Sound and Light

CHAPTER 15

All sound waves

Are caused by vibrations

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All sound waves:

Are longitudinal waves

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All sound waves:

Require a medium -- solid, liquid, or gas.

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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)

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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.

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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

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Loudness is measured in decibels

Logarithmic scale --

70 decibels is twice

as loud as 60

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Frequency determines pitch.

Higher frequency = higher pitchLower frequency = lower pitch

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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

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Resonance

Tendency of an object to vibrate with larger and larger amplitudes

ExamplesOpera singer breaking a glassPushing a kid on a swingRunning 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

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Range of human hearing

20Hz to 20,000 Hz

Ultrasound = Frequency too high for us to hear

Can be used in sonar systemsAbove 20,000 Hz

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Infrasound

Frequencies that are too low to be heard by humans

Below 20 HzExamples:

EarthquakesTornadoesStorms at sea

Light

Sometimes acts

like a waveSometimes acts

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Light acts like a wave when it:Reflects

Refracts

Produces interference

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Light acts like a particle when it:Travels without a mediumDelivers packets of energy (photons) to

solar collectors or chlorophyll

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The energy of light

Is proportional to its frequencyGamma rays have the most energy

because they have the highest frequency.

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The speed of light

Depends on the mediumIn a vacuum, the speed = 3 X 108 m/s.

This speed is known as c

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Light’s brightness depends on wave intensity.

Wave intensity decreases when amplitude decreases

Increases when amplitude increases

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Intensity increases as distance from source decreases

brighter when closer because waves have spread out less

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Electromagnetic spectrum

Light at all possible energies, frequencies, and wavelengths

Entire spectrum given off by sun and other stars

Electromagnetic spectrum

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Radio waves

Longest wavelength, least energyUsed in communication and radar

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microwaves

Next longest wavesUsed in cooking, communication

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Infrared waves

Thermal energy -- heat

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Visible light

Red is longest wavelength of visible light

Violet is shortest

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Ultraviolet (UV) light

9% of sun’s energyCan cause sunburn,

cancerMore dangerous than

visible light because it has a shorter

wavelength

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X-rays

Shorter wavelength than UV light, so more dangerous

Used in medicine, but can disrupt DNA so must limit exposure

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Gamma raysShortest

wavelengths, so most energy (and most dangerous)

Rays can be focused to kill cancer

Given off by radioactive materials

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Reflection

Light bouncing off a surface

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Rough surfaces scatter the light they reflect

so they look dull, not shiny.

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Smooth surfaces reflect the light in the same direction

so they are shiny and can reflect an image.

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Polarization

Filtering of light

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

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Objects have the color of the wavelength they reflect.

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Fiber optics use internal reflection to

transmit light along the length of the fibers.

This is more efficient than transmission through metal wires.

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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.

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Refraction of light

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

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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).

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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.)

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In rainbows, water droplet act as prisms.

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