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Transcript
Unit 6 Waves and Light Waves Wave – a repeating disturbance or movement that transfers energy through matter or space Waves o All waves transfer energy without transporting matter from place to place Waves • Example: Throwing a pebble into a puddle of water Waves • The pebble transfers some of its energy to nearby water molecules, causing them to move • These molecules then pass the energy along to neighboring water molecules, which, in turn, transfer it to their neighbors Waves • The energy moves farther and farther from the source of the disturbance • What you see is energy traveling in the form of a wave on the surface of the water Waves The waves DO NOT carry the water along with them – they ONLY transfer energy Waves • A wave will travel only as long as it has energy to transfer • This is why the ripples on a puddle eventually die out after traveling for a short distance Waves • All waves are produced by something that vibrates Waves Waves Q: What are mechanical waves? A: Waves that require a medium in which to travel. Waves • A medium is the matter that waves travel through • Mediums can be solid, liquid, or gas Waves • Examples of mechanical waves include sound waves, seismic waves, ocean waves, etc… Waves Q: Describe two types of mechanical waves. A: Transverse waves and compressional (or longitudinal) waves Waves Transverse Waves – matter in the medium moves back and forth at right angles to the direction that the waves travels in Waves • Example: ocean waves Waves • A wave in the ocean moves horizontally, but the water that the wave passes through moves up and down • Think of people doing “the wave” at a football game Waves • Compressional Waves – matter in the medium moves back and forth along the same direction the wave travels • Also known as “longitudinal waves” Waves Waves Example: Sound Waves • When you talk, the air molecules are pushed together by the vibrations • The compressions travel through the air to make a wave Waves Q: Are there waves that don’t require a medium in which to travel? A: Yes. Waves • Electromagnetic waves can travel through a vacuum or empty space, as well as through matter Waves • Carry energy from place to place like mechanical waves • Differ from mechanical waves in how they are produced and how they travel • Considered transverse waves Waves Parts of a Wave Q: What are the parts of a wave? A: Crests and troughs or compressions and rarefactions. Parts of a Wave • Transverse waves have crests and troughs Parts of a Wave • Crests – high points in the wave • Troughs – low points in the wave Parts of a Wave Parts of a Wave Parts of a Wave • Compressional waves have compressions and rarefactions Parts of a Wave • Compression – region where the medium becomes crowded together, or more dense • Rarefaction – the less-dense regions of a compressional wave Parts of a Wave Parts of a Wave Measuring Waves Q: How can we measure waves? A: We use wavelength, frequency, amplitude, and speed Measuring Waves • Wavelength – the distance between one point on a wave and the nearest point just like it Measuring Waves Measuring Waves • With transverse waves, we measure wavelength from crest to crest or trough to trough Measuring Waves • 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 Measuring Waves • Frequency – the number of wavelengths that pass a fixed point each second Measuring Waves • Simply count the number of crests or troughs (or compressions or rarefactions) that pass by a given point each second Measuring Waves • The unit for frequency is Hertz (Hz) • One Hz means that one wavelength passes by in one second • Hz = 1/s Measuring Waves • As frequency of a wave increases, wavelength decreases Measuring Waves Measuring Waves • Frequency is always equal to the rate of vibration of the source that creates it Measuring Waves Measuring Waves • Speed – how fast a wave travels Measuring Waves • Speed of a wave depends on the medium it is traveling through • Example: Sound waves generally travel faster in a material when the temperature of the material is greater Measuring Waves • Calculating Wave Speed: Speed (m/s) = Frequency (Hz) x Wavelength (m) or v=fx Measuring Waves Sample Problem What is the speed of a sound wave that has a wavelength of 2 m and a frequency of 170.5 Hz? Measuring Waves What information are you given? Wavelength = 2m Frequency = 170.5 Hz 1. Measuring Waves 2) What is the unknown you are trying to solve for? Speed (velocity) Measuring Waves 3) Write an equation that contains both the given quantities and the unknown variable. v=fx Measuring Waves Substitute in the known quantities and solve the equation. V= 170.5Hz x 2m 4) V =341 m/s Measuring Waves 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? What information are you given? Wavelength = 6m Frequency = .5 Hz What is the unknown you are trying to solve for? Speed (velocity) of wave Write an equation that contains both the given quantities and the unknown variable. v=fx Substitute in the known quantities and solve the equation. V= .5 Hz x 6m V= 3 m/s Measuring 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? Wavelength = 2.5 m Velocity = 4.0 m/s v=fx 4.0 m/s = f x 2.5m f= 4.0m/s 2.5m 1wave/1.6hz f= 1.6 Hz 1 wave passes every .63 seconds Measuring Waves 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? Frequency = 440 Hz Velocity = 350 m/s v=fx 350 m/s = 440 Hz x = 350 m/s 440Hz = .8 m Measuring Waves • Amplitude – related to the energy transferred by a wave Measuring Waves • The greater the wave’s amplitude, the more energy the wave transfers • Amplitude is measured differently for transverse and compressional waves Measuring Waves • Transverse Waves - distance from the crest or trough 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? Measuring Waves Measuring Waves • Compressional Waves - related to how tightly the medium is pushed together at the compressions • The denser the medium at the compressions, the higher the amplitude of the wave and the more energy that is transferred Measuring Waves Waves Q: What’s in a wave? A: Energy, energy, energy… ELECTROMAGNETIC WAVES Q: Remind me again of the basic properties of waves… A: Here’s a short summary on how waves work: ELECTROMAGNETIC WAVES • All waves are produced by something that vibrates • Waves transmit energy from one place to another ELECTROMAGNETIC WAVES • Some types of waves require a medium in which to travel, while other types of waves do not • Sound waves require air particles to travel through ELECTROMAGNETIC WAVES • Water waves must have water molecules • These waves travel because energy is transferred from particle to particle • Without matter, these waves could not move ELECTROMAGNETIC WAVES Electromagnetic waves do NOT require a medium to transfer energy – they can travel through space where no matter is present ELECTROMAGNETIC WAVES Q: How do electromagnetic waves transfer energy without matter? A: They use electric and magnetic fields. ELECTROMAGNETIC WAVES Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating electric and magnetic fields ELECTROMAGNETIC WAVES o Magnetic fields exist around all magnets, even if the space around the magnet contains no matter ELECTROMAGNETIC WAVES • Think about a paper clip being attracted to a magnet without the two even touching – this occurs because of the magnetic field around the magnet ELECTROMAGNETIC WAVES • Electric charges are surrounded by electric fields ELECTROMAGNETIC WAVES This allows the charges to exert forces on each other even when they are far apart ELECTROMAGNETIC WAVES o Electric charges are also surrounded by magnetic fields ELECTROMAGNETIC WAVES • It’s the motion of electrons that generates the magnetic field • An electric current flowing through a wire is surrounded by both an electric field, as well as a magnetic field ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES • A changing magnetic field creates a changing electric field and vice versa • When an electric charge vibrates back and forth, the electric field around it changes ELECTROMAGNETIC WAVES • Because the electric charge is in motion, it also has a magnetic field around it • This magnetic field also changes as the electric charge vibrates ELECTROMAGNETIC WAVES • But how does this become an electromagnetic wave? • The changing electric field around the charge creates a changing magnetic field, which in turn creates a changing electric field ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES • This process continues, with each creating the other • These vibrating electric and magnetic fields travel outward from the moving charge and vibrate at right angles to the direction the wave travels ELECTROMAGNETIC WAVES This makes an electromagnetic wave (which is also a transverse wave) ELECTROMAGNETIC WAVES Q: What are some properties of electromagnetic waves? A: Electromagnetic waves have speed, wavelength, and frequency like mechanical waves. ELECTROMAGNETIC WAVES • Wave Speed • All electromagnetic waves travel at 300,000 km/s in the vacuum of space • This is also known as the speed of light ELECTROMAGNETIC WAVES • Nothing travels faster than the speed of light in nature ELECTROMAGNETIC WAVES • When an electromagnetic wave travels through matter, this speed changes • The speed will depend on the material the wave is passing through ELECTROMAGNETIC WAVES • Electromagnetic waves usually travel most slowly through solids and fastest through gases ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES • Wavelength and Frequency • Like all waves, electromagnetic waves can be described by their wavelengths and frequencies ELECTROMAGNETIC WAVES Wavelength – Distance from one crest to another ELECTROMAGNETIC WAVES • Frequency – number of wavelengths that pass a given point in one second • Also can be described as the number of vibrations made by the electric charge in one second ELECTROMAGNETIC WAVES • The wavelength and frequency of an electromagnetic wave are related • As the frequency increases, the wavelength decreases and vice versa ELECTROMAGNETIC WAVES • Electromagnetic waves can behave as waves and as particles • These particles are referred to as photons ELECTROMAGNETIC WAVES • The amount of energy in a photon is dependent on the frequency of the wave (rather than the amplitude) ELECTROMAGNETIC WAVES • The higher the frequency, the greater the energy in the photon ELECTROMAGNETIC WAVES ELECTROMAGNETIC SPECTRUM Q: Now that I understand electromagnetic waves, what exactly is the electromagnetic spectrum? A: The entire range of electromagnetic wave frequencies. ELECTROMAGNETIC SPECTRUM • The electromagnetic spectrum is composed of 7 types of electromagnetic waves that interact with matter very differently ELECTROMAGNETIC SPECTRUM • They are separated from one another based on their wavelengths and frequencies ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • Radio Waves • Low-frequency electromagnetic waves with wavelengths longer than 1 mm ELECTROMAGNETIC SPECTRUM • Radio waves with wavelengths less than 30 cm are called microwaves ELECTROMAGNETIC SPECTRUM • Microwaves are best known for cooking food in microwave ovens • Microwaves are also used for communication in cellular phones and satellite signals ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • Radio waves also have other uses • To find the movement and position of objects using radar ELECTROMAGNETIC SPECTRUM • To take a picture of the bones and soft tissue on the inside of a patient’s body using MRI ELECTROMAGNETIC SPECTRUM • To carry audio signals from radio stations to your radio • But remember, you can’t hear a radio wave ELECTROMAGNETIC SPECTRUM • The audio signal that the radio wave carries is converted into sound through your radio • What you can hear is the sound wave coming from the radio as it moves through the air ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • Infrared Waves • Have higher frequencies than radio waves and lower frequencies than red light ELECTROMAGNETIC SPECTRUM • Wavelengths vary from 1 mm to 750 nm • 1 nanometer = 10-9 meters ELECTROMAGNETIC SPECTRUM • Often used as a source of heat • Red lamps in cafeterias keep food warm with infrared radiation ELECTROMAGNETIC SPECTRUM • Other uses include: • Controlling televisions through a remote control • Reading CDs via a computer or game console ELECTROMAGNETIC SPECTRUM • Detecting trapped victims or heat loss in a building through a thermogram ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • Visible Light • The range of electromagnetic waves that you can detect with your eyes ELECTROMAGNETIC SPECTRUM • Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular color • These range from long-wavelength red to short-wavelength violet ELECTROMAGNETIC SPECTRUM • If all the colors are present, you see the light as white ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • Ultraviolet Waves • Wavelengths vary from 400 nm to 4 nm ELECTROMAGNETIC SPECTRUM • Ultraviolet waves are energetic enough to enter skin cells • Overexposure can cause skin damage and cancer ELECTROMAGNETIC SPECTRUM • Some ultraviolet waves are helpful • Exposure allows the body to create vitamin D, which is needed for bones and teeth to absorb calcium ELECTROMAGNETIC SPECTRUM • 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 ELECTROMAGNETIC SPECTRUM • This is used by CSI technicians when looking for fingerprints or blood at a crime scene ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM • X-Rays • Very short wavelengths ranging from 12 nm to 0.005 nm ELECTROMAGNETIC SPECTRUM • Have high energy and can penetrate matter that light cannot ELECTROMAGNETIC SPECTRUM • Often used in medicine, industry, and transportation to make pictures of the inside of solid objects ELECTROMAGNETIC SPECTRUM • Gamma Rays • Have the shortest wavelength in the electromagnetic spectrum at about 0.005 nm or less ELECTROMAGNETIC SPECTRUM • Have the highest frequency and therefore the most energy and greatest penetrating ability of all the electromagnetic waves ELECTROMAGNETIC SPECTRUM • Exposure to small amounts of gamma rays are tolerable, but overexposure can be deadly ELECTROMAGNETIC SPECTRUM • Used in the medical field to kill cancer cells and make pictures of the brain based on brain activity ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC SPECTRUM