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Electromagnetic Spectrum Life on Earth is highly dependent on light. Plants need light for the energy to perform photosynthesis, and animals need plants for oxygen and food. Animals need light to see their way around. Have you ever stopped to think about what light actually is? Where does it come from? How does it move from place to place? The Electromagnetic Spectrum The light we see on Earth is a type of electromagnetic radiation. (Radiation refers to energy that spreads out as it travels.) Electromagnetic radiation is energy made up of special particles called photons. This energy travels faster than anything else known in the universe: approximately 300,000,000 meters per second (mps). There are many different types of electromagnetic radiation, but all types travel as waves. Scientists use wavelength to classify electromagnetic waves. Wavelength is a measurement of the length of a single wave of energy. As this diagram shows, wavelength is measured from the trough (or very bottom) of one wave to the trough of the next wave. All the different wavelengths of electromagnetic radiation make up the electromagnetic spectrum. The electromagnetic spectrum includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-Rays, and gamma rays. The electromagnetic spectrum is usually organized with the different types of radiation arranged from longest wavelength to shortest. In this model of the electromagnetic spectrum, longer wavelengths (such as radio waves and microwaves) are on the left and shorter wavelengths (such as X-Rays and gamma rays) are on the right. Regardless of their wavelengths, all types of electromagnetic radiation travel at about the same speed of 300,000,000 mps. In addition to wave speed and wavelength, electromagnetic waves can be measured by frequency. Frequency refers to the number of waves that pass a given point in a given period of time (usually one second). Electromagnetic waves with longer wavelengths have lower frequencies: in other words, fewer waves pass a given point each second. Electromagnetic waves with shorter wavelengths have higher frequencies, which means more waves pass a given point each second. 1 Electromagnetic Spectrum Electromagnetic radiation is made of particles called photons. However, it is important to remember that photons are a special type of particle. They are not like other particles you are familiar with. Unlike protons and electrons, photons have no mass. How the energy of the electromagnetic spectrum behaves as both particles and waves simultaneously is one of the great mysteries of science. Visible Light and Color Visible light is the only part of the electromagnetic spectrum humans can see. This model of the electromagnetic spectrum shows the different colors of visible light. Red light waves, which have the longest wavelengths and lowest frequencies, are on the left. Violet light waves, which have the shortest wavelengths and highest frequencies, are on the right. Visible light is radiated as white light, which is made up of different wavelengths of radiation. Visible light ranges in wavelength from about 400– 700 nanometers (nm). Each of these different wavelengths corresponds to a different color. We see the longest wavelengths of visible light as red light. We see the shortest wavelengths as violet light. A prism separates white light into different colors. White light can be separated into different colors using a special lens called a prism. When sunlight shines through raindrops, the raindrops can act as a prism and create a rainbow. 2 Electromagnetic Spectrum Studying the Universe Using Visible Light Most of our knowledge about the universe comes from observations of visible light. Sometimes we can look directly at an object with our eyes. Sometimes we must use telescopes to aid our eyes. At other times, a more complex analysis is required. For example, scientists have discovered the composition of many objects in the universe— including distant stars—simply by analyzing the visible light these objects emit or reflect. Scientists have done experiments to determine the colors given off by different elements when they are burned. By studying the colors given off by a star, scientists can determine which elements make up that star. This is spectroscopy, the study of the electromagnetic spectrum to determine an object’s composition. When scientists look at stars and other distant objects in space, they are actually looking back in time. Electromagnetic radiation travels faster than anything else in the known universe. However, it still takes some time for the radiation to travel across the vast distances of space. Because of this, scientists measure these vast distances in space using a unit called the light-year. One lightyear equals the distance light travels in one year: about 9.5 trillion kilometers. The closest star to Earth other than the Sun is called Proxima Centauri. Proxima Centauri is located about 4.2 lightyears from Earth. In other words, light emitted by Proxima Centauri takes 4.2 years to reach Earth. When scientists look at Proxima Centauri, they are not observing the star as it is now, but rather as it appeared 4.2 years ago. This means scientists can observe light from across the universe that was emitted millions or even billions of years ago. They could even look at a star that no longer exists! The Doppler Effect Visible light has provided scientists with much information about the objects in the universe. Scientists have also studied visible light to understand the nature of the universe itself. A scientist named Edwin Hubble used visible light to provide evidence that the universe is expanding. He did this through his observations of the Doppler effect. When an object moves toward you, waves from the object appear compressed. When an object moves away from you, waves appear stretched. This is the Doppler effect. The Doppler effect describes how wavelengths of energy shift as objects move. If an object emitting electromagnetic or sound waves is moving toward you, the waves become compressed. In other words, the wavelengths appear to become smaller. If an object emitting electromagnetic or sound waves is moving away from you, the waves become stretched. In other words, the wavelengths appear to become larger. 3 Electromagnetic Spectrum The Doppler effect causes sounds to increase or decrease in pitch. It also affects visible light waves. Recall that we see shorter wavelengths as blue light and longer wavelengths as red light. When a star is moving toward Earth, the light waves emitted by the star become compressed. The shorter wavelengths cause the star’s light to appear blue. This phenomenon is called blueshift. When a star is moving away from Earth, the light waves emitted by the star become stretched. The longer wavelengths cause the star’s light to appear red. This phenomenon is called redshift. Edwin Hubble noticed that all the galaxies we can see from Earth experience redshift. Therefore, they must be moving away from Earth. If all of the objects in the universe are moving away from each other, the universe must be expanding. As the object moves away from the man and toward the woman, it gives off light waves (numbered 1–4 in this diagram). Each observer experiences these light waves differently. Not only is the universe expanding, it appears to be expanding at an increasing rate. Hubble noticed that the farther an object is from Earth, the more dramatic the redshift appears. In other words, these objects are moving even faster. By measuring the intensity of a star’s redshift, scientists can determine the speed at which a star is moving and use this to calculate its distance from Earth. Humans cannot directly perceive any part of the electromagnetic spectrum other than visible light. Most of what we know about the universe and the objects in it comes from the study of visible light. Do you think scientists can use the rest of the electromagnetic spectrum to study the universe? If so, how do you think this is possible? 4 Electromagnetic Spectrum Using High-Energy Electromagnetic Radiation to Study the Universe When objects burn, they give off visible light. Hotter, more energetic objects and events in the universe give off high-energy radiation with wavelengths that are shorter than those of the visible light spectrum. These include ultraviolet radiation, X-Rays, and gamma rays. Scientists can use special instruments, called remote sensing instruments, to detect this radiation and study the objects emitting it. For example, ultraviolet radiation can be used to study very hot areas of the universe, such as the centers of some galaxies where many stars are clustered together. X-Rays are given off by extremely hot gases, so they can be used to study events that are even hotter and more energetic than burning stars. These include supernova explosions caused by the deaths of old stars. After some supernova explosions, the extremely dense, hot core of the dead star remains as a neutron star. Neutron stars emit X-Rays that scientists can detect and study. After other supernova explosions, a black hole is created. Black holes cannot be studied using visible light. This is because black holes are so dense, not even light can escape their gravitational pull. However, scientists can use X-Rays emitted by a supernova explosion to determine where black holes are likely to be created or located. This image shows the remains of a supernova explosion. The lighter areas are X-Rays detected by a remote sensing telescope. Gamma rays have the shortest wavelengths, highest frequencies, and highest energies of the entire electromagnetic spectrum. Bursts of energy from the Sun, called solar flares, generate gamma rays, as do other high- energy events in the universe. High-energy processes on Earth, such as nuclear power generation, can also emit gamma radiation. Using Low-Energy Electromagnetic Radiation to Study the Universe Scientists can also use the low-energy parts of the electromagnetic spectrum to study the universe. This low-energy radiation—including radio waves, microwaves, and infrared radiation— has lower frequencies and longer wavelengths. Special telescopes can pick up these waves from across the universe, allowing scientists to discover distant objects that are impossible to detect through higher-energy waves. Some of these objects are hidden by dust in space. Others are too cool to emit higher-energy waves. 5 Electromagnetic Spectrum Because all objects give off infrared radiation in the form of heat, infrared waves are particularly useful for detecting cooler objects. Special infrared cameras can even be used to detect objects on Earth’s surface. Low-energy waves have even been used to understand the origin of the entire universe. The most widely accepted theory to explain the origin of the universe is the Big Bang theory. The Big Bang theory states that everything in the universe exploded and expanded out of a single point during a single event about 13.7 billion years ago. This massive explosion of energy and matter must have given off electromagnetic radiation. Special cameras reveal the infrared radiation emitted by things, including people! Scientists have been able to detect some of this radiation left over from the Big Bang. This is called cosmic microwave background (CMB) radiation. CMB radiation can be detected everywhere in the universe, using a special instrument called the Wilkinson Microwave Anisotropy Probe (WMAP). The image produced by WMAP shows CMB radiation. Slight differences in temperature, represented by different colors, indicate areas of the universe with more or less matter. Everyday Life: Uses of the Electromagnetic Spectrum You need visible light to see, but did you know you use other parts of the electromagnetic spectrum in your everyday life? Every time you use a cell phone or listen to the radio, you are using radio waves. Radio waves can travel through the air to transmit information, such as music or a conversation. A microwave oven gets its name because it uses microwave radiation to energize the atoms that make up your food. This energy heats up cold food. Doctors and dentists use X-Rays to examine your teeth and bones. X-Rays can pass through your skin and muscles to create images of your bones and teeth. Sometimes electromagnetic radiation can hurt you. When you get a sunburn, it is because of ultraviolet (UV) radiation given off by the Sun. The UV rays can destroy skin cells, causing painful blisters and, in some cases, cancers. 6 Electromagnetic Spectrum What do you know? The electromagnetic spectrum is made up of seven types of radiation of varying wavelength. In the diagram below, place each type of radiation in order from longest wavelength to shortest. Then, decide which characteristic in the following box describes each type of radiation. Write each characteristic beside the appropriate type of electromagnetic radiation. Characteristics of Electromagnetic Radiation • Can be used to transmit information • Can be used to study supernovae, across distances neutron stars, and black holes • The only part of the electromagnetic spectrum humans can see • Emitted by all objects in the form of heat energy • Left over from the Big Bang throughout the universe • Can result in a sunburn • Can be emitted by nuclear power generation 7 Electromagnetic Spectrum Exploring the Doppler Effect To help your child learn more about the electromagnetic spectrum, try experimenting with the Doppler effect together. Sound waves are not a part of the electromagnetic spectrum. However, they are shifted by the Doppler effect the same way that electromagnetic waves are. The unaided human eye is not capable of directly observing the Doppler shift of light on Earth, but the unaided ear can easily detect the Doppler shift of sound waves. A sound’s pitch is related to the frequency of the waves that produce that sound. Sound waves with higher frequencies produce sounds with higher pitches, while sound waves with lower frequencies produce sounds with lower, deeper pitches. The Doppler effect creates an apparent shift in the frequencies of sound waves as the source of the sound moves toward or away from an observer. This is easy to demonstrate at home. First, experiment with different sounds and discuss their frequencies with your child. Try playing notes on a piano or listening to some music together. Listen to the different notes, and ask your child to pick out the sounds that are produced by higher- frequency waves and lower-frequency waves. Then, experiment with the Doppler effect by choosing an object that creates a repetitive sound that has only one pitch. This can be an alarm clock, a beeping timer, a cell phone with a repetitive ring tone, or a portable stereo playing a recording of a repetitive sound. Have your child stand in one spot in a wide, open area, preferably outside. Hold the object producing the sound, and stand several feet away from your child. Tell your child to listen carefully to the sound, paying careful attention to the pitch. Then, quickly run back and forth past your child a few times. Instruct your child to pay attention to the way the pitch changes as you run back and forth. Discuss the phenomenon of the changing pitch as the sound moves closer and farther away. Here are some questions to discuss with your child: • How does the pitch of this sound relate to the frequency of the sound waves? • How does the pitch of the sound change as I move toward you? • How does the pitch of the sound change as I move away from you? • How can you explain this changing pitch? • Sound waves are not a part of the electromagnetic spectrum. How does this experiment relate to what you have learned about the electromagnetic spectrum and the way it is used to study the universe? 8