i s t 1 0 _ c h 1 0 . q x d 7 / 2 2 / 0 9 3 : 5 0 P M P a g e 3 7 4 U N I T Light and Geometric Optics D Light travels through colourful stained glass windows. 374 ist10_ch10.qxd 7/22/09 3:50 PM Page 375 Contents Light is part of the electromagnetic spectrum 10 and travels in waves. 10.1 Light and the Electromagnetic Spectrum 10.2 Producing Visible Light DI 10.3 The Ray Model of Light Ray diagrams model the behaviour of light in 11 mirrors and lenses. 11.1 Mirrors DI 11.2 The Refraction of Light 11.3 Lenses Optical devices help us see farther and more 12 clearly than we can with unaided eyes. 12.1 Human Perception of Light 12.2 Technologies That Use Light DI Unit Task In this unit, you will learn about light, mirrors, and lenses and many devices that use or produce light. For your unit task, you will examine the design of streetlights. The function of streetlights is to illuminate the ground, but they waste light by also illuminating the sky. You will design a shade to reduce the light pollution streetlights cause. Essential Question How can we effectively use the properties of light in technological devices and procedures to enhance society? 375 ist10_ch10.qxd 7/22/09 3:50 PM Page 376 Exploring These doctors are examining a woman’s stomach using a camera on the end of a cable that has been fed down her throat. The screen on the upper left shows the inside of her stomach. Camera on a Pill “Say ahhhhhhhhh...” This is an instruction your doctor might give as she peers into your throat. But no matter how wide you open your mouth, your doctor cannot see very far. To look for problems farther down the digestive tract, doctors use a camera and a tiny light on the end of a flexible cable to take pictures of the inside of your stomach and slightly beyond, as shown in the photograph above. However, the next 8 m of your digestive tract, called the small intestine, can be more difficult to reach. The small intestine is fragile and narrow, with many The pill camera can twists and turns. No camera on a cable can safely reach that far. This is take over 800 000 where the “pill camera” comes in. photographs during The wireless capsule endoscope is a device carrying a miniature an 8-hour trip digital camera that can be swallowed like a pill and will pass through through the your entire digestive system. The pill camera can take over digestive system. 800 000 photographs during an 8-hour trip through the digestive system. The photographs are taken with flash photography, using tiny LED lights on the pill. A computer puts the photographs together like pieces of a jigsaw puzzle into a single image. These images can provide much more information than an X-ray. 376 UNIT D Light and Geometric Optics ist10_ch10.qxd 7/22/09 3:50 PM Page 377 Miniature Technology The most recent capsule endoscopes have the tiny digital camera facing out from the side of the capsule. This gives a good view of the wall of the small intestine. However, the camera can view only one part of the intestine wall at a time. For this reason, the inside of the capsule is designed so that it can spin in a complete circle. A tiny motor drives the camera and lights through a 360-degree rotation. Since the camera and lights are mounted on the inside part of the capsule, the outside part does not have to spin at all. The capsule is just the right size so that it will pass through the small intestine without changing direction and without getting stuck. The capsule is pushed along by the same muscle contractions that move food along. It is specially designed so that it will not be attacked by A capsule endoscope, commonly stomach acids and does not irritate the sensitive lining of the intestine. called a pill camera The pill camera is also disposable. There is no need to recover it after it is excreted to return it to the doctor. As amazing as this technology is, there are still more astounding optical technologies being developed for medical treatment. Optical tweezers use a laser beam to hold and move microscopic objects. Laser micro-scalpels are being refined to target individual cancer cells. Optical textiles can record and transmit a patient’s heart rate and respiration to a technician in another room. Dentists may someday soon be using laser light to detect hidden cracks and early demineralization in a tooth by measuring the light and heat emitted by the tooth. Innovative technologies like these allow us to use the properties of light in new ways. D1 STSE Science, Technology, Society, and the Environment Using Optical Devices In 1610, Italian physicist Galileo pointed one of the that make use of the properties of light and vision to first telescopes ever made into the night sky and enrich our lives. Optics is the study of the behaviour discovered that Jupiter had several moons. Prior to and properties of light, and many devices that we use this time, no one knew of any moons orbiting the each day involve the technology of optics. planet. Today, using more advanced telescopes we 1. In class, brainstorm and record as many optical have found 63 moons orbiting Jupiter. There may be devices as you can that have an impact on our even more moons to discover as we refine our lives. Think of medical, scientific, and personally technology. practical items. Telescopes have given us the ability to see deeper 2. Identify and discuss how each device affects into the universe and with more clarity than ever science, society, and the environment. before. However, telescopes are not the only devices Exploring 377 ist10_ch10.qxd 7/22/09 3:50 PM Page 378 Light is part of the electromagnetic spectrum and travels in waves. 10 378 UNIT D Light and Geometric Optics ist10_ch10.qxd 7/22/09 3:50 PM Page 379 Skills You Will Use In this chapter, you will: • use appropriate terminology related to light and optics Water waves ripple outward from their source. • gather data from laboratory and other sources and record the data using appropriate formats, including tables, flowcharts, and diagrams • communicate ideas, procedures, results, and conclusions in writing Concepts You Will Learn In this chapter, you will: • describe and explain various types of light emissions • identify and label the visible and invisible regions of the electromagnetic spectrum • describe the properties of light and use them to explain naturally occurring optical phenomena Why It Is Important Investigating the properties of light can help you understand the countless ways you use light and interact with light every day. Before Reading The Importance of Graphics Graphics have several purposes in a text: • to support our understanding of the words we read • to add information that is not in the words • to help us see the importance and even beauty of an object or idea Preview this chapter, and match one graphic to each of these purposes. Key Terms • amplitude • bioluminescence • chemiluminescence • electrical discharge • electroluminescence • fluorescent • frequency • incandescent • opaque • phosphorescence • translucent • transparent • triboluminescence • wavelength Light is part of the electromagnetic spectrum and travels in waves. 379 ist10_ch10.qxd 7/22/09 3:50 PM Page 380 Light and the Electromagnetic Spectrum 10.1 Here is a summary of what you will learn in this section: • Light is a form of energy that travels in waves. • Properties of light, such as wavelength, amplitude, and frequency, can be explained using the wave model. • The electromagnetic spectrum includes radio waves, microwaves, and infrared waves, which have wavelengths longer than visible light, and ultraviolet, X-rays, and gamma rays, which have wavelengths shorter than visible light. • Different colours of the visible spectrum have different wavelengths. Figure 10.1 Nighttime soccer Light and Colour It’s a shot to the net! The goalkeeper leaps, the kicker holds his breath, and the crowd roars (Figure 10.1). High above, mostly unnoticed, rows of bright white lights shine down on the game. Nighttime soccer is possible because we can illuminate a stadium using lights that mimic daylight. By positioning spot lights on all sides of the playing field, it is even possible to reduce the shadows that would occur using only one or two bright lights. Multiple stadium lights reduce the shadows, and this makes both watching and playing the game much easier. And then there are the colours. The powerful stadium lights often make colours on the field much more vivid than in regular daylight. The green of the grass appears greener, the red shirts are redder, and the ball — frozen forever in this photograph just above the goal line — is a brighter white. How can white light allow us to see objects of so many different colours? It is because there is more to light than meets the eye. White light is actually composed of a combination of many colours — all the colours of the rainbow, in fact. From the red light of a traffic light to the amber of anti-glare glasses to the violet light used in dentistry, our world is brighter and more colourful thanks to our many sources of light (Figures 10.2 and 10.3 on the next page). 380 UNIT D Light and Geometric Optics ist10_ch10.qxd 7/22/09 3:50 PM Page 381 Figure 10.2 Specially coloured anti-glare Figure 10.3 A dentist uses ultraviolet light to set a filling. glasses help people who have difficulty reading or driving at night. D2 Quick Lab What Is White Light Made Of? Purpose To observe the components of white light Materials & Equipment • ray box with one slit • equilateral glass prism • white paper CAUTION: Do not shine bright light into anyone’s eyes. Figure 10.4 An equilateral prism Procedure 1. Set the prism upright on the desk so that the Questions rectangular sides are vertical. 6. (a) What colours did you see when light from the 2. Place the ray box about 20 cm away from the ray box shone through the prism? prism so that the ray shines on the prism. (b) What is the order of the colours? 3. Slowly rotate the prism. Observe the direction of light that emerges from the prism. (c) How easily could you determine where one colour ended and another colour began? 4. Hold a piece of white paper in the path of the light emerging from the prism about 50 cm away 7. Where do you think the colours came from in from the prism. Observe. step 4? 5. If you do not see anything interesting, try rotating 8. Where have you seen prisms or objects that the prism again. remind you of prisms in your day-to-day life? Light is part of the electromagnetic spectrum and travels in waves. 381 ist10_ch10.qxd 7/22/09 3:50 PM Page 382 During Reading Energy in a Wave Graphics Support Text A wave is a disturbance that transfers energy from one point to another without transferring matter. In a water wave, energy passes Examine the graphics on this through water from one point to another as the wave rises and falls. page and the next page carefully. This movement of energy allows the wave to do work. Imagine that a How do the graphics support your understanding of the text duck sits on the surface of a lake. The duck moves up and down with explanations? Share your thoughts the wave, which means that the wave transfers energy to the duck with a partner. (Figure 10.5). The wave moves the water up and down, but the water does not move forward with the wave. Only energy moves forward. Energy moves Water and duck forward. move up and down with each wave. Figure 10.5 The duck moves up and down with the wave, but does not move forward or back as the wave passes beneath it. Properties of Waves Several terms will help you discuss how waves transfer energy. The highest point in a wave is called a crest, and the lowest point is called a trough (Figure 10.6 on the next page). The level of the water when there are no waves is called the rest position. Three important properties of all waves are wavelength, amplitude, and frequency. • Wavelength is the distance from one place in a wave to the next similar place on the wave; for example, the distance from crest to crest. The standard symbol for wavelength is ␭, the Greek letter lambda. Wavelength is measured in metres. • Amplitude is the wave height from the rest position of the wave to the crest or the wave depth from the rest position to the trough. The energy transferred by a wave depends, in part, on its amplitude. The larger the amplitude, the more energy that is carried. The smaller the amplitude, the less energy that is carried. • Frequency is the rate of repetition of a wave. Figure 10.6 shows waves passing a dock. If wave crests pass the dock 10 times in a minute, the frequency of the wave is 10 cycles/minute. The energy transferred by a wave often depends on the frequency of the wave as well as its amplitude. The higher the frequency, the more energy the wave passes along. The standard symbol for frequency is f. Frequency is usually measured in hertz (Hz), which is cycles per second. 382 UNIT D Light and Geometric Optics ist10_ch10.qxd 7/22/09 3:50 PM Page 383 wavelength dock crest amplitude amplitude trough rest position wavelength Figure 10.6 All waves have a wavelength and amplitude. Relationship between Frequency and Wavelength Imagine you had a pan of water and you began gently tapping the surface of the water (Figure 10.7). You would create a series of wave crests. Suppose you made one new wave crest every second. Would it take more energy or less energy to create three wave crests every second? It would take more energy because you would need to tap much faster. When you create more wave crests per second, the frequency of the wave increases. As the frequency increases, the crests are closer together. So, as more energy is put into making a wave, the frequency of the wave increases and the wavelength shortens. Frequency and wavelength have an inverse relationship, which means that when one value increases, the other decreases. As frequency increases, wavelength decreases. As frequency decreases, wavelength increases. There is a mathematical relationship among the speed, v, the frequency f, and the wavelength ␭ of the wave: v = f ⫻ ␭. For example, if the Figure 10.7 As the frequency of the wave increases, wavelength of a wave is 10 cm and the frequency wavelength decreases. is 5 cycles/s, then the speed is 50 cm/s. Learning Checkpoint 1. Draw a wave and label: (a) crest (b) trough (c) rest position (d) wavelength (e) amplitude Light is part of the electromagnetic spectrum and travels in waves. 383