U N I T 3 Light and Optical Systems What is light? You know it allows you to see, but do you know how? Do you know what causes the shimmering colours in the northern lights shown here? Why do we see in colour? For centuries, scientists around the world have tried to find answers to questions such as these. They developed models to try to explain their observations about light. Some early scientists thought that light might consist of particles that enter our eyes and make us see. However, when light shines on an object, the object’s mass does not increase. Therefore, light must be a form of energy, but what form? After hundreds of years of observations and experiments, scientists have not found one simple model that explains all of the characteristics of light. One helpful model is the ray model. A ray is an imaginary line that represents the direc- tion in which light energy travels. While it explains how light forms images in a mirror and shadows on the wall, the ray model cannot explain colours or the way that light behaves when it travels through a very tiny slit. The wave model explains these characteristics. In this unit, you will learn how to use these models to develop an understanding of how light works. You will also learn how we can use light technologies to assist us in everyday life. For example, an understanding of light has made it possible to develop advanced technologies such as lasers (used in eye surgery) and fibre optics (used in communications). 172 Unit Contents T O P I C 1 What Is Light? 176 T O P I C 2 Reflection 188 T O P I C 3 Refraction 200 T O P I C 4 Lenses and Vision 208 T O P I C 5 Extending Human Vision 221 T O P I C 6 The Source of Colours 228 T O P I C 7 The Wave Model of Light 237 T O P I C 8 Beyond Light 249 U N I T 3 In how many different ways have you used • What is light? light today? How often have you used a mirror or seen reflections off a shiny surface? In • What inventions use Topics 1–3, you will learn just what light is and light? how it behaves in predictable • What do these inventions ways. reveal about the nature of light? We have opened up the universe to our eyes. How? With the help of powerful telescopes, astronomers can study the birth of stars, such as the Orion Nebula shown here. In Topics 4–5, you will investigate some fantastic technologies that extend human vision in ways unimaginable even a few decades ago. 174 MHR • Light and Optical Systems What do you think is causing the bands of colour in these photographs? What exactly is colour? Do you know what a spectrum is? In Topics 6–8, you wil find out, if you don’t yet know… Unit 3 Preview • MHR 175 Read pages 260–261, “Design Your Own Investigation.” Here is a chance to display your skills in designing controlled experiments. You can start planning your investigation well in advance… Start sharing ideas with your teammates. Save all your ideas in an “Experiment Planning File.” Begin collecting the materials you will need for your experiment. Think about how you might present your experimental results in a multimedia presentation. T O P I C 1 What Is Light? In the simplest terms, light is the form of energy that you can see. The Sun is a natural light source. It is the source of the most abundant and least expensive light in the world. Fire is another natural source of light (see Figure 3.1). Light from the Sun is The Sun is a star; all stars in the universe are sources of light. Light produced by nuclear spreads out, or radiates, from the Sun and other stars, in all directions, fusion of hydrogen like the spokes of a bicycle wheel. This type of energy transfer does particles. The Sun is not require matter; it is known as radiation. Energy such as light that composed of about travels by radiation is often called radiant energy. 70 percent hydrogen. When hydrogen Less than one tenth of one millionth of a percent of the Sun’s energy particles fuse (combine), actually reaches Earth. Nonetheless, our lives are totally dependent they form another upon this energy. Plants, people, and other animals could not live with- particle called helium. out light from the Sun. Because sunlight is not always available, people During this process, an have developed light-producing technologies, or artificial lights. A light enormous amount of bulb is an example of an artificial light source. Like the Sun, light energy is released as from a bulb radiates in all directions. heat and light. The temperature inside What else can produce light? Think about what happens when you the Sun is about strike a match. Chemicals on the tip of the match react to produce heat 16 000 000˚C! and light. Once the chemical energy is used up, the match is no longer useful. Like the match, all other sources of light require energy. Flashlights use electrical energy from batteries. Light bulbs glow when you switch on electricity. The light that leaves the Sun is formed through a process called nuclear fusion. Look around the room. List all the items you can that either give off or reflect light. You will discover a great deal about how light behaves as you study this unit. Figure 3.1 Besides the Sun and the stars, flames and sparks are natural sources of light. 176 MHR • Light and Optical Systems Find Out Is Light Energetic? In the introduction, you read that light is a form 2. What change did you notice about the of energy. Scientists often tell us that energy canister after it had been exposed to a has no matter but you can tell that it exists bright light? because it can cause changes in matter. Can 3. Analyzing and Interpreting What evidence do you use this concept to show that light is a you have that light caused a change in both form of energy? steps 1 and 2 of the procedure? Materials Extensions solar-powered calculator 4. When light energy is absorbed by solar 2 identical black film canisters cells, into what form of energy does it aluminum foil change so that the calculator can use bright light source, such as a 100 W bulb the energy? Procedure 5. In your notebook, complete the following 1. Find the solar cells on a calculator. Enter sentence: “Light can be changed into some digits, then completely cover the energy forms such as …” solar cells with your finger to block the light. Observe what happens to the digits. 6. In step 2 of the procedure, what is the manipulated variable? What is the 2. Feel the inside wall of the canister. Replace responding variable? What variables the lid of the canister and place it in a should be controlled to obtain bright light, such as sunlight or light from a meaningful results? 100 W bulb. Wait a few minutes, then 7. Repeat procedure remove the lid of the canister and feel the step 2 with two canis- inside surface. ters. Before placing What Did You Find Out? the canisters in a bright light, cover one 1. What happened to the digits when you of the canisters with prevented the light from reaching the solar aluminum foil (shiny cells on the calculator? Some solar calcula- side out). What can tors have a second source of power, such you infer about the as a battery. What can you infer from your effect of the foil? observations, about the power source of the calculator you tested? Satellites use solar cells The First Basic Principle of Light to power their electronic equipment. Someday, we You have seen that light is a form of energy. This is the first basic might all use sunlight to principle of light. When light is absorbed by a surface, it can be trans- produce the electrical formed into several different forms of energy. Light can be transformed energy we need. In 1987, into thermal energy, electrical energy, or chemical energy. For example, the Sunraycer, a test car the absorption of sunlight by a black sweater causes the garment to covered with solar cell panels, drove across gain thermal energy. Solar cells change light into electricity. Trees in Australia powered only your neighbourhood absorb sunlight to make chemical energy (sugars). by energy from the Sun. What is Light? • MHR 177 Figure 3.2 Sunlight is The brightness, or intensity, of light absorbed by the pavement indicates how much energy a surface on this runway and will receive. A surface can absorb transformed into thermal energy. You can see the more energy if the brightness of the effect of heated air rising light intensifies. For instance, pave- from the pavement on a ment may feel hot to the touch on a road or a runway on a hot, sunny summer day (see Figure 3.2). sunny summer day. However, the pavement will feel only warm if the clouds block out the sunlight. In the activity below, explore further the concepts of light, intensity, and radiant energy. Find Out Reading with Intensity Light intensity is determined by how much 4. Repeat steps 1 to 3 using a 100 W bulb. energy is received on a surface. In this activity, What Did You Find Out? Analyzing and Interpreting you will observe how distance affects the intensity (brightness) of light striking an object. 1. How does increasing the distance from the bulb affect the intensity of the light striking Safety Precautions the book’s pages? 2. Describe the difference between reading the book using the 60 W bulb and reading Materials the book using the 100 W bulb. book lamp with the shade removed 3. Draw two diagrams, one showing light 60 W bulb leaving the 60 W bulb and one showing 100 W bulb light leaving the 100 W bulb. Think of a measuring tape way to represent the amount of energy striking the book at each distance you Procedure measured. You might try drawing different 1. Ask an adult to place a 60 W bulb in the numbers of lines to represent different lamp. CAUTION Turn off the electricity intensities of light. Remember that light before the bulb is changed. Remember, radiates in all directions from the bulb, just bulbs become hot when they are turned on. as it radiates from the candle flame shown in the diagram below. 2. Darken the room. Turn on the lamp and stand about 60 cm away from it while hold- ing this book. Read a sentence from the book at this distance. 3. Performing and Recording Move about 3 m from the lamp. Read a sentence from the book at this distance. Record how your observations compare with those in step 2. 178 MHR • Light and Optical Systems Sources of Light How would your life be different if the Sun and stars were the only sources of light available to you? You would probably go to bed very In your Science Log, early, especially in the winter, because there would not be much that describe several ways in you could do after dark. Without artificial sources of light, there would which plants and animals be no television, no lamps for reading, no computers. All the rooms in respond to changes in buildings would probably have windows or skylights. the intensity of light. For We are lucky to have many sources of light available to us. In earlier example, how do your times, once the Sun had set, people found their way around outside eyes react to a bright light? How do roosters with the aid of torches and lanterns. Candles and oil lamps were com- behave when the Sun monly used indoors. Imagine trying to study by the light of a candle! rises? What do you think Today, we have so much light in our cities that light pollution can birds do during an wash out our view of the skies at night. That is why many observato- eclipse of the Sun? ries, such as the one shown in Figure 3.3, are located far from urban areas. However, some communities are taking steps to conserve light energy. For example, new types of streetlights are designed to direct their light downward, so that they illuminate the ground or the street and not the sky. In addition, these lights are comparatively energy- efficient. For example, the yellow sodium vapour lights shown in Figure 3.4 on the next page are much more efficient than typical white lights. The following pages will compare different types of light sources, both natural and artificial. Figure 3.3 This photograph is a time exposure image of star trails over the dome of the Mayall telescope at the Kitt Peak National Observatory in Arizona, USA. The telescope’s high- altitude location (over 2000 m) and the clear desert skies reduce atmospheric interference to incoming light. What is Light? • MHR 179 Figure 3.4 These bright yellow lights contain sodium vapour. Electricity makes the gas glow, producing a very intense yellow light. Incandescent Sources An object can be heated to such a high temperature that it emits visible light. Such an object is called an incandescent source of light. The emission of visible light by a hot object is called incandescence. Both candle flames and light bulbs are examples of incandescent sources. In the light bulbs used most commonly in our homes, electricity heats a metal wire filament in the bulb (see Figure 3.5). This filament becomes so hot that it glows white. The change from electrical energy to visible light energy involves the following energy transformation: Electrical energy Thermal energy Visible light energy Have you ever touched an incan- descent bulb right after you turned off the light? If so, you probably burned your fingers! About filament 95 percent of the energy given off by incandescent light bulbs is released as heat. In a way, an incan- descent source of light is like having a small electric heater in the room. The filament in an incan- base descent light bulb is usually made of the element tungsten. Figure 3.5 An incandescent light bulb 180 MHR • Light and Optical Systems Fluorescent Sources You may have noticed that when you stand under a so-called “black Ultraviolet light can cause light,” some of your clothing glows, especially white socks! In this eye damage. Never stare process, high-energy, invisible ultraviolet light is absorbed by the directly at an ultraviolet particles in the fabric. (You will learn more about ultraviolet light in light source. Topic 8.) These particles then emit some of this energy as light that you can see, making the clothing glow. This glow is called fluorescence. You can summarize this energy transformation as follows: Ultraviolet light Energy absorbed Visible light energy by particles energy A fluorescent source of light makes use of this energy transformation process. Figure 3.6 shows the typical parts of a fluorescent tube. An electric current from the lead-in wires and electrodes causes the mer- cury vapour inside the tube to give off ultraviolet radiation. A phosphor coating on the inside of the tube absorbs the ultraviolet energy. This causes the coating to glow, thus producing light that you can see. The energy pathway for a fluorescent source is summarized as follows: Electrical energy Energy absorbed by mercury particles ultraviolet light energy Energy absorbed by phosphor particles Visible light energy lead-in wire electrode mercury vapour pin pin tube mercury phosphor coating Figure 3.6 A fluorescent tube Fluorescent tubes use a device called a ballast Fluorescent tubes have a few disadvantages compared to incandescent resistor. One function of light bulbs. They are much more expensive to manufacture and more this device is to limit the amount of electricity difficult to dispose of than incandescent bulbs. Also, both the phosphor flowing to the tube. After coating and the mercury vapour of fluorescent tubes are toxic. a few years of use, parts However, if you compare the energy pathways for the fluorescent of this device can tube and the incandescent light bulb, you will notice a definite advan- become loose and begin tage for fluorescent sources. Thermal energy is not as much involved to vibrate. These vibrat- in the operation of a fluorescent light source. You can even touch the ing parts cause the annoying hum that you tubes when they are lit. As a result, fluorescent lighting wastes much sometimes hear in your less energy as heat than incandescent lighting. In other words, classroom. fluorescent lighting is more energy-efficient. What is Light? • MHR 181