# WAVES Q: What's In A Wave? A: Energy, Energy, Energy

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WAVES Q: What’s in a wave? A: Energy, energy, energy Wave – a repeating disturbance or movement that transfers through matter or space o All waves transfer energy without transporting from place to place Example: throwing a pebble into a puddle of water The pebble transfers some of its energy to nearby molecules, causing them to move These molecules then pass the energy along to neighboring water molecules, which, in turn, transfer it to their neighbors The energy moves farther and farther from the source of the disturbance What you see is energy traveling in the form of a on the surface of the water The waves DO NOT carry the water along with them – they ONLY transfer o A wave will travel only as long as it has energy to transfer This is why the ripples on the puddle eventually die out after traveling for a short distance o All waves are produced by something that Q: What are mechanical waves? A: Waves that require a medium in which to travel. A medium is the that waves travel through o Mediums can be solid, liquid, or gas Examples of mechanical waves include sound waves, seismic waves, ocean waves, etc Q: Describe two types of mechanical waves. A: Transverse waves and compressional (or longitudinal) waves Transverse Waves – matter in the medium moves back and forth at to the direction that the waves travels in o Example: ocean waves 1

A wave in the ocean moves horizontally, but the water that the wave passes through moves up and down o Think of people doing “the wave” at a football game Compressional Waves – matter in the medium moves back and forth along the the wave travels o Also known as “longitudinal waves” o Example: sound waves When you talk, the air molecules are pushed together by the The compressions travel through the air to make a wave Q: Are there waves that don’t require a medium in which to travel? A: Yes. Electromagnetic waves can travel through a or empty space, as well as through matter o Carry energy from place to place like mechanical waves o Differ from mechanical waves in how they are produced and how they travel o Considered PARTS OF WAVES AND MEASURING WAVES Q: What are the parts of a wave? A: Crests and troughs or compressions and rarefactions. Transverse waves have o Crest – high points in the wave o Trough – low points in the wave Compressional waves have compressions and rarefactions o Compression – region where the medium becomes crowded together, or more o Rarefaction – the less-dense regions of a compressional wave Q: How can we measure waves? A: We use wavelength, frequency, amplitude, and speed 2

Wavelength – the between one point on a wave and the nearest point just like it o With transverse waves, we measure wavelength from crest to crest or trough to trough o With compressional waves, we measure wavelength from the start of one compression to the start of the next compression, or from the start of one rarefaction to the start of the next rarefaction Frequency – the number of that pass a fixed point each second o Simply count the number of crests or troughs (or compressions or rarefactions) that pass by a given point each second o The unit for frequency is (Hz) One Hz means that one wavelength passes by in one Hz 1/s o As frequency of a wave , wavelength o Frequency is always equal to the rate of of the source that creates it o Speed – how fast a wave travels 3

Speed of a wave depends on the it is traveling through Example: sound waves generally travel faster in a material when the temperature of the material is o Calculating wave speed Speed (in m/s) frequency (in Hz) x wavelength (in m) OR v fx Sample Problem What is the speed of a sound wave that has a wavelength of 2 m and a frequency of 170.5 Hz? 1. What are the known values? 2. What is the unknown that we are trying to solve for? 3. What formula contains both the known values and the unknown we are solving for? 4. Substitute your values into the formula and solve. 5. Does your answer make sense? Practice Problem #1: Waves in a lake have a wavelength of 6 m apart and pass a person on a raft with a frequency of 0.5 Hz. What is the speed of the waves? Practice Problem #2: A buoy bobs up and down in the ocean. The waves have a wavelength of 2.5 m, and they pass the buoy at a speed of 4.0 m/s. What is the frequency of the waves? How much time does it take for one wave to pass under the buoy? 4

Practice Problem #3: The musical note A above middle C has a frequency of 440 Hz. If the speed of sound is known to be 350 m/s, what is the wavelength of this note? Amplitude – related to the transferred by a wave o The greater the wave’s amplitude, the energy the wave transfers o Amplitude is measured differently for transverse and compressional waves Transverse waves – distance from the of the wave to the rest position of the medium Think about the difference between a tall and a short wave when standing in the water? Which has more energy to knock you over? Compressional waves – related to how the medium is pushed together at the compressions The the medium at the compressions, the higher the amplitude of the wave and the more energy that is transferred 5

Higher Amplitude Wave Lower Amplitude Wave ELECTROMAGNETIC WAVES Q: Remind me again of the basic properties of waves A: Here’s a short summary on how waves work: All waves are produced by something that Waves transmit from one place to another Some types of waves require a in which to travel, while other types of waves do not o Sound waves require air particles to travel through o Water waves must have water molecules These waves travel because energy is transferred from particle to particle Without matter, these waves Electromagnetic waves do NOT require a medium to transfer energy – they can travel through space where no is present Q: How do electromagnetic waves transfer energy without matter? A: They use electric and magnetic fields. Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating and o Magnetic fields exist around all magnets, even if the space around the magnet contains no matter 6

Q: What are some properties of electromagnetic waves? A: Electromagnetic waves have speed, wavelength, and frequency like mechanical waves. Wave speed o All electromagnetic waves travel at in the of space This is also known as the Nothing travels faster than the speed of light in nature o When an electromagnetic wave travels through matter, this speed changes The speed will depend on the material the wave is passing through Electromagnetic waves usually travel most slowly through and fastest through Wavelength and frequency o Like all waves, electromagnetic waves can be described by their wavelengths and frequencies Wavelength – Frequency – number of wavelengths that pass a given point in one second Also can be described as the number of vibrations made by the in one second o The wavelength and frequency of an electromagnetic wave are related As the frequency , the wavelength and vice versa Electromagnetic waves can behave as waves and as particles o These particles are referred to as The amount of energy in a photon is dependent on the (rather than the amplitude) The higher the frequency, the the energy in the photon 7

THE ELECTROMAGNETIC SPECTRUM Q: Now that I understand electromagnetic waves, what exactly is the electromagnetic spectrum? A: The entire range of electromagnetic wave frequencies. The electromagnetic spectrum is composed of of electromagnetic waves that interact with matter very differently They are separated from one another based on their wavelengths and frequencies o Low-frequency electromagnetic waves with wavelengths longer than 1 mm Radio waves also have other uses To find the movement and position of objects using To take a picture of the bones and soft tissue on the inside of a patient’s body using To carry audio signals from radio stations to your radio o But remember, o The audio signal that the radio wave carries is converted into sound through your radio o What you can hear is the sound wave coming from the radio as it moves through the air o wavelengths less than 30 cm Microwaves are best known for cooking food in microwave ovens 8

Microwaves are also used for communication in cellular phones and satellite signals o Have higher frequencies than microwaves and lower frequencies than red light Wavelengths vary from 1 mm to 750 nm 1 10-9 meters Often used as a source of Red lamps in cafeterias keep food warm with infrared radiation Other uses include: Controlling televisions through a remote control Reading CD’s via a computer or game console Detecting trapped victims or heat loss in a building through a thermogram o The range of electromagnetic waves that you can detect with your Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular These range from long-wavelength to short-wavelength If all the colors are present, you see the light as o Wavelengths vary from 400 nm to 4 nm 9

Ultraviolet waves are energetic enough to enter Overexposure can cause skin damage and Some ultraviolet waves are helpful Exposure allows the body to create vitamin D, which is needed for bones and teeth to absorb Also have the ability to kill bacteria on food or medical supplies Can make some materials fluoresce – absorbs the UV rays and reemits the energy as visible light o This is used by CSI technicians when looking for fingerprints or blood at a crime scene o ranging from 12 nm to 0.005 nm Have and can penetrate matter that light cannot Often used in medicine, industry, and transportation to make pictures of the inside of solid objects o Have the in the electromagnetic spectrum at about 0.005 nm or less Have the and therefore the and greatest penetrating ability of all the electromagnetic waves Exposure to small amounts of gamma rays are tolerable, but overexposure can be deadly Used in the medical field to kill cancer cells and make pictures of the brain based on brain activity 10

DIY NOTES: PROPERTIES OF LIGHT Directions: Instead of listening to a lecture and taking notes, you will complete the lecture notes below using the information found on the various websites listed below. Follow the instructions to each of the websites and answer the questions or fill in the missing information. Remember: these are your lecture notes for this section, so don’t hurry through this assignment just to get it completed. Make sure you are answering the questions correctly so that you have the information to study later on for your unit test. Section 1 – Light and Other Materials Without light, nothing is visible. Look around the classroom – what do you see? You probably see desks, posters on the walls, other students what you are actually seeing is light. You can see the desks and other students in the classroom because light passes through the air between those objects and your eyes. Close your eyes. You can’t see the desks or other students anymore because the light can’t pass through your eyelids. For you to see an object, it must reflect or emit some light that reaches your eyes. How light behaves when it strikes an object depends on many factors. Objects can absorb light, reflect light, or transmit light – allow light to pass through them. The type of matter in an object determines the amount of light it absorbs, reflects, or transmits. Visit the following website: ent-translucent-opaque.html 1. What does it mean for an object to be transparent? What happens to light when it encounters transparent objects? 2. What does it mean for an object to be translucent? What happens to light when it encounters translucent objects? 3. Objects on the other side of a translucent object appear and . 4. What does it mean for an object to be opaque? What happens to light when it encounters opaque objects? 11

5. Classify each of the following objects as TP (transparent), TL (translucent), or OP (opaque). Cardboard – Saran Wrap – Styrofoam plate – Tissue paper – Window glass – Waxed paper – Felt – Clean water – 6. Most of the materials we come into contact with are considered (transparent, translucent, opaque). Section 2 – Interactions of Light Every object reflects some light and absorbs some light. When light is transmitted, it can be refracted, polarized, or scattered. To describe reflection, refraction, and many other effects of light, it is useful to use the light ray as a model for light. A light ray is an imaginary line running in the direction that the light travels. Using light rays, the path of light can be traced in geometrical drawings called ray diagrams. Visit the following website: eflectionintro.html 1. What has to happen in order for the reflection of light to occur? 2. When light strikes a surface, the incoming light wave is referred to as the , and the wave that is bounced away from the surface is termed the . 3. Using the interactive tutorial, the information from the third paragraph, and Figure 2, explain the Law of Reflection. 4. Describe the properties of the best surfaces for reflecting light. Give a few examples of these surfaces. 12

5. Label the parts of Figure 2 on the right: 6. According to the Law of Reflection, the is always equal to the when light strikes a smooth surface. 7. The amount of light reflected by an object, and how it is reflected, is highly dependent on the of the object. 8. Describe what occurs when light encounters an object with a rough surface (Use the diagram in Figure 3 or the interactive tutorial). Visit the following website: efraction.html 9. When does the refraction of light occur? 10. Refractive index refers to the at which light moves through a medium. 11. The speed at which refracted light travels is dependent upon what? 12. Circle the correct word to complete the sentence based on the diagram below: When light passes from a (less, more) dense medium to a (less, more) dense medium, the speed of the light wave decreases. In this case, the path of the light ray is bent, or refracted, toward the normal (a line perpendicular to the point where the light hits the object). Air less dense, so light can travel at a higher speed 13

Glass more dense, so light travels at a slower speed 13. Circle the correct word to complete the sentence based on the diagram below: On the other hand, when light travels from a (less, more) dense medium to a (less, more) dense medium, the speed of the light wave is increased. In this case, the path of the light ray is refracted away from the normal. Glass more dense, so the light travels at a slower speed Air less dense, so the light can travel at a higher speed 14. Describe why this drinking straw looks “bent” in the water. Do a little internet research on your own to answer the following questions. Either look around on the websites you already visited or you can find a different website to use. 15. Describe a concave lens and list a few examples of where you might find one. 16. Describe a convex lens and list a few examples of where you might find one. 17. What happens to light rays when they strike a concave lens? What about a convex lens? 14

18. Label each of the diagrams below as a concave lens or a convex lens. Continue drawing in the light rays based on what you know about the behavior of light as it strikes each of these types of lenses. Visit the following website: http://www.hk-phy.org/iq/skycolor/skycolor e.html 19. Use the information provided on this website to describe why the sun and sky appear red at sunrise and sunset. Putting it all together Using what you have learned from the various websites listed above, answer the following questions. 20. Draw a ray diagram for each of the following situations: Light reflecting off of calm water Light reflecting off of the sand on the beach The refraction of light (in general) Light as it strikes a piece of thick, black plastic 15

Q: What are mechanical waves? A: Waves that require a medium in which to travel. A medium is the _ that waves travel through o Mediums can be solid, liquid, or gas Examples of mechanical waves include sound waves, seismic waves, ocean waves, etc Q: Describe two types of mechanical waves.

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