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Physics 1230: Light and Color

Instructor: Joseph MaclennanCLASS 1 - Waves and theElectromagnetic Spectrum

http://www.colorado.edu/physics/phys1230

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Sources of Lightin Nature

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Sources of Lightin Nature

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TodaysTopics

Describing waves Light waves - the Electromagnetic Spectrum Speed of light

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Waves

We are familiar with waves in everyday life -

waves on a rope, waves in the ocean, sound waves,shock waves

x

Propagating waves- rope is medium

Propagating waves- air is medium

rope

these are calledmechanical waves

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More about Waves

A wave is a propagating disturbanceof an equilibriumstate - for example ripples on water, waves on a rope,light in vacuum etc.

The mediumdoes not have to move far - but thedisturbancemoves, often at some characteristic speed

(e.g. cork on water)

Wave on a rope

Light waves

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Waves on a Rope

http://phet.colorado.edu/simulations/

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Concept Question - Waves on a Rope

The direction of motion of the rope is -

A.Parallel to the propagation of the waveB.Perpendicular to the wave propagationC.Other

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Waves

Both the waves in the flag and the ocean waves are waves that you

can see - where we see the medium move. There are other kindsof waves that we cannot see, but which we experience every day.These waves are called electromagnetic waves.

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Sound Waves

Sound is also a type of wave that we cannot see. Like ocean waves,sound waves need a medium to travel through. Sound can travelthrough air because air is made of molecules. These molecules

carry the sound waves by bumping into each other, like Dominoesknocking each other over. Sound can travel through anything madeof molecules - even water! There is no sound in space becausethere are no molecules there to transmit the sound waves!

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Alien Poster

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Waves can be either Periodic or Aperiodic

Periodic wave

Aperiodicwave

Periodic wave

space

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Wavelength

For periodicwaves, we can identify a wave length,

lambda is the repeat distance of the wave

Periodic wave

Periodic wave

space

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Period and Frequency

For periodicwaves, we can identify a period, T, bymeasuring

the time taken for a complete disturbance to pass a given point -

The frequency, ,

(nu) is the inverse of theperiod i.e.

= 1/T

and is the number of times per second that an

oscillation occurs at any fixed point in space

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Wavelength of a WaveWhat is the wavelength of the red wave?

A) 1mB) 2mC) 3m

meters

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Period of a Wave

(sec)

What is the period of the green wave?

A) 1 secondB) 1.5 secondsC) 2 seconds

50 10

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Wavelength, Frequency, and Velocity

For periodicwaves, we can identify a speed, v, bySpeed = distance/time

Speed = Wavelength/Period

Speed = Wavelength x frequency

v =

So => c = or =c/ or =c/

So knowing the frequency, we can calculate the wavelength

Or knowing the wavelength, we can calculate the frequency

For light waves, the speed in air or vacuum is3 x 108 meters/sec

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Wavelength

What is the wavelength of this wave?A) 4 cmB) 8 cm/secondC) 8 cmD) 8 mE) None of above

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Frequency

What is the frequency of this wave?A) 2 cm/secondB) 8 cm/secondC) 4 secondsD) 2 cycles/secondE) 0.25 cycles/second

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Light Waves -

part of the EMSpectrum

Electromagnetic waves are unlike sound or rope waves because they

do not need molecules to travel. This means that electromagneticwaves can travel through air and solid materials - but they can alsotravel through empty space. This is why astronauts on spacewalksuse radios to communicate. Radio waves are a type ofelectromagnetic wave.

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Electromagnetic Waves

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/electromagnetic/index.html

EM waves are periodic, with speedgiven bySpeed = Wavelength x frequency

c =

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If watch train go by and 3 cars pass by sign post in one second andeach car is 10 meters long, how to calculate speed of train?(which eqn makes sense?)

a. Speed of train = (length of car)/(# of cars per second)b. Speed of train = (length of car) x (# of cars per second)c. Speed of train = (# of cars per second) / (length of car)b. Speed of train = (length of car) x (# of cars per second)

Speed of light = Wavelength of wave x Frequency OR c = Speed of light = 3 x 108 m/s

Wavelength of wave ~ length of carFrequency of wave ~ # of cars (oscillations) per second

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Visible light is only a small part of the electromagnetic spectrum.

Most of the electromagnetic spectrum is not accessible to us,unless we're aided by special detectors tuned to the desired

energies, much like our eyes are "tuned" to the energy of visiblelight.

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Scientific Notation

mega, written M, means 106: a 100 MHz FM station is generating waves at 108 Hz

milli, written m, means 10-3: 1/1000 of a meter is a millimeter, or 1 mm

micro, written , means 10-6: the wavelength of light is 0.5 micrometers = 0.5 m

nano, written n, means 10-9: the wavelength of green light is 500 nanometers = 500 nm

400 nm 500 nm 650 nm 0.4 m 0.5 m 0.65 m

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http://imagers.gsfc.nasa.gov/ems/visible.html

Waves - the Electromagnetic Spectrum

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Waves - the Electromagnetic Spectrum

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Electromagnetic Waves critical to life as we know it!Communications radio, TV, cell phones, portable phonesFood prep - microwavesVision (and Life!) visible lightAM radio 530 to 1600 kHz.FM is 88 to 108 MHz.TV is 54-206 MHz (each station gets 6 MHz band (Station 1, 54-60 MHz))Microwaves - same thing but few x 109 Hz (oscillations/s),Light - same thing but several x 10

14

HzAll are electromagnetic waves, but different frequencies:AM radio 530 to 1600 kHz.FM is 88 to 108 MHz.TV broadcasting is 54-206 MHz - each station gets 6 MHz

wide band (Station 1 54-60 MHz)Microwaves - same thing but few x 109 Hz (oscillaitons/s),Light - same thing but several x 1014 Hz

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What is a resonance?

Many objects oscillateorvibrateat special frequenciescalled resonant frequenciesorresonances

When these objects are hitor "shaken" by an externalagent at a frequency = totheir resonant frequencythey will oscillate at their

resonant frequency. Hand moving back and forth atsame frequency as pendulumsresonant frequency (or hit)

Tacoma narrows bridge in thewind

Car on a dirt road with regularbumps (washboard effect)

The oscillations of the objectare largest when the"shaking" occurs at theobjects resonant frequency. We then say that a resonance

has occurrede.g. girl on swing being pushed byher mother (mothers pushfrequency = swing frequency)

Energy is transferred froman external agent to theobject during resonance. Wineglass broken by an opera

singers voice

due to resonance between voicesound frequency and natural

frequency of wineglass

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What do resonances have to do

with light? When light is absorbed

by atoms we can thinkof this as a resonance The light frequency may

match a certain frequencyof resonant vibration in theatom.

When this happens, theenergy of the light istransferred to the atomand the light disappears.

For example, we see lightrays of 470 nm coming intoour eyes because this lightexcites a resonance incertain atoms inside oureyes

When light is emittedby atoms we can thinkof this as a resonance For example when an

electron hits an atom theatom can gain energy inthe form of resonances.

This energy in the atomcan then be released byanother resonant

interaction in which lightis emitted and the atomloses energy.

Each color of light emittedcorresponds to aparticular atomicresonance.

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Resonance

and theCreation of Light

Emission of light Absorption of light

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How does the light from a light bulb

depend on temperature? Light from ideal sources is

generally a mixtureofdifferent wavelengths

Think of the light fromthe sun, which is brokenup by a prism

(Such light is calledblack- body radiation)

The mixture ofwavelengths can beunderstood by asking howbright is the mixture ateach wavelength?

The result is a curvewhichpeaks at a certain wavelengthand falls off at higher orlower wavelengths

The hotter the source, thelower the wavelength atwhich the peak brightnessoccurs

Demo using incandescentbulb with controlled

current (and hencetemperature) This is important in movie-

making

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The hotter the source the bluer the white light.

The cooler the source the redder the white light

Black Body Radiation

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What can we see?

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Concept Question

What is the wavelength of red light?

A) 650 nmB) 0.650 mC) 6.5 mD)

65 mm

E) none of the above

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Concept Question

X-ray wavelengths are

A) longerB) shorterC) same

than/as visible light?

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How can we "see" using the Infrared?

Since the primary source of infrared radiation is heat or thermal radiation, anyobject which has a temperature radiates in the infrared. Even objects that we

think of as being very cold, such as an ice cube, emit infrared. When an objectis not quite hot enough to radiate visible light, it will emit most of its energy inthe infrared. For example, hot charcoal may not give off light but it does emitinfrared radiation which we feel as heat. The warmer the object, the moreinfrared radiation it emits.

Humans, at normal body temperature, radiate most strongly in the infrared at awavelength of about 10 microns.

http://imagers.gsfc.nasa.gov/ems/infrared.html

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Ultraviolet Light

Ultraviolet (UV) lighthas shorter wavelengthsthan visible light.Though these waves areinvisible to the humaneye, some insects, like

bumblebees, can seethem!

http://imagers.gsfc.nasa.gov/ems/

Our Sun emits light at all thedifferent wavelengths inelectromagnetic spectrum, but it

is ultraviolet waves that areresponsible for causing sunburns.At right is a satellite image ofthe Sun taken at an ExtremeUltraviolet wavelength (171Angstroms).

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X-Rays

X-rays were first observed and

documented in 1895 by WilhelmConrad Rntgen, a Germanscientist who found them quiteby accident when experimentingwith vacuum tubes.

A week later, he took an X-rayphotograph of his wife's handwhich clearly revealed herwedding ring and her bones. Thephotograph electrified thegeneral public and aroused great

scientific interest in the newform of radiation. Rntgen calledit "X" to indicate it was anunknown type of radiation. Theyare still often referred to asRntgen rays in German-speaking

countries. http://imagers.gsfc.nasa.gov/ems/

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Lets check the relationship betweenwavelength and frequency

Your 101.5 FM radio station broadcasts at a frequency around101.5 MHz = 101.5 x 106 cycles/second

=> wavelength = velocity /frequency= (3 x 108)/(101.5 x 106) 3 meters

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Lets check the relationship betweenwavelength and frequency

Your 1490 AM radio station broadcasts at a frequency of1490 kHz = 1490 x 103 cycles/second

What is the wavelength? A) 200 meters B) 2 meters C) 2 mm

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1. If increase amplitude of an electromagnetic wave, wavewill get from a transmitter to a receiver -a. sooner than small amplitude wave, b. same time, c. faster

ans. b. Since the speed of light is the same at c, if the wavelengthincreases, the frequency MUST decrease

2. If increase wavelength of a wave, it will -a. move up and down with higher frequencyb.move up and down with lower frequencyc. frequency stays the samee. Speed stays the same

ans. b. ALL electromagnetic waves travel at c = speed of light.

Concept Questions

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Speed of Light - Jupiter's Moon Io

(see http://www.phys.virginia.edu/classes/109N/lectures/spedlite.html)

1676, Ole Rmer, a Danish astronomer,working at the Paris Observatory. He hadmade a systematic study of Io, one of themoons of Jupiter, which was eclipsed byJupiter at regular intervals, as Io wentaround Jupiter in a circular orbit at asteady rate.

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Michelson Measures the Speed of Light (1875)(see http://www.phys.virginia.edu/classes/109N/lectures/spedlite.html)

Michelson in 1875 is commissioned and becomes an instructor in physics andchemistry at the Naval Academy.

Lecture demonstrations had just been introduced at Annapolis and it was suggestedthat it would be a good demonstration to measure the speed of light by Foucault'smethod. Michelson soon realized, on putting together the apparatus, that he couldredesign it for much greater accuracy. Instead of Foucault's 60 feet to the farmirror, Michelson had about 2,000 feet along the bank of the Severn, a distance he

measured to one tenth of an inch. He invested in very high quality lenses and mirrorsto focus and reflect the beam.

His final result was 186,355 miles persecond, with possible error of 30miles per second or so. This wastwenty times more accurate than

Foucault, made the New York Times,and Michelson was famous while stillin his twenties. In fact, this wasaccepted as the most accuratemeasurement of the speed of lightfor the next forty years, at whichpoint Michelson measured it again.

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Measuring the speed of light(Michelson, 1926)

Rotating mirror

Reflecting mirror

Telescope

22 miles

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Measuring the speed of light

Rotatingmirror

ReflectingmirrorTelescope

22 miles

At proper speed, mirror moved 1/8 turn during the 44 mile path of light,and therefore the return light was visible through the telescope. Thishappened at 530 revolutions per second.

=> Speed = distance/time = 44 miles/(1/8 x 1/530) seconds

= 186,000 miles/second

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Speed of Light as a Constant since 1983 The speed of light in the

vacuum of free space is animportant physical constantusually denoted by the symbol

c. The metre is defined suchthat the speed of light in freespace is exactly 299,792,458metres per second (m/s).

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The Globe