© 2001-2005 shannon w. helzer. all rights reserved. unit 16 & 17 waves (light & sound)

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Unit 16 & 17Waves (Light & Sound)


Rotations

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As the Wheel Turns Watch how the sine function (which demonstrates a wave) traces

out as a wheel turns. The vertical axis represents horizontal position and the horizontal

axis represents time.

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Oscillation Graphs When these oscillations between two extremes are graphed wrt time,

we see the following profile emerge.

The wavelength () is the distance between the same position on consecutive “humps.”

The Amplitude (A) is the maximum displacement from zero.

Simple Harmonic Motion

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Simple Harmonic Motion The same pattern is traced out with a swinging pendulum. The motion of the wheel and the pendulum traces out a pattern

known as a Sine Wave. The motion itself is known as Simple Harmonic Motion.

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Waves When these oscillations between two extremes are graphed wrt time, we see the

following profile emerge.

The Wavelength () is the distance from the “same” point on two consecutive oscillations.

The Amplitude (A) is the maximum displacement from zero.

The Period (T) is the time between the same position on consecutive “humps.”

The Frequency (f) describes how often an oscillation occurs.

The high points on the wave are known as “crests.”

The low points on the wave are known as “troughs.”

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Interference of Sine Waves When two or more waves occur in close proximity to one another, they produce

interference patterns. Constructive Interference - If two waves interact with each other and produce a wave

that has a larger amplitude than the original waves, then we say that these waves completely or partially Reinforce each other.

Destructive Interference - If two waves interact with each other producing a wave that has a smaller amplitude than the original waves, then we say that these waves completely or partially cancel each other.

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Longitudinal Waves• A longitudinal wave is one in which the individual atoms or particles

vibrate in a direction parallel to the direction of motion of the wave.

• Notice how the atom in the box below never leaves the box even though the wave is obviously traveling to the right.

• This observation is the key characteristic of a longitudinal wave.

Animation courtesy of Dr. Dan Russell, Kettering University


Transverse Mechanical Waves• A transverse wave is one in which the individual atoms or particles

vibrate in a direction perpendicular to the direction of motion of the wave.

• Notice how the atoms in the box below never leave the box even though the wave is obviously traveling to the right.

• This observation is the key characteristic of a transverse wave.

Animation courtesy of Dr. Dan Russell, Kettering University


Light Waves Light travels in waves known as “Transverse” waves. A wave in which the motion of the medium is at right angles to the

direction in which the wave travels. Identical to the Sine Waves already discussed. Interferes in exactly the same manner.

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More About Light Previously, we learned that light travels in waves known as

“Transverse” waves. Light is composed of massless particles known as photons. The spectrum of electromagnetic radiation spans several

wavelengths (frequencies) of the photons. Of particular interest to us is the visible spectrum. To remember the visible spectrum, use the name “ROY G BIV.”

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VisibleInfrared Ultra Violet GammaRadio Waves Microwaves X-rays

Red

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Frequency

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Speed, Wavelength, and Frequency Each color of light has a different wavelength and frequency. The frequency of light increases as we move to the right in the

“ROYGBIV” spectrum. However, the speed increases as we move to the left in the

“ROYGBIV” spectrum. There is a mathematical relationship between the speed and

frequency of light.

In this equation v is the speed of the light wave, is the wavelength of the light, and f is the frequency of the light.

1

Red

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False Colors There are two “colors” that are not really colors. Which ones? White reflects all colors. Black absorbs all colors. If we placed a white and a black cylinder on a table and shined blue

light on them, then what color would they appear to be? Why?

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Color Filters If we shine light on a white surface, then white light will reflect back to the eye. Indeed, any color of light shined on a white surface will return to the eye because white reflects

all colors. If we shine white light on a colored surface, then the eye would see only the color corresponding

to the color of the surface because all other colors would be absorbed by the material. What would the eye see if we shined red light on a green screen? The eye would see black because the red would be absorbed by the green material and no light

would be reflected.

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Color Filters You have color filters on many devices in your every day life. Some examples are car taillights, stop lights, Light Bright, and retail signs. Color filters absorb every color of the visible spectrum except that which matches

the color of the filter. When these filters absorb the other colors, they heat up. When you look at your taillights, they appear red. When you touch them, they

are warm.

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Color Filters Of the three colors shown, which one will the eye see? Why? Now suppose we change the color of the filter. Of the three colors shown, which one will the eye see? Why?

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Filter


Polarization Recall, light travels as a transverse wave. Light is directional. Now imagine that the light is traveling towards you instead of past

you. There would be several directional orientations of light. These waves are polarized in the directions shown.

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Polarization Sometimes it is desirable to block all but one orientation of

incoming light. For instance, you can wear polarizing sun glasses in order to reduce

glare. Think “Stars and Bars” in order to understand how polarization

works.

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Stars and Bars Imagine you are behind bars. A Ninja comes along and decides to throw stars at you. Are you safe

from his stars? As you saw, the first two stars would not hit you. Why? Were you safe from the third star? Why? The first star was horizontally polarized. The second was “other” polarized. Therefore, they could not

make it through the vertical bars. The third star was vertically polarized; therefore, it was

able to pass through the vertical bars and nail you!

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Polarization Watch the animations below. Which wave is horizontally

polarized? Why? Which one is vertically polarized? Why?

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Polarization Watch the animations below. Which wave is horizontally

polarized? Why? Which one is vertically polarized? Why?

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Cross Polarization Watch the wave below. What is its polarization? Why? Would it be able to pass through if we rotated one polarizer by 90

degrees? Why? Explain what happened.

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Cross Polarization Polarizing filters can be used to demonstrate cross polarization. Watch what happens as we first align the polarizers and then

gradually rotate one of them.

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Sound Waves

Sound travels in waves known as “Longitudinal” Waves. A wave in which the particles in the medium move back and forth in the

same direction in which the wave travels. The picture below shows a Longitudinal wave formed in air molecules due

to a sound. The higher density areas of a Longitudinal wave are known as

compressions. The lower density areas of a Longitudinal wave are known as rarefactions.

When studying interference in sound waves, treat the compressions as

crests and the rarefactions as troughs.

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Fluorescent Light Bulbs Label the following parts of the cutaway view of a fluorescent bulb shown below:

glass tube, mercury blob, contact pins, electrode, and the inert gas. Here is the way a fluorescent bulb works. Electric switch moved from “off” to “on” position heating an electrode (filament)

which heats the inside of the bulb. Due to the added energy, the Mercury changes from a liquid to a gas. Due to a large voltage difference in the bulb, electrons migrate from one end of

the tube to the other. Electrons and mercury gas collide bumping the electrons to higher energy levels. Electrons return to lower energy states releasing light photons. Thousands of these photons are released simultaneously causing the bulb to

produce light.

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Laser Operation LASER stands for Light Amplification by the Stimulated Emission of Radiation. Lasers function in the following matter. A light pump excites electrons in a ruby crystal to higher energy levels. These excited electrons deexcite releasing photons that oscillate between a mirror and a

partially silvered mirror. Some of these photons stimulate other electrons to excited levels, and these electrons

deexcite emitting additional photons. The process continues until the light intensity is bright enough to shine through the

partially silvered mirror. A beam of light emerges from the laser as a laser beam.

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Sound Waves A good approximation for a sound wave (a

Longitudinal wave) can be the wave produced by plucking a slinky.

Notice how the wave travels in the plane of the slinky.

This wave was a reflecting Longitudinal wave.

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Sound Waves Sound is caused by the vibration of material objects.

If the drummer below hits the base drum once, then she will produce a single large wave front.

This wave carries the sound to the conductor as shown below.

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Sound Waves A vibrating object sends one pulse for every vibration.

Once the cymbals below are crashed together, they will vibrate and send many pulses to the listener’s ear.

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Sound Waves Each pulse (crest) of a longitudinal wave is caused by the “outward”

vibration of the cymbal. The same is true for a tuning fork.

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Sound Waves - Resonance Resonance occurs when the vibration of one object

causes the subsequent vibration of another object. If two tuning forks are tuned to the same musical

note, then striking one of the forks causes the second one to vibrate.

This phenomena in known as resonance.

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Sound Wave Destructive Interference Sound waves, like light waves, also interfere. When two identical tuning forks are struck in a

certain way, their waves will cancel each other out.

This phenomena demonstrates destructive interference.

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Sound Wave Constructive Interference When two identical tuning forks are struck at

exactly the same time, their waves will reinforce each other producing a louder sound.

This phenomena demonstrates constructive interference.

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Sound Wave Interference: Beats When two similar tuning forks are struck at exactly the

same time, their waves will periodically reinforce and cancel each other producing a louder sound in a rhythmic pattern.

This pattern is known as Beats. Constructive interference is shown in red and destructive interference is shown in blue.

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Density Variations in Sound Sound travels faster in a more dense medium than in a less dense one.

Why? You can hear sound in water better than you can in air.

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Periodic Waves and Wave Speed• Now instead of just twitching the string and sending one pulse, suppose PhysicsBot repetitively shook the string

sending multiple waves down the string’s length.• The pattern formed is known as a periodic wave.• Notice how this wave looks like a sine or cosine wave.• As a result, this wave type is also known as a sinusoidal wave.• Recall that the wavelength () of a wave is the distance between repeating units of a wave pattern.• Also recall that the period (T) of oscillation of the wavelength of a wave is the time required to complete one

oscillation.• Since speed is distance divided by time, we can determine the speed of the wave by dividing the wavelength by

the period.

vT

v f


Sound Wave Interference: Beats a

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© 2001-2005 shannon w. helzer. all rights reserved. unit 16 & 17 waves (light & sound)

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