ________ the light the telescope gathers. Special ______________ attached to the telescope make that possible.
photographic plate, photometers, charge-coupled devices, sensitive, digitized, wavelengths, negatives
The ______________ ________ was the first image-recording device used with telescopes. Brightness of objects imaged on a photographic plate can be measured with a lot of hard work, yielding only moderate precision. Astronomers also build _______________, sensitive light meters to measure the brightness of individual objects very precisely. Most modern astronomers use ________-________ ________ (CCDs) as both image-recording devices and photometers. A CCD is a specialized computer chip that can contain more than a million microscopic light detectors arranged in an array about the size of a postage stamp. CCD and other types of array detectors can be used like a small photographic plate, but they have dramatic advantages over both photometers and photographic plates. CCDs can detect both bright and faint objects in a single exposure and are much more ___________ than a photographic plate. CCD images are ___________, or converted to numerical data, and can be read directly into a computer memory for later analysis. Although CCDs for astronomy are extremely sensitive and therefore expensive, less sophisticated CCDs are now used in commercial video and digital cameras. Infrared astronomers also use array detectors similar in operation to optical CCDs. At some other _____________, photometers are still used for measuring brightness of celestial objects. The digital data representing an image from a CCD, or other array detector, are easy to manipulate to bring out details that would not otherwise be visible. For example, astronomical images are sometimes reproduced as ______________, with the sky white and the stars dark. This makes the faint parts of the image easier to see (Figure 4-11).
nanometer, Ångstrom, visible spectrum
The average wavelength of visible light is about 0.0005 mm. Two hundred light waves would fit end-to-end across the thickness of a sheet of paper. It is too awkward to measure such short distances in millimeters, so physicists and astronomers describe the wavelength of light using either the unit of _____________ (nm), one-billionth of a meter (10-9m), or the _____________ (Å), equal to 10^-10 m or 0.1 nm. The wavelength of visible light ranges between about 400 nm and 700 nm, or, equivalently, 4000 Å and 7000 Å. Some astronomers refer to wavelengths using units of microns (10-6m), millimeters, centimeters, or meters. Figure 4-2 shows how the ___________ ____________ makes up only a small part of the electromagnetic spectrum.
convenient, same, electromagnetic radiation, light, wave, particles, photon, wavelength, shorter, more, longer, less, visible, more
The distinction between these wavelength ranges is mostly arbitrary—they are simply ____________ human-invented labels. For example, the longest-wavelength IR radiation and the shortest-wavelength microwaves are the ________. Similarly, very short-wavelength UV light can be considered to be X-rays. Nonetheless, it is all ___________________ ______, and you could say we are "making light" of it all, because all these types of radiation are the same phenomenon as _______: Some types your eyes can see, some types your eyes can't see. Although light behaves as a ________, under certain conditions it also behaves as a particle. A particle of light is called a ________, and you can think of a photon as a minimum-sized bundle of electromagnetic waves. The amount of energy a photon carries depends on its ___________. ________-wavelength photons carry ________ energy, and ________-wavelength photons carry ________ energy. A photon of ___________ light carries a small amount of energy, but an X-ray photon carries much ________ energy and a radio photon carries much less. more waves= higher frequency= shorter wave length less waves= lower frequency= longer wave length <-------------(gamma)----(visual)---(radio)---------------> high frequency..........................................low frequency
observations, telescopes, natural light, faint, atmosphere
The science of astronomy is based on _____________. Astronomers cannot visit distant galaxies and far-off worlds, so they have to study them using ______________. Eleven chapters remain in your exploration, and every one will present information gained by astronomers using telescopes. __________ ___________ is a treasure that links you to the sky. The quest of astronomers is to study light from the Sun, planets, moons, asteroids, comets, stars, nebulae, and galaxies and extract information about their natures. Most celestial objects are very ______ sources of light, so large telescopes, such as the ones shown in this chapter, are built to collect the greatest amount of light possible. Some types of telescopes gather light that is invisible to the human eye, but all telescopes work by the same basic principles. Some telescopes are used on Earth's surface, but others must go high into Earth's ____________, or even above the atmosphere into space, to work properly. This chapter focuses on telescopes, instruments, and techniques used to analyze light for astronomical research. Astronomy is more than technology and scientific analysis, it helps us understand what we are. In the quotation that opens this chapter, the poet Robert Frost suggests that someone in every town should own a telescope to help us look upward and outward.
