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Unit 5: and

Definitions: Electromagnetic Spectrum regions: Amplitude Frequency Radio waves Absorption Photon Speed of light (c) Microwave Electromagnetic radiation Radio Telescope Infrared Electromagnetic spectrum Reflection Radiation Visible light Emission Refraction Wavelength Ultraviolet X-rays rays

Can you…? 1. Match the vocabulary terms provided on the unit plan to their definitions. 2. Read the assigned readings and answer the questions provided. 3. Memorize the speed of light as 3.0x10 8 m/s. 4. If given wavelength or frequency, calculate the missing variable. 5. Characterize the different areas of the electromagnetic spectrum. 6. Construct a diagram of the electromagnetic spectrum and label the regions and their wavelengths and frequencies. 7. If given wavelength or frequency, determine the radiation type. 8. Explain why the sky appears blue. 9. Use a telescope to make observations of the sky. 10. Describe how observing methods that use radiation beyond the visual range have affected astronomy and our comprehension of the universe. 11. Describe the roles of telescopes and photography in advancing our knowledge of astronomy.

Readings: • Light (ART5.1) • The Electromagnetic Spectrum (ART 5.2)

Electromagnetic Spectrum and Telescopes Questions to answer LIGHT ART 5.1

1. What was the difference between the theory of light proposed by Christian Huygens and the theory proposed by Issac Newton?

2. What evidence supported the wave theory?

3. What did James Clerk Maxwell show?

4. Visible light is a form of what type of radiation?

5. Why does the quantum theory of light successfully explain all aspects of the behavior of light?

6. How was the speed of light determined and what is its value?

7. Is the speed of light the same in all mediums? Explain.

8. What is the difference between a luminous body and an illuminated body?

9. What causes the colors of opaque objects?

The Electromagnetic Spectrum and Telescope ART 5.1: LIGHT

The Nature of Light The wave theory received additional support from the electromagnetic theory of James Clerk The scientific study of the behavior of light is Maxwell (1864), who showed that electric and called optics and covers reflection of light by magnetic fields were propagated together and that a mirror or other object, refraction by a lens or their speed was identical with the speed of light. It prism, diffraction of light as it passes by the thus became clear that visible light is a form of edge of an opaque object, and interference electromagnetic radiation, constituting only a patterns resulting from diffraction. Also small part of the electromagnetic spectrum. studied is the polarization of light. Any Maxwell's theory was confirmed experimentally successful theory of the nature of light must with the discovery of radio waves by Heinrich be able to explain these and other optical Hertz in 1886. phenomena. Modern Theory of the Nature of Light The Wave, Particle, and Electromagnetic Theories of Light With the acceptance of the electromagnetic theory of light, only two general problems remained. One The earliest scientific theories of the nature of of these was that of the luminiferous ether, a light were proposed around the end of the 17th hypothetical medium suggested as the carrier of cent. In 1690, Christian Huygens proposed a light waves, just as air or water carries sound theory that explained light as a wave waves. The ether was assumed to have some very phenomenon. However, a rival theory was unusual properties, e.g., being massless but offered by Sir Isaac Newton in 1704. Newton, having high elasticity. A number of experiments who had discovered the visible spectrum in performed to give evidence of the ether, 1666, held that light is composed of tiny notably by A. A. Michelson in 1881 and by particles, or corpuscles, emitted by luminous Michelson and E. W. Morley in 1887, failed to bodies. By combining this corpuscular theory support the ether hypothesis. With the publication with his laws of mechanics, he was able to of the special theory of relativity in 1905 by explain many optical phenomena. Albert Einstein, the ether was shown to be unnecessary to the electromagnetic theory. For more than 100 years, Newton's corpuscular theory of light was favored over The second main problem, and the more serious the wave theory, partly because of Newton's of the two, was the explanation of various great prestige and partly because not enough phenomena, such as the photoelectric effect, that experimental evidence existed to provide an involved the interaction of light with matter. adequate basis of comparison between the two Again the solution to the problem was proposed theories. Finally, important experiments were by Einstein, also in 1905. Einstein extended the done on the diffraction and interference of quantum theory of thermal radiation proposed by light by Thomas Young (1801) and A. J. Max in 1900 to cover not only vibrations Fresnel (1814–15) that could only be of the source of radiation but also vibrations of the interpreted in terms of the wave theory. The radiation itself. He thus suggested that light, and polarization of light was still another other forms of electromagnetic radiation as well, phenomenon that could only be explained by travel as tiny bundles of energy called light the wave theory. Thus, in the 19th cent. the quanta, or photons. The energy of each photon is wave theory became the dominant theory of directly proportional to its frequency. the nature of light.

5: The Electromagnetic Spectrum and Telescope Page 1 of 2 ART 5.1: LIGHT

With the development of the quantum theory relativity predicts that the speed of light in a of atomic and molecular structure by Niels vacuum is the limiting velocity for material Bohr and others, it became apparent that light particles; no particle can be accelerated from rest and other forms of electromagnetic radiation to the speed of light, although it may approach it are emitted and absorbed in connection with very closely. Particles moving at less than the energy transitions of the particles of the speed of light in a vacuum but greater than that of substance radiating or absorbing the light. In light in some other medium will emit a faint blue these processes, the quantum, or particle, light known as Cherenkov radiation when they nature of light is more important than its wave pass through the other medium. This phenomenon nature. When the transmission of light is has been used in various applications involving under consideration, however, the wave nature elementary particles. dominates over the particle nature. In 1924, Louis de Broglie showed that an analogous Luminous and Illuminated Bodies picture holds for particle behavior, with moving particles having certain wavelike In general, vision is due to the stimulation of the properties that govern their motion, so that optic nerves in the eye by light either directly there exists a complementarity between from its source or indirectly after reflection from particles and waves known as particle-wave other objects. A luminous body, such as the sun, duality. The quantum theory of light has another star, or a light bulb, is thus distinguished successfully explained all aspects of the from an illuminated body, such as the moon and behavior of light. most of the other objects one sees. The amount and type of light given off by a luminous body or The Speed of Light reflected by an illuminated body is of concern to the branch of physics known as photometry (see An important question in the history of the also lighting). Illuminated bodies not only reflect study of light has been the determination of its light but sometimes also transmit it. Transparent speed and of the relationship of this speed to objects, such as glass, air, and some liquids, allow other physical phenomena. At one time it was light to pass through them. Translucent objects, thought that light travels with infinite speed— such as tissue paper and certain types of glass, i.e., it is propagated instantaneously from its also allow light to pass through them but diffuse source to an observer. Olaus Rømer showed (scatter) it in the process, so that an observer that it was finite, however, and in 1675 cannot see a clear image of whatever lies on the estimated its value from differences in the other side of the object. Opaque objects do not time of eclipse of certain of Jupiter's satellites allow light to pass through them at all. Some when observed from different points in the transparent and translucent objects allow only earth's . More accurate measurements light of certain wavelengths to pass through them were made during the 19th cent. by A. H. L. and thus appear colored. The colors of opaque Fizeau (1849), using a toothed wheel to objects are caused by selective reflection of interrupt the light, and by J. B. L. Foucault certain wavelengths and absorption of others. (1850), using a rotating mirror. The most accurate measurements of this type were made by Michelson. Modern electronic methods have improved this accuracy, yielding a value of 2.99792458 × 108 m (c.186,000 mi) per sec for the speed of light in a vacuum, and less for its speed in other media. The theory of

5: The Electromagnetic Spectrum and Telescope Page 2 of 2 ART 5.2 Electromagnetic Spectrum

Measuring the electromagnetic spectrum

You actually know more about it than you may think! The electromagnetic (EM) spectrum is just a name that scientists give a bunch of types of radiation when they want to talk about them as a group. Radiation is energy that travels and spreads out as it goes-- visible light that comes from a lamp in your house and radio waves that come from a radio station are two types of electromagnetic radiation. Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events create higher energy radiation than cool objects. Only extremely hot objects or particles moving at very high velocities can create high-energy radiation like X-rays and gamma-rays.

Here are the different types of radiation in the EM spectrum, in order from lowest energy to highest:

Radio: Yes, this is the same kind of energy that radio stations emit into the air for your boom box to capture and turn into your favorite Mozart, Madonna, or Justin Timberlake tunes. But radio waves are also emitted by other things ... such as stars and gases in space. You may not be able to dance to what these objects emit, but you can use it to learn what they are made of.

Microwaves: They will cook your popcorn in just a few minutes! Microwaves in space are used by astronomers to learn about the structure of nearby galaxies, and our own !

Infrared: Our skin emits infrared light, which is why we can be seen in the dark by someone using night vision goggles. In space, IR light maps the dust between stars.

Visible: Yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars ... also by fast-moving particles hitting other particles.

Ultraviolet: We know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn! Stars and other "hot" objects in space emit UV radiation.

X-rays: Your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X-rays .

5: The Electromagnetic Spectrum and Telescope Page 1 of 8 ART 5.2 Electromagnetic Spectrum

Gamma-rays: Radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma- ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.

A Radio Wave is not a Gamma-Ray; a Microwave is not an X-ray ... or is it?

We may think that radio waves are completely different physical objects or events than gamma-rays. They are produced in very different ways, and we detect them in different ways. But are they really different things? The answer is 'no'. Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing. They are all electromagnetic radiation.

Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing, electromagnetic radiation.

Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma- rays.

Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each way of thinking about the EM spectrum is related to the others in a precise mathematical way. So why do we have three ways of describing things, each with a different set of physical units? After all, frequency is measured in cycles per second (which is called a Hertz), wavelength is measured in meters, and energy is measured in volts.

5: The Electromagnetic Spectrum and Telescope Page 2 of 8 ART 5.2 Electromagnetic Spectrum

The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency.

The answer is that scientists don't like to use big numbers when they don't have to. It is much easier to say or write "two kilometers or 2 km" than "two thousand meters or 2,000 m". So generally, scientists use whatever units are easiest for whatever they are working with. In radio astronomy, astronomers tend to use wavelengths or frequencies. This is because most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km (30 gigahertz (GHz) to 100 kilohertz (kHz)). The radio is a very broad part of the EM spectrum. Infrared astronomers also use wavelength to describe their part of the EM spectrum. They tend to use microns (or millionths of meters) for wavelengths, so that they can say their part of the EM spectrum falls in the range 1 to 100 microns. Optical astronomers use wavelengths as well. Scientists use both angstroms (0.00000001 cm, or 10 -8 cm in scientific notation) and nanometers (0.0000001, or 10 -7, cm). In the newer "SI" version of the metric system, we think of visible light in units of nanometers or 0.000000001 meters (10 -9 m). In this system, the violet, blue, green, yellow, orange, and red light we know so well has wavelengths between 400 and 700 nanometers. This range is only a small part of the entire EM spectrum, so you can tell that the light we see is just a little fraction of all the EM radiation around us! By the time you get to the ultraviolet, X-ray, and gamma-ray regions of the EM spectrum, lengths have become too tiny to think about any more. So scientists usually refer to these photons by their energies, which are measured in electron volts. Ultraviolet radiation falls in the range from a few electron volts (eV) to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.

5: The Electromagnetic Spectrum and Telescope Page 3 of 8 ART 5.2 Electromagnetic Spectrum

Why Do We Have to Go to Space to See All of the Electromagnetic Spectrum?

Electromagnetic radiation from space is unable to reach the surface of the Earth except at a very few wavelengths, such as the visible spectrum, radio frequencies, and some ultraviolet wavelengths. Astronomers can get above enough of the Earth's atmosphere to observe at some infrared wavelengths from mountain tops or by flying their telescopes in an aircraft. Experiments can also be taken up to altitudes as high as 35 km by balloons which can operate for months. Rocket flights can take instruments all the way above the Earth's atmosphere for just a few minutes before they fall back to Earth, but a great many important first results in astronomy and astrophysics came from just those few minutes of observations. For long-term observations, however, it is best to have your detector on an orbiting satellite ... and get above it all!

Space Observatories in Different Regions of the EM Spectrum

Radio observatories

Radio waves CAN make it through the Earth's atmosphere without significant obstacles (In fact radio telescopes can observe even on cloudy days!). However, the availability of a space radio observatory complements radio telescopes on the Earth in some important ways.

There are a number of radio observatories in space. These include Polar, Cluster II, ISEE 1, ISEE 2, GOES 9 and Voyager 1. Most of these study the ionospheres of the planets down to 3 x 10 -4 Hz. Some have been used to monitor radio signals given off by earthquakes.

One is a special technique used in radio astronomy called "interferometry". Radio astronomers can combine data from two telescopes that are very far apart and create images which have the same resolution as if they had a single telescope as big as the distance between the two telescopes! That means radio telescope arrays can see incredibly small details. One such array is called the Very Large Baseline Array (VLBA): it consists of ten radio telescopes which reach all the way from Hawaii to Puerto Rico: nearly a third of the way around the world! By putting a radio telescope in orbit around the Earth, radio

5: The Electromagnetic Spectrum and Telescope Page 4 of 8 ART 5.2 Electromagnetic Spectrum astronomers could make images as if they had a radio telescope the size of the entire planet! The Very Long Baseline Interferometry (VLBI) Space Observatory Program (VSOP) attempts to do just that. This Japanese mission was launched in February 1997, and renamed HALCA shortly thereafter.

Microwave observatories The sky is a source of microwaves in every direction, most often called the microwave background. This background is believed to be the remnant from the "Big Bang" scientists believe our Universe began with. It is believed that a very long time ago all of space was scrunched together in a very small, hot ball. The ball exploded outward and became our Universe as it expanded and cooled. Since the Big Bang, 13.7 billion years ago, it has cooled all the way to just three degrees above zero. It is this "three degrees" that we measure as the microwave background.

From 1989 to 1993 the Cosmic Background Explorer (COBE), made very precise measurements of the temperature of this microwave background. COBE mapped out the entire microwave background, carefully measuring very small differences in temperatures from one direction to another. Astronomers have many theories about the beginning of the Universe and their theories predict how the microwave background would look. The very precise measurements made by COBE eliminated a great many of the theories about the Big Bang.

The Wilkinson Microwave Anisotropy Probe (WMAP), launched in the summer of 2001, measures the temperature fluctuations of the cosmic microwave background radiation over the entire sky with even greater precision. WMAP is answering such fundamental questions as:

• What are the values of the cosmological parameters of the Big Bang theory? • How did structures of galaxies form in the Universe? • When did the first structure of galaxies form?

Infrared observatories The most recent infrared observatory currently in orbit was the Infrared Space Observatory (ISO), launched in November 1995 by the European Space Agency, and operated until May 1998. It was placed in an elliptical orbit with a 24 hour period which kept it in view of the ground stations at all times, a necessary arrangement since ISO transmitted observations as it made them rather than storing information for later playback. ISO observed from 2.5 to 240 microns.

In August 2003, NASA launched the Spitzer (formerly SIRTF, the Space Infrared Telescope Facility). Spitzer uses a passive cooling system ( i.e. it radiates away its own heat rather than requiring an active refrigerator system like most other space infrared observatories) and it was placed in an earth-trailing, heliocentric orbit, where it will not have to contend with Earth occultation of sources nor with the comparatively warm environment in near-Earth space.

Another major infrared facility coming soon will be the Stratospheric Observatory for Infrared Astronomy (SOFIA). Although SOFIA will not be an orbiting facility, it will carry a large telescope within a 747 aircraft flying at an altitude sufficient to get it well above most of the Earth's infrared absorbing atmosphere. SOFIA will be replacing the Kuiper Airborne Observatory.

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The James Webb Space Telescope (JWST) will be a large infrared telescope with a 6.5-meter primary mirror. Launch is planned for 2013. JWST will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of systems capable of supporting life on planets like Earth, to the evolution of our own Solar System. JWST was formerly known as the "Next Generation Space Telescope" (NGST).

Visible spectrum observatories The only visual observatory in orbit at the moment is the (HST). Like radio observatories in space, there are visible observatories already on the ground. However, Hubble has several special advantages over them.

HST's biggest advantage is, because it is above the Earth's atmosphere, it does not suffer distorted vision from the air. If the air was all the same temperature above a telescope and there was no (or the wind was perfectly constant), telescopes would have a perfect view through the air. Alas, this is not how our atmosphere works. There are small temperature differences, wind speed changes, pressure differences, and so on. This causes light passing through air to suffer tiny wobbles. It gets bent a little, much like light gets bent by a pair of glasses. But unlike glasses, two light beams coming from the same direction do not get bent in quite the same way. You've probably seen this before -- looking along the top of the road on a hot day, everything seems to shimmer over the black road surface. This blurs the image telescopes see, limiting their ability to resolve objects. On a good night in an observatory on a high mountain, the amount of distortion caused by the atmosphere can be very small. But the Space Telescope has NO distortion from the atmosphere and its perfect view gives it many many times better resolution than even the best ground-based telescopes on the best nights.

Another advantage of the Space Telescope is that without the atmosphere in the way, it can see more than just the visible spectrum. The Space Telescope can also see ultraviolet light which normally is absorbed by the Earth's atmosphere and cannot be seen by regular telescopes. So the Space Telescope can see a much wider portion of the spectrum.

Ultraviolet observatories Right now there are no dedicated ultraviolet observatories in orbit. The Hubble Space Telescope can perform a great deal of observing at ultraviolet wavelengths, but it has a very fairly small field of view. Until September 1996, the International Ultraviolet Explorer (IUE) was operating and observing ultraviolet radiation. Its demise, although unfortunate, was hardly premature: IUE was launched in January, 1978 with planned operations of three years. IUE functioned more or less like a regular ground based observatory save that the telescope operator and scientist did not actually visit the telescope, but sent it commands from the ground. Other than some care in the selection of materials for filters, a UV telescope like IUE is very much like a regular visible light telescope.

In addition to IUE, there have been fairly important recent UV space missions. A reusable shuttle package called Astro has been flown twice in the cargo bay of the space shuttle: it consisted of a set of three UV telescopes. Unlike HST, the Astro UV telescopes had very large fields of view and so could take images of larger objects in the sky -- like galaxies. For instance, if the Hubble Space Telescope and the Astro telescopes were used to look at the Comet Hale-Bopp, Hubble would be able to take spectacular pictures

5: The Electromagnetic Spectrum and Telescope Page 6 of 8 ART 5.2 Electromagnetic Spectrum of the core of the comet. The Astro telescopes would be able to take pictures of the entire comet, core and tail.

Extreme Ultraviolet observatories There are two extreme ultraviolet observatories in space at the moment. One of them is the very first extreme ultraviolet observatory ever, the Extreme Ultraviolet Explorer (EUVE). Astronomers have been somewhat reluctant to explore from space at the extreme ultraviolet wavelengths since all theory strongly suggests that the interstellar medium (the tiny traces of gases and dust between the stars) would absorb radiation in this portion of the spectrum. However, when the Extreme Ultraviolet Explorer (EUVE) was launched, observations showed that the solar system is located within a bubble in the local interstellar medium. The region around the Sun is relativity devoid of gas and dust which allows the EUVE instruments to see much further than theory predicted.

Another extreme ultraviolet observatory currently operating is the Array of Low Energy X-ray Imaging Sensors (ALEXIS). Although its name indicates that it is an X-ray observatory, the range of energy ALEXIS is exploring is at the very lowest end of the X-ray spectrum and often considered to be extreme ultraviolet. ALEXIS was launched on 25 Apr 1993 on a Pegasus rocket. During launch, a hinge plate supporting one of the solar panels broke. However, the satellite survived, and the panel remains connected to the satellite via the electrical cables and a tether, and it still provides the requisite power to the satellite. ALEXIS is spinning about an axis pointed approximately toward the sun. ALEXIS provides sky maps on a daily basis whenever the satellite is not in a 100% sunlight orbit. These sky maps are used to study diffuse x-ray emission, monitor the brightness of known EUV objects, and to detect transient objects.

X-ray observatories There are several X-ray observatories currently operating in space with more to be launched in the next few years.

The Rossi X-ray Timing Explorer (RXTE) was launched on December 30, 1995. RXTE is able to make very precise timing measurements of X-ray objects, particularly those which show patterns in their X-ray emissions over very short time periods, such as certain systems and pulsars.

Other X-ray observatories currently operating in space include ROSAT, a joint venture between the United States, Germany, and the United Kingdom; the Advanced Satellite for Cosmology and Astrophysics (ASCA), a joint U.S.-Japan venture; the Kvant astrophysics module attached to the Russian space station , and Beppo SAX, an Italian X-ray satellite.

NASA launched another major new X-ray astronomy satellite, the Chandra X-ray Observatory (CXO), in mid 1999.

Gamma-ray observatories The Compton Gamma-Ray Observatory (CGRO) was launched by the space shuttle in April 1991. The observatory's instruments are dedicated to observing the gamma-ray sky, including locating gamma-ray burst sources, monitoring solar flares, and other highly energetic astrophysical phenomenon. An unexpected discovery which Compton has made was the observation of gamma-ray burst events coming from the Earth itself at the top of thunderstorm systems. The cause of this phenomenon is not known, but it is currently suspected to be related to "Sprites": lightning flashes which are occasionally seen jumping upward from cloud tops to the upper stratosphere.

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The Russian gamma-ray observatory Granat has exhausted its control fuel. Its last maneuver in 1994 was to initiate a roll which allowed it to perform a continuous all-sky survey until Nov 1998.

The European mission INTEGRAL was launched in October 2002. It is studying gamma-ray bursts, and sources within our galaxy.

SWIFT is a part of the NASA medium explorer program designed with help from American universities and NASA's international partners. It launched in November 2004. SWIFT will study gamma-ray bursts, and be capable of quickly pointing narrow field X-ray and optical detectors in the direction of gamma-ray bursts detected by its large field detectors.

The next major gamma-ray mission in the near future is the Gamma-Ray Large Area Space Telescope (GLAST). GLAST will have a field of view twice as large as that of the Compton Gamma-Ray Observatory, and a sensitivity of up to 50 times greater than Compton's EGRET instrument. GLAST will study a wide range of gamma-ray objects, including pulsars, black holes, active galaxies, diffuse gamma- ray emission, and bursts.

Questions

1. List the regions of the electromagnetic spectrum in order of lowest to highest frequency

2. What type of emission maps the dust between stars?

3. Does gas in space emit radio waves?

4. What type of emission can come from radioactive materials?

5. Do the photons that make up radio waves travel at the same speed as the photons that make up visible light? Explain.

6. What are the particles of light called?

7. Why is light considered both particle and wave?

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