Sound and Light CHAPTER 15

All sound waves  Are caused by vibrations

All sound waves:  Are longitudinal waves

All sound waves:  Require a medium -- solid, liquid, or gas.

The speed of sound  Depends on the medium and temperature  Fastest in most solids (except ones like rubber that DAMPEN or weaken the sound waves -- these make good soundproofing materials)

The speed of sound is  Slowest in gases  The colder the gas, the slower the speed  Speed of sound in room temperature air: 346 m/s (760 mi/hour)

Why does sound travel fastest in solids?  The molecules are closer together than in a liquid or gas so they pass on the vibrations more quickly

Loudness of sound depends on wave intensity Intensity in turn is determined by the amplitude and distance from the source of the sound.

Intensity  Increases when amplitude increases  Decreases when amplitude decreases

Intensity increases as distance from source decreases Louder when closer because waves have spread out less

Loudness is measured in decibels  Logarithmic scale decibels is twice as loud as 60

Frequency determines pitch.  Higher frequency = higher pitch  Lower frequency = lower pitch

Beat Frequencies  Interference of two or more frequencies to form a new wave  Frequencies must be close to one another but not the same

Doppler effect  Frequency (and pitch) go up when source of sound is approaching because wavelength is shortened  Go down when source is retreating

Resonance  Tendency of an object to vibrate with larger and larger amplitudes  Examples  Opera singer breaking a glass  Pushing a kid on a swing  Running fingers on rim of a glass

Sonar  Measures distance by measuring time for sound wave to reflect off a surface, calculated distance using d = v x t

Range of human hearing  20Hz to 20,000 Hz

Ultrasound = Frequency too high for us to hear  Can be used in sonar systems  Above 20,000 Hz

Infrasound  Frequencies that are too low to be heard by humans  Below 20 Hz  Examples:  Earthquakes  Tornadoes  Storms at sea

Light  Sometimes acts like a wave  Sometimes acts like a particle

Light acts like a wave when it:  Reflects  Refracts  Produces interference

Light acts like a particle when it:  Travels without a medium  Delivers packets of energy (photons) to solar collectors or chlorophyll

The energy of light  Is proportional to its frequency  Gamma rays have the most energy because they have the highest frequency.

The speed of light  Depends on the medium  In a vacuum, the speed = 3 X 10 8 m/s. This speed is known as c (as in E=mc 2 ).

Light’s brightness depends on wave intensity.  Wave intensity decreases when amplitude decreases  Increases when amplitude increases

Intensity increases as distance from source decreases brighter when closer because waves have spread out less

Electromagnetic spectrum  Light at all possible energies, frequencies, and wavelengths  Entire spectrum given off by sun and other stars

Electromagnetic spectrum

Radio waves  Longest wavelength, least energy  Used in communication and radar

microwaves  Next longest waves  Used in cooking, communication

Infrared waves  Thermal energy -- heat

Visible light  Red is longest wavelength of visible light  Violet is shortest

Ultraviolet (UV) light  9% of sun’s energy  Can cause sunburn, cancer  More dangerous than visible light because it has a shorter wavelength

X-rays  Shorter wavelength than UV light, so more dangerous  Used in medicine, but can disrupt DNA so must limit exposure

Gamma rays  Shortest wavelengths, so most energy (and most dangerous)  Rays can be focused to kill cancer  Given off by radioactive materials

Reflection  Light bouncing off a surface

Rough surfaces scatter the light they reflect so they look dull, not shiny.

Smooth surfaces reflect the light in the same direction so they are shiny and can reflect an image.

Polarization  Filtering of light

Curved mirrors distort images, because different parts of the mirror reflect the light in different directions.

Objects have the color of the wavelength they reflect.

Fiber optics use internal reflection to transmit light along the length of the fibers.  This is more efficient than transmission through metal wires.

Refraction of Light Light waves can bend (refract) when they pass from one medium into another. The waves bend because the new medium changes the waves’ speed.

Refraction of light  The bending of light waves changes the apparent position of objects.

Because lenses diffract light, they can:  magnify images (microscope, magnifying glass)  focus images (eye, telescope).  Correct the poor focus of a near- or far- sighted eye (glasses).

Prisms separate light into its component colors. This happens because the different colors travel at different speeds and therefore bend different amounts. (Red is fastest and bends the least, so it appears at the top of a rainbow.)

In rainbows, water droplet act as prisms.