Question

UNIT 2 THE ELECTROMAGNETIC SPECTRUM

Answer 1

Introduction 

Contrary to popular belief, outer space is not empty space. It is filled with electromagnetic radiation that crisscrosses the universe. This radiation comprises the spectrum of energy ranging from radio waves on one end to gamma rays on the other. It is called the electromagnetic spectrum because this radiation is associated with electric and magnetic fields that transfer energy as they travel through space. Because humans can see it, the most familiar part of the electromagnetic spectrum is visible light—red, orange, yellow, green, blue, and violet.

Like expanding ripples in a pond after a pebble has been tossed in, electromagnetic radiation travels across space in the form of waves. These waves travel at the speed of light—300,000 kilometers per second. Their wavelengths, the distance from wave crest to wave crest, vary from thousands of kilometers across (in the case of the longest radio waves) to fractions of a nanometer, in the cases of the smallest x-rays and gamma rays. Electromagnetic radiation has properties of both waves and particles. What we detect depends on the method we use to study it. The beautiful colors that appear in a soap film or in the dispersion of light from a diamond are best described as waves. The light that strikes a solar cell to produce an electric current is best described as a particle. When described as particles, individual packets of electromagnetic energy are called photons. The amount of energy a photon of light contains depends upon its wavelength. Electromagnetic radiation with long wavelengths contains little energy. Electro-magnetic radiation with short wavelengths contains a great amount of energy. Scientists name the different regions of the electromagnetic spectrum according to their wavelengths. (See figure 1.) Radio waves have the longest wavelengths, ranging from a few centimeters from crest to crest to thousands of kilometers. Micro-waves range from a few centimeters to about 0.1 cm. Infrared radiation falls between 700 nanometers and 0.1 cm. (Nano means one billionth. Thus 700 nanometers is a distance equal to 700 billionths or 7 x 10-7 meter.) Visible light is a very narrow band of radiation ranging from 400 to 700 nanometers. For comparison, it would take 50 visible light waves arranged end to end to span the thickness of a sheet of household plastic wrap. Below visible light is the slightly broader band of ultraviolet light that lies between 10 and 300 nanometers. X-rays follow ultraviolet light and diminish into the hundred-billionth of a meter range. Gamma rays fall in the trillionth of a meter range. The wavelengths of x-rays and gamma rays are so tiny that scientists use another unit, the electron volt, to describe them. This is the energy that an electron gains when it falls through a potential difference, or voltage, of one volt. It works out that one electron volt has a wavelength of about 0.0001 centimeters. X-rays range from 100 electron volts (100 eV) to thousands of electron volts. Gamma rays range from thousands of electron volts to billions of electron volts.

These two views of the constellation Orion dramatically illustrate the difference between what we are able to detect in visible light from Earth’s surface and what is detectable in infrared light to a spacecraft in Earth orbit. Photo Credits: Akira Fujii—visible light image; Infrared Astronomical Satellite— infrared image.

Answer 2

Using the Electromagnetic Spectrum

 All objects in space are very distant and difficult for humans to visit. Only the Moon has been visited so far. Instead of visiting stars and planets, astronomers collect electromagnetic radiation from them using a variety of tools. Radio dishes capture radio signals from space. Big telescopes on Earth gather visible and infrared light. Interplanetary spacecraft have traveled to all the planets in our solar system except Pluto and have landed on two. No spacecraft has ever brought back planetary material for study. They send back all their information by radio waves. Virtually everything astronomers have learned about the universe beyond Earth depends on the information contained in the electromagnetic radiation that has traveled to Earth. For example, when a star explodes as in a supernova, it emits energy in all wavelengths of the electromagnetic spectrum. The most famous supernova is the stellar explosion that became visible in 1054 and produced the Crab Nebula. Electromagnetic 

radiation from radio to gamma rays has been detected from this object, and each section of the spectrum tells a different piece of the story. For most of history, humans used only visible light to explore the skies. With basic tools and the human eye, we developed sophisticated methods of time keeping and calendars. Telescopes were invented in the 17th century. Astronomers then mapped the sky in greater detail––still with visible light. They learned about the temperature, constituents, distribution, and the motions of stars. In the 20th century, scientists began to explore the other regions of the spectrum. Each region provided new evidence about the universe. Radio waves tell scientists about many things: the distribution of gases in our Milky Way Galaxy, the power in the great jets of material spewing from the centers of some other galaxies, and details about magnetic fields in space. The first radio astronomers unexpectedly found cool hydrogen gas distributed throughout the Milky Way. Hydrogen atoms are the building blocks for all matter. The remnant radiation from the Big Bang, the beginning of the universe, shows up in the microwave spectrum. Infrared studies (also radio studies) tell us about molecules in space. For example, an infrared search reveals huge clouds of formaldehyde in space, each more than a million times more massive than the Sun. Some ultraviolet light comes from powerful galaxies very far away. Astronomers have yet to understand the highly energetic engines in the centers of these strange objects. Ultraviolet light studies have mapped the hot gas near our Sun (within about 50 light years). The high energy end of the spectrum—x-rays and gamma rays—provide scientists with information about processes they cannot reproduce here on Earth because they lack the required power. Nuclear physicists use strange stars and galaxies as a laboratory. These objects are pulsars, neutron stars, black holes, and active galaxies. Their study helps scientists better understand the behavior of matter at extremely high densities and temperatures in the presence of intense electric and magnetic fields. Each region of the electromagnetic spectrum provides a piece of the puzzle. Using more than one region of the electromagnetic spectrum at a time gives scientists a more complete picture. For example, relatively cool objects, such as star-forming clouds of gas and dust, show up best in the radio and infrared spectral region. Hotter objects, such as stars, emit most of their energy at visible and ultraviolet wavelengths. The most energetic objects, such as supernova explosions, radiate intensely in the x-ray and gamma ray regions. There are two main techniques for analyzing starlight. One is called spectroscopy and the other photometry. Spectroscopy spreads out the different wavelengths of light into a spectrum for study. Photometry measures the quantity of light in specific wavelengths or by combining all wavelengths. Astronomers use many filters in their work. Filters help astronomers analyze particular components of the spectrum. For example, a red filter blocks out all visible light wavelengths except those that fall around 600 nanometers (it lets through red light). Unfortunately for astronomical research, Earth’s atmosphere acts as a filter to block most wavelengths in the electromagnetic spectrum. (See Unit 1.) Only small portions of the spectrum actually reach the surface. (See figure 2.) More pieces of the puzzle are gathered by putting observatories at high altitudes (on mountain tops) where the air is thin and dry, and by flying instruments on planes and balloons. By far the best viewing location is outer space.

Answer 3

Unit Goals 

• To investigate the visible light spectrum. • To demonstrate the relationship between energy and wavelength in the electromagnetic spectrum.

Teaching Strategy

Because of the complex apparatus required to study some of the wavelengths of the electromagnetic spectrum and the danger of some of the radiation, only the visible light spectrum will be studied in the activities that follow. Several different methods for displaying the visible spectrum will be presented. Some of the demonstrations will involve sunlight, but a flood or spotlight may be substituted. For best results, these activities should be conducted in a room where there is good control of light. 

ACTIVITY: Simple Spectroscope

Description: A basic hand-held spectroscope is made from a diffraction grating and a paper tube. 

Objective: To construct a simple spectroscope with a diffraction grating and use it to analyze the colors emitted by various light sources.

National Education Standards:

Mathematics

Measurement

Connections

Science

Systems, order, & organization

Change, constancy, & measurement

Abilities necessary to do scientific inquiry

Abilities of technological design

Technology

Understand engineering design

Materials:

Diffraction grating, 2-cm square (See management and tips section.)

Paper tube (tube from toilet paper roll)

Poster board square (5 by 10-cm)

Masking tape

Scissors

Razor blade knife

2 single-edge razor blades

Spectrum tubes and power supply (See

management and tips section.)

Pencil

Procedure: 1. Using the pencil, trace around the end of the paper tube on the poster board. Make two circles and cut them out. The circles should be just larger than the tube’s opening. 2. Cut a 2-centimeter square hole in the center of one circle. Tape the diffraction grating square over the hole. If students are making their own spectroscopes, it may be better if an adult cuts the squares and the slot in step 4 below. 3. Tape the circle with the grating inward to one end of the tube. 4. Make a slot cutter tool by taping two single-edge razor blades together with a piece of poster board between. Use the tool to make parallel cuts about 2 centimeters long across the middle of the second circle. Use the razor blade knife to cut across the ends of the cuts to form a narrow slot across the middle of the circle. 5. Place the circle with the slot against the other end of the tube. While holding it in place, observe a light source such as a fluorescent tube. Be sure to look through the grating end of the spectroscope. The spectrum will appear off to the side from the slot. Rotate the circle with the slot until the spectrum is as wide as possible. Tape the circle to the end of the tube in this position. The spectroscope is complete.

6. Examine various light sources with the spectroscope. If possible, examine nighttime street lighting. Use particular caution when examining sunlight. Do not look directly into the Sun.

Background: Simple spectroscopes, like the one described here, are easy to make and offer users a quick look at the color components of visible light. Different light sources (incandescent, fluorescent, etc.) may look the same to the naked eye but will appear differently in the spectroscope. The colors are arranged in the same order but some may be missing and their intensity will vary. The appearance of the spectrum displayed is distinctive and can tell the observer what the light source is.

Management and Tips: The analytical spectroscope activity that follows adds a measurement scale to the spectroscope design. The scale enables the user to actually measure the colors displayed. As will be described in greater detail in that activity, the specific location of the colors are like fingerprints when it comes to identifying the composition of the light source. Refer to the background and management tips section for the Analytical Spectroscope activity for information on how diffraction gratings produce spectra. 

Spectroscopes can be made with glass prisms but prisms are heavy. Diffraction grating spectroscopes can do the same job but are much lighter. A diffraction grating can spread out the spectrum more than a prism can. This ability is called dispersion. Because gratings are smaller and lighter, they are well suited for spacecraft where size and weight are important considerations. Most research telescopes have some kind of grating spectrograph attached. Spectrographs are spectroscopes that provide a record, photographic or digital, of the spectrum observed. Many school science supply houses sell diffraction grating material in sheets or rolls. One sheet is usually enough for every student in a class to have a piece of grating to build his or her own spectroscope. Holographic diffraction gratings work best for this activity. Refer to the note on the source for holographic grating in the next activity. A variety of light sources can be used for this activity, including fluorescent and incandescent lights and spectra tubes with power supplies. Spectra tubes and the power supplies are available from school science supply catalogs. It may be possible to borrow tubes and supplies from another school if your school does not have them. The advantage of spectrum tubes is that they provide spectra from different gases such as hydrogen and helium. When using the spectroscope to observe sunlight, students should look at reflected sunlight such as light bouncing off clouds or light colored concrete. Other light sources include streetlights (mercury, low-pressure sodium, and high-pressure sodium), neon signs, and candle flames.

Assessment: Compare student drawn spectra from different light sources. 

Extensions: • How do astronomers measure the spectra of objects in space? What do those spectra tell us about these objects? • Investigate other applications for the electromagnetic spectrum. 

Answer 4

ACTIVITY: Projecting Visible Spectra

Description: Two methods for projecting the visible spectrum are explained. 

Objective: To study the range of colors in the visible spectrum.

National Education Standards:

Mathematics

Measurement

Connections

Science

Change, constancy, & measurement

Abilities necessary to do scientific inquiry

Materials:

Method 1

Flashlight (focusing kind)

Stiff poster board

2 single-edge razor blades

tape

Glass prism

Projection screen

Method 2

Overhead projector

Holographic diffraction grating (See next

page for sources.)

2 sheets of opaque paper

Tape

Projection screen

Procedure: Method 1 

1. Make a partition with a narrow slot in its center to block all but a narrow beam from the flashlight. Cut out a 4 by 1-centimeter vertical rectangle out from a 10 by 10-centimeter piece of poster board. Tape the two single-edge razor blades to the poster board so that their edges face each other and there is a 1- to 2-millimeter gap between them. 2. Darken the classroom (the darker the better). 3. Brace the partition so that it stands upright with the gap in the vertical direction. 4. Aim the flashlight beam at the screen and focus it into a tight beam. Direct the beam of the flashlight directly through the gap in the partition so that a narrow vertical slot of light falls on the screen. 5. Stand the glass prism upright and place it in the narrow beam of light on the opposite side of the partition. 6. Slowly rotate the prism until the narrow slot of light disperses the visible spectrum. Depending upon the exact alignment, the spectrum may fall on a wall rather than on the screen. Adjust the setup so that the spectrum is displayed on the projection screen.

Procedure: Method 2 

1. For this method, you must obtain a piece of holographic diffraction grating—a grating produced by accurate holographic techniques. See page 33 for the source of the grating. Note: Method 2 will not work well with a standard transmission grating. 2. Place two pieces of opaque paper on the stage of an overhead projector so that they are almost touching. There should be a narrow gap between them that lets light through. Aim the projector so that a narrow vertical beam of light falls on the projection screen. 3. Hang a square of holographic grating over the projector lens with tape. 4. Darken the classroom (the darker the better). 5. Look for the color produced by the grating. It will fall on the screen or the wall on both sides of the center line of the projector. You may have to adjust the aiming of the projector to have one of the two spectra produced fall on the screen. 6. If the spectra produced is a narrow line of color, rotate the holographic film 90 degrees and remount it to the projector lens so that a broad band of color is projected.

Background: Visible light, passing through a prism at a suitable angle, is dispersed into its component colors. This happens because of refraction. When visible light waves cross an interface between two media of different densities (such as from air into glass) at an angle other than 90 degrees, the light waves are bent (refracted). Different wavelengths of visible light are bent different amounts and this causes them to be dispersed into a continuum of colors. (See diagram.) Diffraction gratings also disperse light. There are two main kinds of gratings. One transmits light directly. The other is a mirror-like reflection grating. In either case, diffraction gratings have thousands of tiny lines cut into their surfaces. In both kinds of gratings, the visible colors are created by constructive and destructive interference. Additional information on how diffraction gratings work is found in the Analytical Spectroscope activity and in many physics and physical science textbooks.

Management and Tips: When projecting spectra, be sure to darken the room as much as possible. If it is not possible to darken the room, a large cardboard box can be used as a light shield for method 1. Cut a small peep-hole to examine the spectra. Method 2 produces a much larger spectra than method 1. In both cases, the size of the spectral display can be enlarged by increasing the distance from the prism or diffraction grating to the screen. The disadvantage of enlarging the display is that only so much light is available from the light source and increasing its dispersion diminishes it intensity. A better light source for method 1 is the Sun. If you have a window with direct sunlight, you can block most of the light except for a narrow beam that you direct through the gap in the partition. You will probably have to place the partition with the slot on its side to display a visible spectra. A slide projector can also be used as a light source for method 1. Refer to the Analytical Spectroscope activity for more information on how the diffraction grating works.

Assessment: Have students use crayons or marker pens to sketch the visible spectrum produced. Ask students to identify each color present and to measure the widths of each color band. Have them determine which colors bend more and which bend less as they come through the prism or diffraction grating.

Extensions: • Who discovered the visible spectrum? How many colors did the scientist see? • A compact disk acts like a reflection diffraction grating. Darken the room and shine a strong beam of white light from a flashlight on the disk. The beam will be dispersed by the grating and be visible on a wall. • Construct a water prism out of four sheets of glass. Glue the sheets together as shown in the illustration with clear silicone aquarium cement. When the cement is dry, fill the Vshaped trough with water and check for leaks. Set the finished water prism in a window with direct sunlight. A visible spectrum will appear somewhere in the classroom. You can reposition the visible spectrum by bouncing the sunlight off a mirror before it enters the prism in order to change the sunlight angle. • A pocket mirror placed in a shallow pan of water can also project a spectrum. Set up the mirror and pan as shown in the illustration. 

Sources:

Diffraction gratings are available from most

school science catalogs. Holographic diffraction

grating are available from:

Learning Technologies, Inc.

40 Cameron Avenue

Somerville, MA 02144

Phone: 1-800-537-8703

Reference:

Sadler, P. “Projecting Spectra for Classroom

Investigations,” The Physics Teacher, October

1991, 29(7), pp423–427.