light waves what we call light is a small portion of the electromagnetic spectrum what we call light...

Light WavesLight Waves

What we call light is a small portion What we call light is a small portion of the of the electromagnetic spectrumelectromagnetic spectrum

All the different colors are All the different colors are electromagnetic waves with different electromagnetic waves with different wave lengthswave lengths

Wave speed = frequency x Wave speed = frequency x wavelengthwavelength

EM radiation is alternating electric EM radiation is alternating electric and magnetic fieldsand magnetic fields

EM SpectrumEM Spectrum

long

low f

low energ

y

short

high f

high energ

y

Electromagnetic WavesElectromagnetic Waves

Changing magnetic/electric field Changing magnetic/electric field induces electric/magnetic fieldinduces electric/magnetic field

Electromagnetic Electromagnetic SpectrumSpectrum

Remember, the lower the frequency Remember, the lower the frequency the longer the wavelength and vice the longer the wavelength and vice versaversa

Wave speed of light is 300,000,000 Wave speed of light is 300,000,000 m/sm/s

Visible LightVisible Light

Visible LightVisible Light small part of small part of

the spectrum the spectrum we can seewe can see

ROY G. BIV - ROY G. BIV - colors in colors in order of order of increasing increasing energyenergy

R O Y G. B I V

red orange yellow green blue indigo violet

ColorsColors

Lowest frequencies of visible light Lowest frequencies of visible light are redare red

Highest frequencies are violetHighest frequencies are violet The order is red, orange, yellow, The order is red, orange, yellow,

green, blue, indigo, violetgreen, blue, indigo, violet

ColorsColors

Colored objects are a result of Colored objects are a result of selective reflection and absorption of selective reflection and absorption of lightlight

Sunlight is a mixture of colorsSunlight is a mixture of colors We call this mixture white lightWe call this mixture white light When white light strikes a red object, When white light strikes a red object,

the red frequencies are reflected and the red frequencies are reflected and the other frequencies are absorbedthe other frequencies are absorbed

SunlightSunlight

ColorsColors

White objects reflect all frequenciesWhite objects reflect all frequencies Black objects absorb all frequenciesBlack objects absorb all frequencies Objects can only reflect the colors of Objects can only reflect the colors of

light that shine on themlight that shine on them If you shine blue light on a red If you shine blue light on a red

object it will look blackobject it will look black

ColorsColors

We can also consider shining light We can also consider shining light through semi-transparent objectsthrough semi-transparent objects

Filters used on the lights in a theater Filters used on the lights in a theater are materials that absorb some are materials that absorb some frequencies and let other frequencies and let other frequencies pass throughfrequencies pass through

Red filters transmit red light through Red filters transmit red light through the material while other frequencies the material while other frequencies are absorbedare absorbed

Mixing ColorsMixing Colors

Our eyes and brains are very Our eyes and brains are very complicated color processing complicated color processing sensors and computerssensors and computers

Artists have learned how to mix Artists have learned how to mix colors to make pleasing colors to make pleasing combinations and to create a huge combinations and to create a huge array of different colorsarray of different colors

All based on what the sensors in our All based on what the sensors in our eyes respond toeyes respond to

Visual ResponseVisual Response


Primary Primary light colorslight colors red, green, bluered, green, blue additive colorsadditive colors combine to form combine to form

white lightwhite light EXEX: computer RGBs: computer RGBs


FilterFilter transparent transparent

material that material that absorbs all absorbs all light colors light colors except the except the filter colorfilter color


PigmentPigment colored material that absorbs colored material that absorbs

and reflects different colorsand reflects different colors

Primary pigment colorsPrimary pigment colors cyan, magenta, yellowcyan, magenta, yellow subtractive colorssubtractive colors combine to form blackcombine to form black

EXEX: color ink cartridges: color ink cartridges


Light Pigment

When mixing pigments, the color of the mixture is the color of light that both

pigments reflect.

Light and MatterLight and Matter

OpaqueOpaque absorbs or reflects all lightabsorbs or reflects all light

TransparentTransparent allows light to pass through allows light to pass through

completelycompletely

TranslucentTranslucent allows some light to pass throughallows some light to pass through

Selective ScatteringSelective Scattering

This is why the sky is blue!This is why the sky is blue! The higher frequencies are scattered The higher frequencies are scattered

more by the molecules in the more by the molecules in the atmosphereatmosphere

Reds and oranges just pass through, Reds and oranges just pass through, so the sky appears blueso the sky appears blue

Blue Sky & Red SunsetsBlue Sky & Red Sunsets

NOON• less atmosphere• less scattering• blue sky, yellow

sun

SUNSET• more atmosphere• more scattering• orange-red sky & sun

• Molecules in atmosphere scatter light rays.

• Shorter wavelengths (blue, violet) are scattered more easily.

DiffractionDiffraction

Waves bend when they encounter an Waves bend when they encounter an objectobject

Stand in the water and watch the wake Stand in the water and watch the wake from a boat hit youfrom a boat hit you

Look behind you and you will see that Look behind you and you will see that after a short distance, the wake after a short distance, the wake continues oncontinues on

It filled in the hole by bending around It filled in the hole by bending around youyou

This bending is called This bending is called diffractiondiffraction


DiffractionDiffraction bending of waves bending of waves

around a barrieraround a barrier

longer longer wavelengths (red) wavelengths (red) bend more - bend more - opposite of opposite of refractionrefraction


Diffraction is the third way to bend Diffraction is the third way to bend lightlight

The other two are reflection and The other two are reflection and refractionrefraction

The amount of bending that occurs The amount of bending that occurs depends on the relative sizes of the depends on the relative sizes of the object and the wavelength of the object and the wavelength of the wavewave

Longer wavelengths bend easier than Longer wavelengths bend easier than short onesshort ones

InterferenceInterference

InterferenceInterference constructive constructive brighter light brighter light destructive destructive dimmer light dimmer light


When the waves are hitting the When the waves are hitting the edges of something, the new edges of something, the new bending waves tend to interfere bending waves tend to interfere with each other and we get some with each other and we get some new patternsnew patterns

Recall the principle of Recall the principle of superpositionsuperposition

We simply add the amplitudesWe simply add the amplitudes


Wave nature of light was Wave nature of light was demonstrated by Youngdemonstrated by Young


The bright and dark areas result The bright and dark areas result from differences in path lengths from differences in path lengths from the slits to the screenfrom the slits to the screen

This changes the where the This changes the where the peaks and troughs appearpeaks and troughs appear

Remember the principle of Remember the principle of superpositionsuperposition


Can get interference from a single Can get interference from a single slitslit

Waves coming through one side of Waves coming through one side of the slit interfere with waves coming the slit interfere with waves coming through the other sidethrough the other side

Extend the idea to three, four… slitsExtend the idea to three, four… slits Make something with hundreds of Make something with hundreds of

slitsslits Called a diffraction gratingCalled a diffraction grating

Diffraction GratingsDiffraction Gratings

Used in spectrometers (devices Used in spectrometers (devices to separate light into colorsto separate light into colors

It spreads the spectrumIt spreads the spectrum Since different colors have Since different colors have

different wavelengths, the different wavelengths, the constructive interference occurs constructive interference occurs at different locationsat different locations

Diffraction GratingsDiffraction Gratings

Diffraction GratingsDiffraction Gratings glass or plastic made glass or plastic made

up of many tiny up of many tiny parallel slitsparallel slits

may also be reflectivemay also be reflective spectroscopes, spectroscopes,

reflective rainbow reflective rainbow stickers, CD surfacesstickers, CD surfaces

Thin FilmsThin Films

Look at an oil slick floating on Look at an oil slick floating on the surface of a pondthe surface of a pond

The slick has a rainbow of colors The slick has a rainbow of colors depending on the angle of depending on the angle of viewingviewing

This results from interference in This results from interference in the very thin film of oilthe very thin film of oil

Called Called iridescenceiridescence

Thin FilmsThin Films

Thin Films - Bubbles & Oil SlicksThin Films - Bubbles & Oil Slicks interference results from double interference results from double

reflectionreflection

PolarizationPolarization

A phenomenon that occurs in A phenomenon that occurs in transverse waves onlytransverse waves only


These waves are These waves are plane-polarizedplane-polarized All the motion is confined to a All the motion is confined to a

planeplane


Shake an electron up and down Shake an electron up and down and you create an and you create an electromagnetic wave that is electromagnetic wave that is plane-polarized in the vertical plane-polarized in the vertical directiondirection

Shake an electron side-to-side Shake an electron side-to-side and you create an EM wave that and you create an EM wave that is plane-polarized in the is plane-polarized in the horizontal directionhorizontal direction


A standard incandescent bulb A standard incandescent bulb emits light that is unpolarizedemits light that is unpolarized

The electrons are shaking in The electrons are shaking in random directionsrandom directions

So, the light has its electric field So, the light has its electric field shaking in different directions shaking in different directions for different wavesfor different waves


Some transparent crystalline Some transparent crystalline materials have a remarkable materials have a remarkable propertyproperty

These materials have their atoms These materials have their atoms arranged in non-cubic structuresarranged in non-cubic structures

These crystals effectively divide These crystals effectively divide the light into two beams that are the light into two beams that are plane-polarized at right angles to plane-polarized at right angles to each othereach other


Any polarization direction can be split into a horizontal and vertical component. Like adding two separate beams together.


Some crystals strongly absorb one Some crystals strongly absorb one of these beams while letting the of these beams while letting the other beam pass right throughother beam pass right through

These materials are called These materials are called polarizerspolarizers

Take a thin sheet of such a Take a thin sheet of such a material and imbed it between two material and imbed it between two sheets of glass or cellulose, and sheets of glass or cellulose, and you have a Polaroid filteryou have a Polaroid filter


What happens if we work with What happens if we work with two polarizers?two polarizers?

If we place them so they are If we place them so they are aligned, then the light passing aligned, then the light passing through the first polarizer, will through the first polarizer, will also pass through the second onealso pass through the second one

But if the polarizers are at right But if the polarizers are at right angles, the second one will angles, the second one will absorb all the lightabsorb all the light


Most of the light reflected from Most of the light reflected from non-metallic surfaces becomes non-metallic surfaces becomes polarizedpolarized

Consider the glare from glass or Consider the glare from glass or waterwater

Reflected wave has more Reflected wave has more vibrations parallel to the surfacevibrations parallel to the surface

Analogous to skipping a rock Analogous to skipping a rock across a pondacross a pond


Sunglasses are polarized to block Sunglasses are polarized to block rays reflecting from horizontal rays reflecting from horizontal surfaces light the road or a lakesurfaces light the road or a lake

This means the direction of This means the direction of polarization of the sunglasses is polarization of the sunglasses is vertical so it blocks horizontally vertical so it blocks horizontally polarized rayspolarized rays

light waves what we call light is a small portion of the electromagnetic spectrum what we call light...

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