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Chapter 15: Sound and Light

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Chapter 15: Sound and Light Section 1: Sound Section 2: The Nature of Light Section 3: Reflection and Color Section 4: Refraction, Lenses, and Prisms Sound Key Terms: Sound Wave Pitch Infrasound Ultrasound Resonance Sonar Sound Properties of Sound Sound wave are longitudinal waves, in which particles of air vibrate in the same direction the wave travels. Sound waves have compressions and rarefractions. Sound waves may be produced differently, but in all cases a vibrating object sets the medium around it in motion. Sound The Speed of sound depends on the medium (pg 491 Table 1) The speed of sound in a particular medium depends on how well the particles can transmit the compressions and rarefractions. Sound waves travel faster in solids and liquids than through gases. However, some solids, such as rubber; dampen vibrations so that sound does not travel well. (Soundproofing) Sound Loudness is determined by intensity The loudness of a sound depends partly on the energy contained in the sound wave. Intensity describes the rate at which a sound wave transmits energy though a given area of the medium. It depends on the amplitude of the sound wave as well as the distance from the source. Sound A sound with twice the intensity of another sound does not seem twice as loud. For a sound to seem twice as loud the intensity would have to be 10 times the intensity of another sound. Relative intensity is found by comparing the intensity of a sound with the intensity of the quietest sound a human can hear; the threshold of hearing. It is measured in decibels (dB) 0 dB Threshold of Hearing 120 dB Threshold of Pain Sound Pitch is determined by frequency The pitch of a sound is related to the frequency of sound waves. Higher-pitch = higher frequency (Shorter Wavelengths) Lower-pitch = lower frequency (Longer Wavelenghts) Sound Humans hear sound waves in a limited frequency range. Humans hear sound waves with a frequency between 20 Hz and 20,000 Hz Below 20 Hz is call infrasound Above 20,000 Hz is called ultrasound Sound Musical Instruments Musical instruments rely on standing waves By changing the length of the standing wave the frequency will change. The primary standing wave has a wavelength that is twice the length of the string or column. This is the fundamental frequency Sound Harmonic give every instrument a unique sound Harmonic is the fundamental frequency and a certain whole-number multiples of that frequency. Every musical instrument has a characteristic sound quality resulting from the mixture of harmonics. Instruments use resonance to amplify sound Sound Resonance is a phenomenon that occurs when two objects naturally vibrate at the same frequency These two vibrations are the natural frequency an instrument has plus a forced vibration. Natural frequency depends on the shape, size, mass, and material an object is make of. Sound Hearing and the Ear The human ear is a very sensitive organ that senses vibrations in the air, amplifies them, and then transmits signals to the brain. Vibrations pass through three regions of the ear Outer, Middle, and Inner (pg 496 Figure 7) Sound Sound waves pass though the ear canal and strike the eardrum, they cause the eardrum to vibrate. These vibrations pass from the eardrum through the three small bones of the middle ear (hammer, anvil, and stirrup). When the vibrations reach the stirrup, the stirrup strikes a membrane at the opening of the inner ear, sending waves through the cochlea. Resonance occurs in the inner ear Sound The cochlea contains a long, flexible membrane called the basilar membrane. Different parts of the basilar membrane vibrate at different natural frequencies. A wave of a particular frequency cause only a small portion of the basilar membrane to vibrate. Sound anvil - (also called the incus) a tiny bone that passes vibrations from the hammer to the stirrup. cochlea - a spiral-shaped, fluid-filled inner ear structure; it is lined with cilia (tiny hairs) that move when vibrated and cause a nerve impulse to form. eardrum - (also called the tympanic membrane) a thin membrane that vibrates when sound waves reach it. hammer - (also called the malleus) a tiny bone that passes vibrations from the eardrum to the anvil. Sound nerves - these carry electro-chemical signals from the inner ear (the cochlea) to the brain. outer ear canal - the tube through which sound travels to the eardrum. stirrup - (also called the stapes) a tiny, U-shaped bone that passes vibrations from the stirrup to the cochlea. This is the smallest bone in the human body (it is 0.25 to 0.33 cm long). Sound Sound Ultrasound and Sonar Sonar is used for underwater location Sonar – Sound Navigation and Ranging – is a system that uses acoustic signals and echo returns to determine the location of objects or to communicate. Ultrasound is above 20,000 Hz Sound Ultrasound imaging is used in medicine Echoes of very high frequency ultrasound waves, between 1 million and 15 billion Hz are used to produce computerized images called sonograms Some ultrasound waves are reflected at boundaries The Nature of Light Key Terms Photon Intensity Radar The Nature of Light Waves and Particles Two models of Light 1. Waves 2. Steam of Particles Light produces interference patterns like water waves The Nature of Light 1801 Thomas Young devised an experiment to test the nature of light. He was able show that light produces a striped pattern similar the pattern caused by water waves. Light can be modeled as a wave Young concluded that light must consist of waves. The model describes light as transverse waves that do not require a medium to travel. The Nature of Light Light waves consist of electric and magnetic fields. Because of this they are called electromagnetic waves. We can describe transverse waves by amplitude, wavelength, and frequency. The wave model also explains why light may reflect, refract, or diffract when it meets and object. Light wave can also interfere with one another to produce standing waves. The Nature of Light The wave model of light cannot explain some observations One example is when light strikes a piece of metal electrons may fly off the metal’s surface. According to the wave model, very bright red light should have more energy then dim blue light because in bright light waves should have a greater amplitude. The Nature of Light Light can be modeled as a stream of particles Energy from light is contained in small packets. A packet of blue light carries more energy than a packet of red light. In the particle model of light, these packets are called photons, and a beam of light is considered t be a stream of photons. The Nature of Light Photons do not have mass; they are more like little bundles of energy. The model of light used depends on the situation The energy of light is proportional to frequency Higher the frequency more energy Lower the frequency less energy The Nature of Light The speed of light depends on the medium (pg 501 Table 2) In a vacuum, all light travels at the same speed, called the speed of light 3 x 108 m/s (186,000 mi/s). Light is the fastest signal in the universe. When light travels through a medium its speed slows down. The Nature of Light The brightness of light depends on intensity Intensity is the rate at which energy flows through a given area of space. Like the intensity of sound, the intensity of light source decreases as the light spreads out in spherical wave fronts. The Nature of Light The Electromagnetic Spectrum We can detect light from 400nm (violet) to 700nm (red) This is the visible spectrum and only makes up a small part of the electromagnetic spectrum The spectrum consists of light at al possible energies, frequencies, and wavelengths. The Nature of Light Many modern tools take advantage of the different properties of electromagnetic waves. (Radar guns to cancer treatment) Sunlight contains ultraviolet light (UV) UV light has higher energy and shorter wavelengths than visible light. The Nature of Light X rays and gamma rays are used in medicine X rays have wavelengths less than 10-8 m and gamma rays have wavelengths as short as 10-14 m. Because both x rays and gamma rays have very high energies, they may kill living cells or turn them into cancer cells. The Nature of Light Infrared light can be felt as warmth (IR) Microwaves are used in cooking and communication Microwave ovens use waves with a frequency of 2450 MHz (12.2 cm wavelenght). The Nature of Light Radio waves are used in communications and radar Radar – Radio Detection and Ranging – a system that uses reflected radio waves to determine the velocity and location of objects. Reflection and Color Reflection of Light A light ray is an imaginary line running in the direction that the light travels. Rough surfaces reflect light in many directions The reflection of light off a rough surface is called diffused reflection Reflection and Color Smooth surfaces reflect light rays in one direction The reflected light is reflected off a surface at the same angle that the incoming light struck the surface. Law of Reflection The angle of incidence equals the angle of reflection. The point where light hits the surface is called the normal. The normal forms a perpendicular line to the surface Reflection and Color Mirrors Flat mirrors create virtual images. an image that forms at a location from which light rays appear to come but don not actually come. Behind the mirror. Curved mirrors can distort images Reflection and Color Because the surface of a curved mirror is not flat, the line perpendicular to the mirror (the normal) points in many directions.
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