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Homework

• Turn in your homework at the beginning of class next lecture. It will be collected shortly after lecture starts.

• Put your homework in the appropriate alphabetized box on the right side of the back of the lecture hall (the right as I would see it from the front)

• Please consider coming to office hours before emailing us with homework questions.

Clickers

• Register your clicker at iclicker.com

• No exceptions if you are absent, forget your clicker, run out of batteries, etc.

• You do get more points for a correct answer, so try to figure it out!

Chapter 1, continued

• Electromagnetic waves– Is there a medium?– What is “wiggling”?– Electric and magnetic

fields

• EM Spectrum– Visible light– Radio waves– Microwaves– Xrays

• Resonance– Why do we only see some

EM waves?– Examples and demos

• Light sources– Incandescent bulbs– Fluorescent bulbs– Other sources

White Light through a Prism

A prism spreads out the over- lapping wavelengths in white light into different spatial locations where they can be seen as colors.

400 nm400 nm

700 nm700 nm

Clicker Question• Picture a rope with a pulse disturbance moving horizontally.

How does the distance and direction traveled by pieces of the rope compare to the distance and direction traveled by the pulse?

a) The rope piece travels more than the pulse, in the same direction

b) The rope piece travels more than the pulse, at a right angle to the pulse

c) The rope piece travels less than the pulse, in the same direction

d) The rope piece travels less than the pulse, at a right angle to the pulse

Clicker Question• Which is the correct light ray to describe how we

“see” the apple? (the color of the arrow is not relevant)

AA

BB

CCDDEE Remember the light Remember the light

has to come from the has to come from the bulb first, so B is not bulb first, so B is not correct.correct.C is the correct light rayC is the correct light ray

Electromagnetic (EM) Waves

• Light is part of the Electromagnetic Spectrum

• Unlike sound or rope waves, EM waves do not need a material to transmit them, they can travel through empty space

• So what is actually “wiggling” in an EM wave?

• Magnetic field linesMagnetic field lines

Electric and Magnetic Fields

• Static electricity

Electromagnetic Waves

• Made up of oscillating electric and magnetic fields

EM Waves: Key Points

• Waves can travel in empty space

• The oscillating fields are at right angles (perpendicular) to the direction of wave motion

Electromagnetic Spectrum

Electromagnetic Spectrum

Clicker Question

The speed of light is 3 x 108 m/sA radio signal takes 2.5 x 10-3 s (2.5 milliseconds) to travel from Boston to DC. What is the distance between these cities?a)1200 kmb)750 kmc)7500 kmd)500 kme)1000 km

distance =speed * time

3 x 108 m/s * 2.5 x 10-3 s

7.5 x 105 m

750 km

Images at different frequencies

RadarRadar

IRIR

UVUV White lightWhite light

XrayXray

Millimeter Wave Scanning

Waves and Resonance• Oscillating systems have a special frequency at which

they like to vibrate, called the resonant frequency

• External driving of these systems at the resonant frequency will result in the most efficient transfer of energy

• Ex: Swings, rocking your car out of a snowbank, washboard ripples on a dirt road, shattering a crystal glass, dye bleaching, light perception in your eye, washing machine

Example: Pendulum

• What happens when we change the length of the string?

• What happens when I change how fast I move my hand?

• The same process occurs in our eyes, which are not sensitive to frequencies outside what we call “visible light”

Effect of resonance produced by military helicopter blade going around at frequency resonant with the

helicopter body

What do resonances have to do with light?• When light is absorbed by atoms we can think of

this as a resonance– For example, we “see” light rays of 470 nm coming into

our eyes because this light excites a resonance in certain atoms inside our eyes

– EM waves with the wrong frequency do not excite the resonance, and we don’t “see” them.

• Light can also be emitted by atoms, which is also a form of resonance– This is how fluorescent lights, lasers and LEDs work.

Incandescent Light Bulbs

Incandescent Light Bulbs

Filament with current of electrons which hit into atoms causing light to be emitted

Atom

Electrons

Light emitted at many different resonance frequencies of atoms appears as white light

Atom

Atom

Incandescent Light Bulbs

• A A continuouscontinuous light source light source• Almost 90% of its emission is Almost 90% of its emission is

invisible to the human eye, invisible to the human eye, producing heat and wasting producing heat and wasting energyenergy

Clicker Question

The light from an ordinary incandescent light bulb appears white because

a) A current of electrons excites a resonance in atoms at the frequency of white light.

b) A current of electrons excites resonances in atoms at many different frequencies

c) A current of electrons wiggles at the frequency of white light

Fluorescent Light Bulbs

• Fluorescent bulbs have a lower current and power usage for the same light output in the visible range

• How do they do this?

The atoms inside a fluorescent bulb have ultraviolet resonant frequencies

Atom

Electrons

Invisible ultra-violet light

Atom

Atom

Phosphors

white light

Fluorescent Light Bulbs• Because the phosphors emit at very specific resonant

frequencies, the spectrum is not continuous

Incandescent vs. Fluorescent

Neon lights

• Produced the same way, but with a different set of atoms in the tube to produce the different colors.

Light Emitting Diodes (LEDs)

• A semiconductor systemA semiconductor system• Charges are initially separatedCharges are initially separated• An applied current pushes An applied current pushes

them “up the hill”, where they them “up the hill”, where they can recombine and emit lightcan recombine and emit light

Clicker Question

• What kind of bulbs do you think they use to keep food hot at a buffet bar?

a) Incandescentb) Fluorescentc) Neond) LEDs

Color Temperature• Light from ideal sources is generally a mixture of

different wavelengths– Think of the rainbow generated by sunlight– The spectra of different light bulbs

• The mixture of wavelengths can be understood by asking how bright is the mixture at each wavelength

• How do these different formulations change the color of the light?

The hotter the source the more bluish the white light. The cooler the source the more reddish the white light

These are curves of the intensity of each of the wavelengths present in an "off-white" light as the (color) temperature of the filament in the light bulb increases.

Color Temperature in Fluorescent LightsFluorescent light bulbs all Fluorescent light bulbs all feel feel the same temperature the same temperature when you touch them, but the phosphors can be when you touch them, but the phosphors can be selected to selected to simulatesimulate a specific color temperature. a specific color temperature.

From www.usa.phillips.com

Here is how a picture changes under lighting with different color temperatures

HigherHighercolor tempcolor tempmore bluishmore bluish

HigherHighercolor tempcolor tempmore bluishmore bluish

LowerLowercolor tempcolor tempmore reddishmore reddish

LowerLowercolor tempcolor tempmore reddishmore reddishYou can also change the color You can also change the color

temperature in imaging temperature in imaging processing softwareprocessing software

It's a good idea to remember some rough wavelengths associated with colors

• Violet and blue are what we see when shorter wavelength visible rays enter our eyes.– They have

relatively higher frequencies

• Red is how we see longer wavelength visible rays– Red has a relatively

smaller frequency