Medical Physics Learning Module PDF

LM01-FPHY 0425 Learning Module Medical Physics...

LM01-FPHY 0425 Learning Module Medical Physics ity rs ive e of LE Knowledge Area Code : SCI Un at ty St er a p SA Course Code : FPHY0425 ul ro ns p Learning Module Code : LM01-FPHY0425 ni s a R Pe is i FO an Th T ta O Ba N  Netiquette Guide for Online Courses Netiquette Guide for Online Courses It is important to recognize that the online classroom is in fact a classroom, and certain behaviors are expected when you communicate with both your peers and your instructors. These guidelines for online behavior and interaction are known as netiquette. Security Remember that your password is the only thing protecting you from pranks or more serious harm. Don't share your password with anyone. Change your password if you think someone else might know it. Always log out when you are finished using the system. Appearance Bear in mind that you are attending a class, dress appropriately. General Guidelines When communicating online, you should always: Treat your instructor and classmates with respect in email or any other communication. Always use your professors’ proper title: Dr. or Prof., or if in doubt use Mr. or Ms. Unless specifically invited, don’t refer to your instructor by first name. Use clear and concise language. Remember that all college level communication should have correct spelling and grammar (this includes discussion boards). Avoid slang terms such as “wassup?” and texting abbreviations such as “u” instead of “you.” Use the prescribed font Palatino Linotype and use a size 10-point font. Avoid using the caps lock feature AS IT CAN BE INTERPRETTED AS YELLING. Limit and possibly avoid the use of emoticons like :) or J. Be cautious when using humor or sarcasm as tone is sometimes lost in an email or discussion post and your message might be taken seriously or sound offensive. Be careful with personal information (both yours and other’s). Do not send confidential information via e-mail. Email Netiquette When you send an email to your instructor, teaching assistant, or classmates, you should: Use a descriptive subject line. Be brief. Avoid attachments unless you are sure your recipients can open them. Avoid HTML in favor of plain text. Sign your message with your name and return e-mail address. Think before you send the e-mail to more than one person. Does everyone really need to see your message? Be sure you REALLY want everyone to receive your response when you click, “reply all.” Be sure that the message author intended for the information to be passed along before you click the “forward” button. Learning Module: Medical Physics iii  Netiquette Guide for Online Courses Netiquette Guide for Online Courses Message Board Netiquette and Guidelines When posting on the Discussion Board in your online class, you should: Make posts that are on topic and within the scope of the course material. Take your posts seriously and review and edit your posts before sending. Be as brief as possible while still making a thorough comment. Always give proper credit when referencing or quoting another source. Be sure to read all messages in a thread before replying. Don’t repeat someone else’s post without adding something of your own to it. Avoid short, generic replies such as, “I agree.” You should include why you agree or add to the previous point. Always be respectful of others’ opinions even when they differ from your own. When you disagree with someone, you should express your differing opinion in a respectful, non-critical way. Do not make personal or insulting remarks. Be open-minded. (Source: http://teach.ufl.edu/wp-content/uploads/2012/08/NetiquetteGuideforOnlineCourses.pdf) Learning Module: Medical Physics iv  Table of Contents Cover Table of Contents Copyright Page Statement on Copyright o Learning Module Development Team § Writers § Reviewers o Quality Management Team Netiquette Guide for Online Courses Preliminaries o Course Overview § Introduction § Key Learning Competencies § Course Details Course Code Course Title No. of Units Classification Pre-requisite / Co-requisite Semester and Academic Year Schedule Name of Faculty Contact Details Consultation Schedule § Assessment with Rubrics § Final Requirement with Rubrics § Grading System § Course Policy Packet 1 o Introduction o Learning Outcomes o Minimum Technical Skills Requirement o Learning Management System o Duration o Delivery Mode o Module Requirement with Rubrics Pre-Assessment Content o Topic 1 (Course Packet 1) § Readings § Lesson Proper Review Activity Processing of the Activity Brief Lesson Enhancement Activity Generalization Application o Topic 2 (Course Packet 2) Learning Module: Medical Physics vi  § Readings § Lesson Proper Review Activity Table of Contents Processing of the Activity Brief Lesson Enhancement Activity Generalization Application o Topic 3 (Course Packet 3) § Readings § Lesson Proper Review Activity Processing of the Activity Brief Lesson Enhancement Activity Generalization Application Course Packet Discussion Forum Post-Assessment Additional Activities (can include self-testing, discussion questions and group activities) Annexes o Answer Key o References o Feedback Form Brief Introduction of the Succeeding Learning Module Learning Module: Medical Physics vii  Course Overview Course Overview Introduction An introductory physics course for health care professionals. This course provides an introduction to the topics of, waves, sounds, optics, atomic and nuclear physics, ionizing radiation and radiation safety. It aims to provide a basic understanding of physical processes and physical problem solving techniques in order to form a basis of future studies in health sciences. Key Learning Competencies 1. Articulate the ability to think critically and to use appropriate concepts to analyze qualitatively situations or problems involving physics. 2. Demonstrate the ability to use appropriate mathematical techniques and concepts to obtain quantitative solutions to problems in physics. 3. Communicate the technical knowledge lucidly, demonstrate the use of the computer for analyzing and processing the experimental data, and to prepare coherent reports of your findings. Course Details: Course Code: FPHY0425 Course Title: Medical Physics No. of Units: 3 units lecture and 2 units laboratory Classification : Lecture/Laboratory Pre-requisite / Co-Requisite: None Semester and Academic Year: 1st sem. 2020-2021 Schedule Name of Faculty Contact Details Email: Mobile Number: Viber: Messenger: Consultation Day: Time: Learning Management System Google Classroom Assessment with Rubrics Two types of assessment may be given: an objective test that provides limited set of options for the student’s response or a projective test that requires the student to generate free responses. For objective tests, one item is equivalent to one point unless indicated in the activity. For course requirements, the rubrics below will be the basis of the student’s score: Learning Module: Medical Physics 1  LM01-FPHY 0425 Module Overview Introduction Module Overview Waves: Sound and Light is taken up during the 1st part of the module in Medical Physics. It will take the student through the basic principles of waves, light and sound with different application in medicine. Instructional activities are included such as mini-lab, small group discussion, video review activity and laboratory experiment. Topic 01: Waves Topic 02: Sounds Topic 03: Electromagnetic Waves Topic 04: Lights Learning Outcomes 1. Articulate the ability to think critically and to use appropriate concepts to analyze qualitatively situations or problems involving physics. 2. Demonstrate the ability to use appropriate mathematical techniques and concepts to obtain quantitative solutions to problems in physics. 3. Communicate the technical knowledge lucidly, demonstrate the use of the computer for analyzing and processing the experimental data, and to prepare coherent reports of your findings. Minimum Technical Skills Requirement Computer skills. Learning Management System Google Classroom Duration The total number of learning hours for this module is 48 hours, which is equally divided into four topics. For the specific distribution of hours per topic, please refer to the list below: Topic 01: Waves = 12 hours Topic 02: Sound = 12 hours Topic 03: Electromagnetic Waves = 12 hours Topic 04: Light = 12 hours Delivery Mode online (synchronous or asynchronous) modular Learning Module: Medical Physics 7  Course Packet LM01-FPHY 01 0425 Learning Module Medical Physics Course Packet 01 Waves y Knowledge Area Code : SCI sit r ive e of Course Code : FPHY0425 LE Un at ty St er Learning Module Code : LM-FPHY0425 a p SA ul ro Course Packet Code : LM-FPHY0425-01 ns p ni is a R Pe is FO an Th Learning Module: Medical Physics 3 T ta O Ba N  Course Packet LM01-FPHY 01 0425 Waves Waves Introduction This packet discusses the following concepts; wave’s description, properties of waves, behavior of waves and computation of quantities such as speed of wave, wavelength and frequency. Instructional activities are included such as mini-lab, small group discussion, video review activity and laboratory experiment Objectives 1. Explain the relationship among waves, energy and matter. 2. Describe the difference between transverse and compressional (longitudinal) waves. 3. Describe the relationship between frequency and wavelength of a wave. 4. Explain why waves travel at different speeds. 5. Explain how waves can reflect from some surfaces. 6. Explain how waves change direction when they move from one material into another. 7. Describe how waves are able to bend around barriers. Learning Management System Google classroom Duration Topic 01: Waves = 12hours (9 hours self-directed learning with practical exercises and 3hours assessment) Delivery Mode online (synchronous or asynchronous). Readings (In order to help learners make sense of and/or actively read required material, it’s recommended to provide them with relevance (why are they reading this?) and guiding questions. This helps them focus their reading and be prepared for activities related to the readings. For web-based or Google Drive-stored readings, you may list them here or embed them in the topic pages.) Introduction Visual and auditory stimuli both occur in the form of waves. Although the two stimuli are very different in terms of composition, wave forms share similar characteristics that are especially important to our visual and auditory perceptions. In this section, we describe the physical properties of the waves as well as the perceptual experiences associated with them. Learning Module: Medical Physics 1  Course Packet LM01-FPHY 01 0425 WAVES WAVES Review: Heat and Thermodynamics When scientists originally studied thermodynamics, they were really studying heat and thermal energy. Heat can do anything: move from one area to another, get atoms excited, and even increase energy. Did we say energy? That's what heat is. When you increase the heat in a system, you are really increasing the amount of energy in the system. Now that you understand that fact, you can see that the study of thermodynamics is the study of the amount of energy moving in and out of systems. Heat of Atoms Now all of this energy is moving around the world. You need to remember that it all happens on a really small scale. Energy that is transferred is at an atomic level. Atoms and molecules are transmitting these tiny amounts of energy. When heat moves from one area to another, it's because millions of atoms and molecules are working together. Those millions of pieces become the energy flow throughout the entire planet. Heat Movement Heat moves from one system to another because of differences in the temperatures of the systems. If you have two identical systems with equal temperatures, there will be no flow of energy. When you have two systems with different temperatures, the energy will start to flow. Air mass of high pressure forces large numbers of molecules into areas of low pressure. Areas of high temperature give off energy to areas with lower temperature. There is a constant flow of energy throughout the universe. Heat is only one type of that energy. Activity. I. Waves and Energy It’s a beautiful day. You are sitting by a pond in a park. Music from a school marching band is carried to your ears by waves. A fish jumps, making waves that spread past a leaf that fell from a tree, causing the leaf to move. In the following procedures, you’ll observe how waves carry energy that can cause objects to move. 1. Add water to a large, clear , plastic plate to a depth of about 1 cm. Learning Module: Medical Physics 2  Course Packet LM01-FPHY 01 0425 2. Use a dropper to release a single drop of water onto the water’s surface. Repeat. 3. Float a cork or straw on the water. WAVES 4. When water is still, repeat step 2 from a height of 10 cm., then again from 20cm. 5. Think Critically. In your notebook, record your observations. How did the motion of the cork depend on the height of the dropper? II. Waves Construct a Venn Diagram as you read this packet, list the characteristics unique to transverse waves under the left tab, those unique to compressional waves under the right tab, and those characteristics common to both under the middle tab. Brief Lesson. What is a wave? When you are relaxing on an air mattress in a pool and someone does a cannonball dive off the diving board, you suddenly find yourself bobbing up and down. You can make something move by giving it a push or pull, but the person jumping didn’t touch your air mattress. How did the energy from the dive travel through the water and move your air mattress? The up- and-down motion was caused by the peaks and valleys of the ripples that moved from where the splash occurred. These peaks and valleys make up water waves. Waves Carry Energy Rhythmic disturbances that carry energy without carrying matter are called waves. Water waves are shown in Figure 1. You can see the energy of the wave from a speedboat traveling outward, but the water only moves up and down. If you’ve ever felt a clap of thunder, you know that sound waves can carry large amounts of energy. Learning Module: Medical Physics 3  Course Packet LM01-FPHY 01 0425 You also transfer energy when you throw something to a friend, as in Figure 1. However, there is a difference between a moving ball and a wave. A ball is made of matter, and when it is thrown, the matter moves from one place to another. So, unlike the wave, throwing a ball WAVES involves the transport of matter as well as energy. Figure 1. The wave and the thrown ball carry energy in different ways. The waves created by a boat move mostly up and down, but energy travels outward from the boat When the ball is thrown, the ball carries energy as it moves forward. Figure 2. As the students pass the ball, the students’ position do not change—only the position of the ball changes. A wave transports energy without transporting matter from place to place. In a water wave, water molecules bump each other and pass energy from molecule to molecule. Learning Module: Medical Physics 4  Course Packet LM01-FPHY 01 0425 A Model for Waves How does a wave carry energy without transporting matter? Imagine a line of people, as shown WAVES in Figure 2. The first person in line passes a ball to the second person, who passes the ball to the next person, and so on. Passing a ball down a line of people is a model for how waves can transport energy without trans- porting matter. Even though the ball has traveled, the people in line have not moved. In this model, you can think of the ball as representing energy. What do the people in line represent? Think about the ripples on the surface of a pond. The energy carried by the ripples travels through the water. The water is made up of water molecules. It is the individual molecules of water that pass the wave energy, just as the people. The water molecules transport the energy in a water wave by colliding with the molecules around them, as shown in Figure 2. Mechanical Waves In the wave model, the ball could not be transferred if the line of people didn’t exist. The energy of a water wave could not be transferred if no water molecules existed. These types of waves, which use matter to transfer energy, are called mechanical waves. The matter through which a mechanical wave travels is called a medium. For ripples on a pond, the medium is the water. A mechanical wave travels as energy is transferred from particle to particle in the medium. For example, a sound wave is a mechanical wave that can travel through air, as well as solids, liquids, and other gases. Without a medium such as air, there would be no sound waves. In outer space sound waves can’t travel because there is no air. Transverse Waves In a mechanical transverse wave, the wave energy causes the matter in the medium to move up and down or back and forth at right angles to the direction the wave travels. You can make a model of a transverse wave. Stretch a long rope out on the ground. Hold one end in your hand. Now shake the end in your hand back and forth. As you shake the rope, you create a wave that seems to slide along the rope. When you first started shaking the rope, it might have appeared that the rope itself was moving away from you. But it was only the wave that was moving away from your hand. The wave energy moves through the rope, but the matter in the rope doesn’t travel. You can see that the wave has peaks and valleys at regular intervals. As shown in Figure 3, the high points of transverse waves are called crests. The low points are called troughs. Figure 3. The high points on the wave are called crests and the low points are called troughs. Learning Module: Medical Physics 5  Course Packet LM01-FPHY 01 0425 WAVES Compressional Waves Mechanical waves can be either transverse or compressional. In a compressional wave, matter in the medium moves forward and backward along the same direction that the wave travels. You can make a compressional wave by squeezing together and releasing several coils of a coiled spring toy, as shown in Figure 4. The coils move only as the wave passes and then return to their original positions. So, like transverse waves, compressional waves carry only energy forward along the spring. In this example, the spring is the medium the wave moves through, but the spring does not move along with the wave. SoundWaves Sound waves are compressional waves. How do you make sound waves when you talk or sing? If you hold your fingers against your throat while you hum, you can feel vibrations. These vibrations are the movements of your vocal cords. If you touch a stereo speaker while it’s playing, you can feel it vibrating, too. All waves are produced by something that is vibrating. Making Sound Waves How do vibrating objects make sound waves? Look at the drum shown in Figure 5. When you hit the drumhead it starts vibrating up and down. As the drumhead moves upward, the molecules next to it are pushed closer together. This group of molecules that are closer together is a compression. As the compression is formed, it moves away from the drumhead, just as the squeezed coils move along the coiled spring toy in Figure 4. When the drumhead moves downward, the molecules near it have more room and can spread farther apart. This group of molecules that are farther apart is a rarefaction. The rarefaction also moves away from the drumhead. As the drumhead vibrates up and down, it forms a series of compressions and rarefactions that move away and spread out in all directions. This series of compressions and rarefactions is a sound wave. Learning Module: Medical Physics 6  Course Packet LM01-FPHY 01 0425 WAVES Figure 5 A vibrating drumhead makes compressions and rarefactions in the air. Activity: Comparing sounds Procedure: 1. Hold a wooden ruler on the edge of your desk so that most of it extends off the edge of the desk. 2. Pluck the free of the ruler so that it vibrates up and down. Use gentle motion at first, then pluck with more energy. 3. Repeat step 2, moving the ruler about 1cm. further onto the desk each time until only about 5 cm. extend off the edge. Analysis: 1. Compare the loudness of the sounds that are made by plucking the ruler in different ways. 2. Describe the differences in the sound as the end of the ruler extended farther from the desk. Electromagnetic Waves Waves that can travel through space where there is no matter are electromagnetic waves. There are different types of electromagnetic waves, including radio waves, infrared waves, visible light waves, ultraviolet waves, X rays, and gamma rays. These waves can travel in matter or in space. Radio waves from TV and radio stations travel through air, and may be reflected from a satellite in space. They then travel through air, through the walls of your house, and to your TV or radio. Learning Module: Medical Physics 7  Course Packet LM01-FPHY 01 0425 WAVES Radiant Energy from the Sun The Sun emits electromagnetic waves that travel through space and reach Earth. The energy carried by electromagnetic waves is called radiant energy. Almost 92 percent of the radiant energy that reaches Earth from the Sun is carried by infrared and visible light waves. Infrared waves make you feel warm when you sit in sunlight, and visible light waves enable you to see. A small amount of the radiant energy that reaches Earth is carried by ultraviolet waves. These are the waves that can cause sunburn if you are exposed to sun- light for too long. Wave Properties 1. Amplitude Can you describe a wave? For a water wave, one way might be to tell how high the wave rises above, or falls below, the normal level. This distance is called the wave’s amplitude. The amplitude of a transverse wave is one-half the distance between a crest and a trough, as shown in Figure 6. In a compressional wave, the amplitude is greater when the particles of the medium are squeezed closer together in each compression and spread farther apart in each rarefaction. Amplitude and Energy.A wave’s amplitude is related to the energy that the wave carries. For example, the electromagnetic waves that make up bright light have greater amplitudes than the waves that make up dim light. Waves of bright light carry more energy than the waves that make up dim light. In a similar way, loud sound waves have greater amplitudes than soft sound waves. Loud sounds carry more energy than soft sounds. If a sound is loud enough, it can carry enough energy to damage your hearing. When a hurricane strikes a coastal area, the resulting water waves carry enough energy to damage almost anything that stands in their path. The large waves caused by a hurricane carry more energy than the small waves or ripples on a pond. Learning Module: Medical Physics 8  Course Packet LM01-FPHY 01 0425 WAVES Figure 6. The energy carried by a wave increases as its amplitude increases The devastating effect that a wave with large amplitude can have is seen in the aftermath of tsunamis. Tsunamis are huge sea waves that are caused by underwater earthquakes along faults on the seafloor. The movement of the seafloor along a fault produces the wave. As the wave moves toward shallow water and slows down, the amplitude of the wave grows. The tremendous amounts of energy tsunamis carry cause great damage when they move ashore. 2. Wavelength Another way to describe a wave is by its wavelength. Figure 7 shows the wavelength of a transverse wave and a compressional wave. For a transverse wave, wavelength is the distance from the top of one crest to the top of the next crest, or from the bottom of one trough to the bottom of the next trough. For a compressional wave, the wavelength is the distance between the center of one compression and the center of the next compression, or from the center of one rarefaction to the center of the next rarefaction. Electromagnetic waves have wavelengths that range from kilometers, for radio waves, to less than the diameter of an atom, for X rays and gamma rays. This range is called the electromagnetic spectrum. Figure 8 shows the names given to different parts of the electromagnetic spectrum. Visible light is only a small part of the electromagnetic spectrum. It is the wavelength of visible light waves that determines their color. For example, the wavelength of red light waves is longer than the wavelength of green light waves. Figure 7. A transverse or a compressional wave has a wavelength Learning Module: Medical Physics 9  Course Packet LM01-FPHY 01 0425 WAVES Figure 8. The wavelengths and frequencies of electromagnetic waves vary. 3. Frequency The frequency of a wave is the number of wavelengths that pass a given point in 1 s. The unit of frequency is the number of wavelengths per second, or hertz (Hz). Recall that waves are produced by something that vibrates. The faster the vibration is, the higher the frequency is of the wave that is produced. A Sidewalk Model For waves that travel with the same speed, frequency and wavelength are related. To model this relationship, imagine people on two parallel moving sidewalks in an airport, as shown in Figure 9. One sidewalk has four travelers spaced 4 m apart. The other sidewalk has 16 travelers spaced 1 m apart. Now imagine that both sidewalks are moving at the same speed and approaching a pillar between them. On which sidewalk will more people go past the pillar? On the sidewalk with the shorter distance between people, four people will pass the pillar for each one person on the other sidewalk. When four people pass the pillar on the first sidewalk, 16 people pass the pillar on the second sidewalk. Figure 9. When people are farther apart on a moving sidewalk, fewer people pass the pillar every minute. Learning Module: Medical Physics 10  Course Packet LM01-FPHY 01 0425 Frequency and Wavelength Suppose that each person in Figure 9 represents the crest of a wave. Then the movement of WAVES people on the first sidewalk is like a wave with a wavelength of 4 m. For the second sidewalk, the wavelength would be 1 m. On the first sidewalk, where the wavelength is longer, the people pass the pillar less frequently. Smaller frequencies result in longer wavelengths. On the second sidewalk, where the wave- length is shorter, the people pass the pillar more frequently. Higher frequencies result in shorter wavelengths. This is true for all waves that travel at the same speed. As the frequency of a wave increases, its wavelength decreases. How are frequency and wavelength related? Color and Pitch Because frequency and wavelength are related, either the wavelength or frequency of a light wave determines the color of the light. For example, blue light has a larger frequency and shorter wavelength than red light. Either the wavelength or frequency determines the pitch of a sound wave. Pitch is how high or low a sound seems to be. When you sing a musical scale, the pitch and frequency increase from note to note. Wavelength and frequency are also related for sound waves traveling in air. As the frequency of sound waves increases, their wavelength decreases. Figure 10 shows how the frequency and wavelength change for notes on a musical scale. Figure 10. The frequency of the notes on a musical scale increases as the notes get higher in pitch, but the wavelength of the notes decreases. Learning Module: Medical Physics 11  Course Packet LM01-FPHY 01 0425 WAVES Wave Speed You’ve probably watched a distant thunderstorm approach on a hot summer day. You see a bolt of lightning flash between a dark cloud and the ground. If the thunderstorm is many kilometers away, several seconds will pass between when you see the lightning and when you hear the thunder. This happens because light travels much faster in air than sound does. Light travels through air at about 300 million m/s. Sound travels through air at about 340 m/s. The speed of any wave can be calculated from this equation: In this equation, the wavelength is represented by the symbol , which is the Greek letter lambda. Variables, units, and symbols: Quantity Symbol Quantity Term Unit Unit Symbol v Wave speed Meters/second m/s l wavelength meter m f frequency Hertz Hz Remember: frequency- number of complete waves passing a point in a given time 𝑐𝑦𝑐𝑙𝑒𝑠 𝑓 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡 If 10 waves pass in 1 second, the frequency is 10 Hz If 6 waves pass in 2 seconds, the frequency is 3 Hz Example: 1. A wave has frequency of 50 Hz and a wavelength of 10 m. What is the speed of the wave? Learning Module: Medical Physics 12  Course Packet LM01-FPHY 01 0425 Solution: f = 50 Hz l =10 m v=? WAVES v=l×f =(10 m)×(50 Hz)=500 m/s 2. A wave has frequency of 5 Hz and a speed of 25 m/s. What is the wavelength of the wave? f = 5 Hz v = 2 5 m/s l=? 3. A wave has wavelength of 10 m and a speed of 340 m/s. What is the frequency of the wave? Problems for you to try: Complete the following practice problems. You MUST show ALL the work outlined in the steps in the example problems. 1. A wave with a frequency of 14 Hz has a wavelength of 3 meters. At what speed will this wave travel? 2. The speed of a wave is 65 m/sec. If the wavelength of the wave is 0.8 meters, what is the frequency of the wave? 3. A wave has a frequency of 46 Hz and a wavelength of 1.7 meters. What is the speed of this wave? 4. A wave traveling at 230 m/sec has a wavelength of 2.1 meters. What is the frequency of this wave? 5. A wave with a frequency of 500 Hz is traveling at a speed of 200 m/s. What is the wavelength? Learning Module: Medical Physics 13  Course Packet LM01-FPHY 01 0425 When mechanical waves, such as sound, and electromagnetic waves, such as light, travel in WAVES different materials, they change speed. Mechanical waves usually travel fastest in solids, and slowest in gases. Electromagnetic waves travel fastest in gases and slowest in solids. For example, the speed of light is about 30 percent faster in air than in water. WAVE BEHAVIOR Reflection What causes the echo when you yell across an empty gymnasium or down a long, empty hallway? Why can you see your face when you look in a mirror? The echo of your voice and the face you see in the mirror are caused by wave reflection. Reflection occurs when a wave strikes an object or surface and bounces off. An echo is reflected sound. Sound reflects from all surfaces. Your echo bounces off the walls, floor, ceiling, furniture, and people. You see your face in a mirror or a still pond, as shown in Figure 11, because of reflection. Light waves produced by a source of light such as the Sun or a lightbulb bounce off your face, strike the mirror, and reflect back to your eyes. When a surface is smooth and even the reflected image is clear and sharp. However, Figure 11 shows that when light reflects from an uneven or rough surface, you can’t see a sharp image because the reflected light scatters in many different directions. What causes reflection? Figure 11. The image formed by reflection depends on the smoothness of the surface. Learning Module: Medical Physics 14  Course Packet LM01-FPHY 01 0425 Activity: Observing How Light Refracts Procedure: WAVES 1. Fill a large, opaque drinking glass or cup with water. 2. Place a white soda straw in the water at an angle. 3. Looking directly down into the cup from above, observe the straw where it meets the water. 4. Placing yourself so that the straw angles to your left or right, slowly back away 1 m. Observe the straw as it appears above, at, and below the surface of the water. Analysis: 1. Describe the straw’s appearance from above. 2. Describe the straw’s appearance above and below the water’s surface in step 4. Refraction A wave changes direction when it reflects from a surface. Waves also can change direction in another way. Perhaps you have tried to grab a sinking object when you are in a swimming pool, only to come up empty-handed. Yet you were sure you grabbed right where you saw the object. You missed grabbing the object because the light rays from the object changed direction as they passed from the water into the air. The bending of a wave as it moves from one medium into another is called Refraction and Wave Speed Remember that the speed of a wave can be different in different materials. For example, light waves travel faster in air than in water. Refraction occurs when the speed of a wave changes as it passes from one substance to another, as shown in Figure 12. A line that is perpendicular to the water’s surface is called the normal. When a light ray passes from air into water, it slows down and bends toward the normal. When the ray passes from water into air, it speeds up and bends away from the normal. The larger the change in speed of the light wave is, the larger the change in direction is. You notice refraction when you look down into a fishbowl. Refraction makes the fish appear to be closer to the surface and farther away from you than it really is, as shown in Figure 13. Light rays reflected from the fish are bent away from the normal as they pass from water to air. Your brain interprets the light that enters your eyes by assuming that light rays always travel in straight lines. As a result, the light rays seem to be coming from a fish that is closer to the surface. Learning Module: Medical Physics 15  Course Packet LM01-FPHY 01 0425 WAVES Figure 12. A wave is refracted Figure 13. When you look at the goldfish in the water, the fish is in a different position than it appears. Color from Refraction Sunlight contains light of various wavelengths. When sunlight passes through a prism, refraction occurs twice: once when sunlight enters the prism and again when it leaves the prism and returns to the air. Violet light has the shortest wavelength and is bent the most. Red light has the longest wavelength and is bent the least. Each color has a different wavelength and is refracted a different amount. As a result, the colors of sunlight are separated when they emerge from the prism. Figure 14 shows how refraction produces a rainbow when light waves from the Sun pass into and out of water droplets. The colors you see in a rainbow are in order of decreasing wavelength: red, orange, yellow, green, blue, indigo, and violet. Diffraction Why can you hear music from the band room when you are down the hall? You can hear the music because the sound waves bend as they pass through an open doorway. This bending isn’t caused by refraction. Instead, the bending is caused by diffraction. Diffraction is the bending of waves around a barrier. Light waves can diffract, too. You can hear your friends in the band room but you can’t see them until you reach the open door. Therefore, you know that light waves do not diffract as much as sound waves do. Light waves do bend around the edges of an open door. However, Learning Module: Medical Physics 16  Course Packet LM01-FPHY 01 0425 for an opening as wide as a door, the amount the light bends is extremely small. As a result, the diffraction of light is far too small to allow you to see around a corner. WAVES Figure14 Light rays refract as they enter and leave each water drop. Each color refracts at different angles because of their different wavelengths, so they separate into the colors of the visible spectrum. Diffraction and Wavelength. The reason that light waves don’t diffract much when they pass through an open door is that the wavelengths of visible light are much smaller than the width of the door. Light waves have wavelengths between about 400 and 700 billionths of a meter, while the width of doorway is about one meter. Sound waves that you can hear have wavelengths between a few millimeters and about 10 m. They bend more easily around the corners of an open door. A wave is diffracted more when its wavelength is similar in size to the barrier or opening. Under what conditions would more diffraction of a wave occur? Diffraction of Water Waves. Perhaps you have noticed water waves bending around barriers. For example, when water waves strike obstacles such as the islands shown in Figure 15, they don’t stop moving. Here the size and spacing of the islands is not too different from the wavelength of the water waves. So the water waves bend around the islands, and keep on moving. They also spread out after they pass through openings between the islands. If the islands were much larger than the water wavelength, less diffraction would occur. What happens when waves meet? Suppose you throw two pebbles into a still pond. Ripples spread from the impact of each pebble and travel toward each other. What happens when two of these ripples meet? Do they collide like billiard balls and change direction? Waves behave differently from billiard balls when they meet. Waves pass right through each other and continue moving. Figure 15. Water waves bend or diffract around these islands. More diffraction occurs when the object is closer in size to the wavelength. Learning Module: Medical Physics 17  Course Packet LM01-FPHY 01 0425 Wave Interference. While two waves overlap a new wave is formed by adding the two waves together. The ability of two waves to combine and form a new wave when they overlap is called interference. After they overlap, the individual waves continue to travel on in their original WAVES form. Learning Module: Medical Physics 18  Course Packet LM01-FPHY 01 0425 The different ways waves can interfere are shown in Figure 16 on the next page. Sometimes when the waves meet, the crest of one wave overlaps the crest of another wave. This is called constructive interference. The amplitudes of these combining waves add together to make a WAVES larger wave while they overlap. Destructive interference occurs when the crest of one wave overlaps the trough of another wave. Then, the amplitudes of the two waves combine to make a wave with a smaller amplitude. If the two waves have equal amplitudes and meet crest to trough, they cancel each other while the waves overlap. Waves and Particles. Like waves of water, when light travels through a small opening, such as a narrow slit, the light spreads out in all directions on the other side of the slit. If small particles, instead of waves, were sent through the slit, they would continue in a straight line without spreading. The spreading, or diffraction, is only a property of waves. Interference also doesn’t occur with particles. If waves meet, they reinforce or cancel each other, then travel on. If particles approach each other, they either collide and scatter or miss each other completely. Interference, like diffraction, is a property of waves. Reducing Noise. You might have seen someone use a power lawn mower or a chain saw. In the past, many people who performed these tasks damaged their hearing because of the loud noises produced by these machines. Loud sounds have waves with larger amplitudes and carry more energy than softer sounds. The energy carried by loud sounds can damage cells in the ear that vibrate and transmit signals to the brain. Damage to the ear from loud sounds can be prevented by reducing the energy that reaches the ear. Ear protectors contain materials that absorb some of the energy carried by sound waves, so that less sound energy reaches the ear. Pilots of small planes have a more complicated problem. If they shut out all the noise of the plane’s motor, the pilots wouldn’t be able to hear instructions from air-traffic controllers. To solve this problem, ear protectors have been developed, as shown in Figure 17, that have electronic circuits. These circuits detect noise from the aircraft and produce sound frequencies that destructively interfere with the noise. They do not interfere with human voices, so people can hear normal conversation. Destructive interference can be a benefit. Figure 17. Some airplane pilots use special ear protectors that cancel out engine noise but don’t block human voices. Learning Module: Medical Physics 19