Waves Sound and Light Chapter 17 18 Mechanical

Waves, Sound and Light Chapter 17 & 18

Mechanical Waves • A mechanical wave is created when a source of energy causes a vibration to travel through a medium. –A mechanical wave is a disturbance in matter that carries energy from one place to another. • The material through which a wave travels is called a medium. • Mechanical waves require a medium to travel through. Solids, liquids, and gases all can act as mediums. • A vibration is a repeating back-and-forth motion.

Mechanical Waves • There are three types of mechanical waves – Transverse wave - a wave that causes the medium to vibrate at right angles to the direction in which the wave travels – Longitudinal wave – a wave in which the vibration of the medium is parallel to the direction the wave travels. – Surface wave - a wave that travels along a surface separating two media.

Mechanical Waves • Transverse waves • The highest point of the wave is the crest. • The lowest point of the wave is the trough. • A single point on the rope vibrates up and down between a crest and trough.

Mechanical Waves • A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels. • The wave carries energy from left to right, in a direction perpendicular to the up-and-down motion of the rope.

Mechanical Waves • Longitudinal Waves • An area where the particles in a medium are spaced close together is called a compression. • An area where the particles in a medium are spread out is called a rarefaction. A. B. A compression starts to move along the spring. A rarefaction follows the compression along the spring.

Properties of Mechanical Waves Any motion that repeats at regular intervals is called periodic motion. • Frequency ( f )– the number of complete cycles in a given time – Frequency is measured in cycles per second or hertz (Hz) – f = 1/T • Period (T)– the time required for one complete cycle • Wavelength (λ) – the distance between a point on one wave and the same point on the next cycle of the wave Note: Increasing the frequency of a wave decreases its wavelength – they are inverses of each other

Properties of Mechanical Waves • Wave Speed –When the wavelength is in meters, and the frequency is in hertz, the units for speed are meters per second. –The speed of a wave is also calculated by dividing its wavelength by its period. Note: v = λf = λ/T

Problem One end of a rope is vibrated to produce a wave with a wavelength of 0. 25 meter. The frequency of the wave is 3. 0 hertz. What is the speed of the wave?

Problem A wave on a rope has a wavelength of 2. 0 m and a frequency of 2. 0 Hz. What is the speed of the wave?

Problem A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a frequency of 4 Hz and a wavelength of 0. 1 m. What is the speed of the wave?

Problem What is the speed of a wave in a spring if it has a wavelength of 10 cm and a period of 0. 2 s?

Problem What is the wavelength of an earthquake wave if it has a speed of 5 km/s and a frequency of 10 Hz?

Properties of Mechanical Waves • The speed of a wave can change if it enters a new medium, or if variables such as pressure and temperature change. • For many kinds of waves, the speed of the waves is roughly constant for a range of different frequencies. • The wave with the lower frequency has a longer wavelength.

Properties of Mechanical Waves • Amplitude – The amplitude of a wave is the maximum displacement of the medium from its rest position. – The more energy the wave has, the greater the wave’s amplitude

Behavior of Waves • Reflection occurs when a wave bounces off a surface that it cannot pass through – Does not change the speed of the wave or frequency, but the wave can be flipped upside down • Refraction is the bending of a wave as it enters a new medium at an angle. – The speed of the wave changes so one side of the wave moves slower than the other side

Behavior of Waves • Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening • A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle

Behavior of Waves • Interference occurs when two or more waves overlap and combine together • Two types of interference: – Constructive = larger displacement – Destructive = smaller displacement

Behavior of waves • Standing wave – A wave that appears to stay in one place – doesn’t move through a medium Note: A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.

Behavior of Waves Standing Waves • A node is a point on a standing wave that has no displacement from the rest position. At the nodes, there is complete destructive interference between the incoming and reflected waves. • An antinode is a point where a crest or trough occurs midway between two nodes.

Properties of Sound Waves – longitudinal waves (undergo compression and rarefaction) • Speed – the speed of a sound wave varies in different media. – Effected by density, temperature, and pressure of the media

Properties of Sound Waves Intensity is the rate at which a wave’s energy flows through a given area. • Sound intensity depends on both the wave’s amplitude and the distance from the sound source. • The decibel (d. B) is a unit that compares the intensity of different sounds. For every 10 -decibel increase, the sound intensity increases tenfold. • A 0 -decibel sound can just barely be heard. • A 20 -decibel sound has 100 times more energy per second than a 0 -decibel sound. • A 30 -decibel sound delivers 1000 times more energy per second than a 0 -decibel sound.

Properties of Sound Waves Loudness is a physical response to the intensity of sound, modified by physical factors. • The loudness depends on sound intensity. • Loudness also depends on factors such as the health of your ears and how your brain interprets sound waves.

Properties of Sound Waves • Frequency and Pitch The frequency of a sound wave depends on how fast the source of the sound is vibrating. The air in the tubing of brass instruments forms a standing wave. Longer tubing makes a standing wave with a longer wavelength and a lower frequency. Pitch is the frequency of a sound as you perceive it. • High-frequency sounds have a high pitch, and lowfrequency sounds have a low pitch. • Pitch also depends on other factors such as your age and the health of your ears.

Properties of Sound Waves Ultrasound Most people hear sounds between 20 hertz and 20, 000 hertz. • Infrasound is sound at frequencies lower than most people can hear. • Ultrasound is sound at frequencies higher than most people hear.

Properties of Sound Waves The Doppler Effect • The Doppler effect is a change in sound frequency caused by motion of the sound source, motion of the listener, or both.

Electromagnetic (EM)Waves Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields. • Like mechanical waves, electromagnetic waves carry energy from place to place. • Electromagnetic waves differ from mechanical waves in how they are produced and how they travel. Electromagnetic waves are produced by constantly changing electric fields and magnetic fields. • An electric field in a region of space exerts electric forces on charged particles. Electric fields are produced by electrically charged particles and by changing magnetic fields. • A magnetic field in a region of space produces magnetic forces. Magnetic fields are produced by magnets, by changing electric fields, and by vibrating charges.

Electromagnetic (EM)Waves Electromagnetic waves are transverse waves because the fields are at right angles to the direction in which the wave travels.

Electromagnetic (EM)Waves Changing electric fields produce changing magnetic fields, and changing magnetic fields produce changing electric fields, so the fields regenerate each other. • Electromagnetic waves do not need a medium. • The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation. Note: speed of light in a vacuum (c) = 3 x 108 m/s

Electromagnetic (EM)Waves The speed of an electromagnetic wave is the product of its wavelength and its frequency. • The speed of electromagnetic waves in a vacuum is constant, so the wavelength is inversely proportional to the frequency. • As the wavelength increases, the frequency decreases.

Problem A radio station broadcasts a radio wave with a wavelength of 3. 0 meters. What is the frequency of the wave?

Problem A global positioning satellite transmits a radio wave with a wavelength of 19 cm. What is the frequency of the radio wave? (Hint: Convert the wavelength to meters before calculating the frequency. )

Problem The radio waves of a particular AM radio station vibrate 680, 000 times per second. What is the wavelength of the wave?

Problem Radio waves that vibrate 160, 000 times per second are used on some train lines for communications. If radio waves that vibrate half as many times per second were used instead, how would the wavelength change?

Electromagnetic (EM) Waves Wave-Particle Duality Interference pattern appears on screen. Card with two slits • Electromagnetic radiation with one behaves as both a wave Card slit Light source and a particle – Evidence of particles – shadows and photoelectric effect • Light is made of packets of energy called photons – Evidence of a wave – double slit experiment Bright bands show constructive interference. Light from single slit produces coherent light at second card. Dark bands show destructive interference.

Electromagnetic (EM) Waves Intensity is the rate at which a wave’s energy flows through a given unit of area. A wave model also explains how intensity decreases. • As waves travel away from the source, they pass through a larger and larger area. • The total energy does not change, so the wave’s intensity decreases.

Electromagnetic Spectrum The full range of frequencies of electromagnetic radiation is called the electromagnetic spectrum. • Visible light is the only part of the electromagnetic spectrum that you can see, but it is just a small part. • Each kind of wave is characterized by a range of wavelengths and frequencies. All of these waves have many useful applications. The electromagnetic spectrum consists of radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

Electromagnetic Spectrum

Electromagnetic Spectrum Radio waves have the longest wavelengths in the electromagnetic spectrum. Wavelengths range from 1 millimeter to as much as thousands of kilometers or longer. Radio waves also have the lowest frequencies in the spectrum— 300, 000 megahertz (MHz) or less.

Electromagnetic Spectrum The shortest-wavelength radio waves are called microwaves. Microwavelengths are from about 1 m to about 1 mm. • Frequencies vary from about 300 MHz to about 300, 000 MHz. • Microwaves cook and reheat food. Microwaves also carry cell phone conversations. The process works much like a radio broadcast. Radar The word radar is an acronym for radio detection and ranging. Radar technology uses a radio transmitter to send out short bursts of radio waves. • The waves reflect off the objects they encounter and bounce back toward where they came from. • The returning waves are then picked up by a radio receiver.

Electromagnetic Spectrum Infrared rays have higher frequencies than radio waves and lower frequencies than red light. Infrared wavelengths vary from about 1 millimeter to about 750 nanometers (10– 9 meter). Your skin senses infrared radiation as warmth. Restaurants use infrared lamps to keep foods warm. Warmer objects give off more infrared radiation than cooler objects.

Electromagnetic Spectrum The visible part of the electromagnetic spectrum is light that the human eye can see. Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular color.

Electromagnetic Spectrum Ultraviolet rays vary from about 400 nm to about 4 nm. • Some exposure to ultraviolet rays helps your skin produce vitamin D, which helps the body absorb calcium from foods. • Excessive exposure can cause sunburn, wrinkles, skin cancer, and eye damage. • Ultraviolet rays are used to kill microorganisms. In winter, plant nurseries use ultraviolet lights to help plants grow.

Electromagnetic Spectrum X-Rays X-rays have very short wavelengths, from about 12 nm to about 0. 005 nm. X-rays have high energy and can penetrate matter that light cannot. Too much exposure to X-rays can kill or damage living tissue.

Electromagnetic Spectrum Gamma rays have the shortest wavelengths in the electromagnetic spectrum, about 0. 005 nm or less. They have the highest frequencies, the most energy, and the greatest penetrating ability of all the electromagnetic waves. Exposure to tiny amounts of gamma rays is tolerable, but overexposure can be deadly.

Electromagnetic Spectrum There are two ways that signals are encoded for radio. • In amplitude modulation, the amplitude of the wave is varied. The frequency remains the same. AM radio stations broadcast by amplitude modulation. • In frequency modulation, the frequency of the wave is varied. The amplitude remains the same. FM stations broadcast by frequency modulation.

Behavior of Light Materials can be transparent, translucent, or opaque • A transparent material transmits light, which means it allows most of the light that strikes it to pass through it. • A translucent material scatters light. If you can see through a material, but the objects you see through it do not look clear or distinct, then the material is translucent. • An opaque material either absorbs or reflects all of the light that strikes it. Most materials are opaque.

Behavior of Light When light strikes a new medium, the light can be reflected, absorbed, or transmitted. Reflection • An image is a copy of an object formed by reflected (or refracted) waves of light. • Regular reflection occurs when parallel light waves strike a surface and reflect all in the same direction. • Diffuse reflection occurs when parallel light waves strike a rough, uneven surface and reflect in many different directions

Behavior of Light When light is transmitted it can be refracted, polarized, or scattered Refraction • A light wave can refract, or bend, when it passes at an angle from one medium into another. • Refraction makes underwater objects appear closer and larger than they really are. • Refraction can also make an object appear to break at the surface of the water. • Refraction can also sometimes cause a mirage, a false or distorted image

Behavior of Light Polarization • Light with waves that vibrate in only one plane is polarized light. Light reflecting from a nonmetallic flat surface, such as a window or the surface of a lake, can become polarized

Behavior of Light In scattering, light is redirected as it passes through a medium.

Color Sunlight is made up of all the colors of the visible spectrum. A prism separates white light into a visible spectrum. • When red light, with its longer wavelength, enters a glass prism, it slows down the least of all the colors. • Red light is bent the least. • Violet light is bent the most. The process in which white light separates into colors is called dispersion. A rainbow forms when droplets of water in the air act like prisms.

Color An object’s color is the color of light that reaches your eye when you look at the object. Primary colors are three specific colors that can be combined in varying amounts to create all possible colors. The primary colors of light are red, green, and blue Each secondary color of light is a combination of two primary colors. The secondary colors of light are cyan, yellow, and magenta. • If you add a primary color to the proper secondary color, you will get white light. • Two colors of light that combine to form white light are complementary colors of light. • A complementary color pair is a combination of one primary color and one secondary color

Color The primary colors of pigments are cyan, yellow, and magenta. A pigment is a material that absorbs some colors of light and reflects other colors. • Paints, inks, photographs, and dyes get their colors from pigments. • Color printers and photocopiers use three colors–cyan, magenta, and yellow– plus black. • You can mix varying amounts of these primary pigment colors to make almost any other color. Any two colors of pigments that combine to. make black pigment are complementary colors of pigments. • Cyan and magenta combine to form blue. • Cyan and yellow combine to form green. • Yellow and magenta combine to form red. • The secondary colors of pigments are red, green, and blue