Name Date Class _. Please turn to the section titled The Nature of Light.

Transcription

1 Please turn to the section titled The Nature of Light. In this section, you will learn that light has both wave and particle characteristics. You will also see that visible light is just part of a wide range of wavelengths that make up the electromagnetic spectrum. Finally, you will learn how electromagnetic waves are used in communication, medicine, and other areas. Most of us see and feel light almost every moment of our lives, from the first rays of dawn to the warm glow of a campfire at night. Even people who cannot see can feel the warmth of the sun on their skin, which is an effect of infrared light. We are very familiar with light, but how much do we understand about what light really is? As it turns out, no single scientific model can simply explain all the properties of light. The two most commonly used models describe light as a wave or as a stream of particles. Light produces interference patterns like water waves. In 1801, the English scientist Thomas Young devised an experiment to test the nature of light. He passed a beam of light through two narrow openings onto a screen on the other side. Figure 10A shows what Young observed. Notice in Figure 10A that the light produced a striped pattern on the screen. Figure 10B shows a similar pattern caused by water waves interfering in a ripple tank. Young concluded that the two beams of light beyond the openings were interfering with each other. Please turn to the next page. Because Young s experiment produced interference patterns, Young concluded that light must consist of waves. The model of light as a wave is still used today to explain many of the basic properties of light and its behavior. You learned in the chapter titled Waves that light waves are transverse waves that do not require a medium to travel. Light waves are also called electromagnetic waves. That s because light waves consist of changing electric and magnetic fields. You will examine these electric and magnetic fields in more detail in other chapters. All you need to know in this chapter is that light has the properties of transverse waves. These properties help you understand the wave model of light. The wave model of light accounts for many common observations of the behavior of light. For example, light waves are reflected when they meet a mirror. Light waves also refract when they pass through a lens and diffract when they pass through a narrow opening. However, the wave model of light cannot explain some observations. In the early part of the twentieth century, physicists began to realize that the wave model of light could not account for some observations. For example, when light strikes a piece of metal, electrons may fly off the metal s surface. In some cases, however, bright red light may not knock any electrons off the plate, while dim blue light does knock electrons off the plate. Figure 11 illustrates this phenomenon. In Figure 11A, bright red light is shining on a metal plate, but no electrons are knocked off the plate. In Figure 11B, you see that dim blue light can knock electrons off the same plate. Holt Science Spectrum English Audio CD Program Script Sound and Light p.8

2 According to the wave model, very bright red light should have more energy than dim blue light. That s because the waves in bright light should have a greater amplitude than the waves in dim light. But this does not explain how the blue light can knock electrons off the plate while the red light cannot. Light can be modeled as a stream of particles. One way to explain the effects of light striking a metal plate is to assume that the energy of light is contained in small packets. A packet of blue light contains more energy than a packet of red light. The energy in the packet of blue light is enough to knock an electron off the plate. Bright red light contains many packets, but no single packet in bright red light has enough energy to knock an electron off the plate. In the particle model of light, these packets are called photons. A photon is a particle of light. A beam of light is considered to be a stream of photons. Photons are considered particles, but photons are not like ordinary particles of matter. Photons do not have mass. Instead, they are more like bundles of energy. However, unlike the energy in a wave, the energy in a photon is located in a particular place. The model of light used depends on the situation being studied. Light can be modeled as either waves or particles. So which explanation is correct? The success of any scientific theory is how well it can explain different observations. Some effects, such as the interference of light, are more easily explained with the wave model. Other effects, such as light knocking electrons off a metal plate, are better explained by the particle model. The particle model also easily explains how light can travel across empty space without a medium. Most scientists currently accept both the wave model and the particle model of light. Scientists usually use one model or the other depending on the situation that they are studying. Some scientists believe that light has a dual nature. These scientists feel that light has different characteristics depending on the situation. In many cases, using either the wave model or the particle model of light gives good results. The energy of light is proportional to frequency. Whether light is modeled as a particle or as a wave, light is also a form of energy. Each photon of light can be thought of as carrying a small quantity of energy. Figure 12 shows that the quantity of this energy is proportional to the frequency of the corresponding light wave. For example, a photon of red light carries a quantity of energy that corresponds to the frequency of waves in red light, which is 4.5 times ten-to-the-fourteenth hertz. Examine Figure 12. How much energy does this wave frequency have? Notice in Figure 12 that the photon energy of a wave in red light is 3.0 times ten-to-the-negative-nineteenth joules. A photon with twice this much energy corresponds to a wave with twice the frequency. This wave lies in the ultraviolet range of the electromagnetic spectrum. Likewise, a photon with half as much energy as a wave of red light corresponds to a wave with half the frequency. This would be a photon of infrared light. The speed of light depends on the medium through which it travels. Holt Science Spectrum English Audio CD Program Script Sound and Light p.9

3 In a vacuum, all light travels at the same speed, called c. The speed of light is very fast, 3 times ten-to-the-eighth meters per second. In fact, light is the fastest thing in the universe; nothing can travel faster than light travels in a vacuum. However, when light travels through air, water, or glass, it travels slower than it does in a vacuum. Table 2 shows the speed of light in several different mediums. Examine Table 2. Of all the mediums shown in the table, in which medium does light travel the slowest? If you said diamond, you are correct. Turn to the next page. You have probably noticed that it is easier to read near a lamp with a 100 watt bulb than near a lamp with a 60 watt bulb. That s because a 100 watt bulb is brighter than a 60 watt bulb. The quantity that measures how much light illuminates a surface is called intensity. Intensity depends on the number of photons or waves that pass through a certain area of space. Like the intensity of sound, the intensity of light decreases as the light spreads out from a light source in spherical wave fronts. Figure 13 shows a series of spheres that are centered around a source of light. As light spreads out from the source, the number of photons passing through a given area on a sphere decreases. Notice in Figure 13 that less light falls on each unit square as the distance from the light source increases. An observer farther away from the light source will therefore see the light as dimmer than will an observer who is closer to the light source. Now that we have spent some time discussing the nature of light, we are ready to take a look at the full range of the electromagnetic spectrum. Light fills the air and space around us. Our eyes can detect light waves ranging from 400 nanometers to 700 nanometers, which correspond to the spectrum of colors from violet to red. But the visible spectrum is only one small part of the entire electromagnetic spectrum. Figure 14 shows frequencies and wavelengths across the entire electromagnetic spectrum. We live in a sea of electromagnetic waves. These waves range from the sun s ultraviolet light to radio waves transmitted by television and radio stations. Examine Figure 14. Which type of light has the highest frequency and the shortest wavelength? If you said gamma rays, you are correct. The electromagnetic spectrum consists of light at all possible energies, frequencies, and wavelengths. All light is similar in some ways, but each part of the electromagnetic spectrum also has unique properties. Many modern technologies are designed to take advantage of these different properties. The invisible light that lies just beyond violet light falls into the ultraviolet, or U-V, portion of the spectrum. Ultraviolet light has waves with higher energy and shorter wavelengths than visible light. About nine percent of the energy emitted by the sun is ultraviolet light. Because of its high energy, ultraviolet light can pass through thin layers of clouds. As a result, you can get a sunburn even on overcast days. X rays and gamma rays are used in medicine. Holt Science Spectrum English Audio CD Program Script Sound and Light p.10

4 Beyond the ultraviolet part of the spectrum lie waves known as X rays. X rays have even higher energy and shorter wavelengths than ultraviolet waves. X rays have wavelengths less than ten-to-the-negative-eighth meters. Electromagnetic waves with the highest energy are gamma rays. Gamma rays have wavelengths as short as ten-to-the-negative-fourteenth meters. An X-ray image at the doctor s office is made by passing X rays through the body. Although most of the X rays pass right through the body, a few X rays are absorbed by bones and other tissues. Figure 15 shows an image that was produced by passing X rays through the body onto a photographic plate. Notice that the image in Figure 15 looks like a photographic negative. Dark areas on the image show where X rays passed through, while bright areas show denser structures, such as bones, which absorb X rays. X rays are useful tools for doctors, but X rays can also be dangerous. Both X rays and gamma rays have very high energy levels. As a result, both these waves may kill living cells or may turn them into cancer cells. On the other hand, gamma rays can actually be used to treat cancer by killing the diseased cells. Turn to the next page. Electromagnetic waves with wavelengths slightly longer than red light fall into the infrared, or I-R, portion of the spectrum. Infrared light from the sun or a heat lamp warms you. Infrared light is also used to keep food warm. You may have noticed reddish lamps positioned above food in a cafeteria line. The energy provided by the infrared light is just enough to keep the food warm without the food continuing to cook. Figure 16 shows an image that was produced with the help of infrared light. Notice in Figure 16 that an infrared camera can reveal the temperatures of different parts of an object, such as a person s head. An infrared sensor can be used to measure the heat that objects radiate. The device can then create images that show temperature variations. By detecting infrared radiation, areas of different temperature can be mapped. For example, remote sensors on weather satellites can record infrared light. These sensors can track the movement of clouds and record temperature changes in the atmosphere. Microwaves are used in cooking and communication. Waves with wavelengths in the range of centimeters are known as microwaves. Microwaves are even longer than infrared waves. The most familiar application of microwaves today is in cooking. Microwave ovens in the United States use microwaves with a frequency of 2,450 megahertz, or a wavelength of 12.2 centimeters. Microwaves are reflected by metals, but they are easily transmitted through air, glass, paper, and plastic. However, water, fat, and sugar all absorb microwaves. Microwaves travel about 3 to 5 centimeters into most foods. As microwaves penetrate into foods, the microwaves are absorbed along with their energy. The rapidly changing electric fields of the microwaves cause water and other molecules to vibrate. The energy of these vibrations is transferred as heat to other parts of the food. Microwaves are also used to carry telecommunication signals, as will be discussed in the chapter on Communication Technology. Radio waves are used in communication and radar. Holt Science Spectrum English Audio CD Program Script Sound and Light p.11

5 Waves longer than microwaves are classified as radio waves. Radio waves have wavelengths that range from tenths of a meter to millions of meters. This portion of the electromagnetic spectrum includes TV signals, AM and FM radio signals, and other radio waves. Air-traffic control towers use radar to determine the location of aircraft. Radar is a system that uses reflected radio waves to determine the location of particular objects and the distances to those objects. Antennas at a control tower emit radio waves in all directions. In some cases, the antennas emit low-frequency microwaves. When the radio signal from an air-traffic control tower reaches an airplane, a transmitter on the plane sends another signal back to the control tower. This signal gives the plane s location and elevation above the ground. Figure 17 shows a screen that displays the locations of various aircraft around an airport. At shorter range, the original radio waves sent by the antenna may reflect off the plane. The signal then goes back to a receiver at the control tower. A computer calculates the distance to the plane based on the time delay between the original signal and the reflected signal. Radar is also used by police to monitor the speed of vehicles. A radar gun fires a radar signal of a known frequency at a moving vehicle. The radar gun then measures the frequency of the reflected waves. Because the vehicle is moving, the reflected signals will have a different frequency. That s because of the Doppler effect, which was described in the chapter titled Waves. A computer chip converts the difference in frequency into a speed and shows the result on a digital display. Now please turn your attention to the key concepts that are listed in the Summary. Light can be modeled as electromagnetic waves or as a stream of particles called photons. The energy of a photon is proportional to the frequency of the corresponding light wave. The speed of light in a vacuum, or c, is 3.0 times ten-to-the-eighth meters per second. Light travels more slowly in a medium. The electromagnetic spectrum includes light at all possible values of energy, frequency, and wavelength. ********************************* Holt Science Spectrum English Audio CD Program Script Sound and Light p.12