ch27_mksb

 =Chapter 27-Light =


 * **__Primary__** source of light is sun
 * __**Secondary**__ source or light is brightness of sky
 * Other common sources: flames, white-hot filaments in lamps, glowing gases in glass tubes
 * Everything made visible by the light it reflects
 * Air, water or window glass allows light to pass straight through
 * Thin paper or frosted glass allow passage of light diffused in directions so that we can’t see objects through them.


 * ==27.1 Early Concepts of Light ==
 * Some of ancient Greek philosophers thought light consisted of tiny particles which could enter the eye to create vision.
 * Socrates and Plato thought vision resulted from streamers or filaments emitted by eye making contact with an object.
 * It was later found that light originated from external source and came to viewers eyes after interacting with other objects, which created vision.
 * Some thought light traveled in waves, other thought it traveled in particles.
 * Particle theory supported by fact that light seemed to move in straight lines instead of spreading out as waves do.
 * Huygens provided evidence when light DOES spread like a wave (diffraction)
 * Wave theory became the accepted theory in 19th century
 * 1905 Einstein published theory explaining the __**photoelectric effect**__- light consists of particles later called **photons** (massless bundles of concentrated electromagnetic energy)
 * __Light has dual nature, part particle part wave__

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 * ==27.2 The Speed of Light ==
 * Not known whether light travels instantaneous or w/ finite speed until latter part of 17th century
 * 1675 Danish astronomer Olaus Roemer made first demonstration that light travels at finite speed by making careful measurements of Jupiter's moons, especially the innermost moon, Io.
 * Huygens interpreted it as when the Earth is farther away from Jupiter, the moon seems like it's late because of the longer amount of time it takes the light. The moon passed into Jupiter's shadow at the predicted time, but the light took much longer to reach Roemer on earth.
 * [[image:http://light.physics.auth.gr/images/history/galileo/fizeau.jpg align="center"]]
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 * Speed of light= extra distance traveled/extra time measured
 * 300 000 000/ 1000s= 300 000 km/s
 * Most famous experiment measuring speed of light performed by Albert Michelson in 1880
 * Light from intense source directed by lens to an octagonal mirror initially at rest. Mirror carefully adjusted so that beam of light reflected to stationary mirror located on mountain 35km away, then reflected back to octagonal mirror and into eye of observer.
 * Distance the light had to travel to the distant mountain carefully surveyed, so Michelson had only to find the time it took to make a round trip. Accomplished by spinning the octagonal mirror at high rate
 * Conducted experiments to find that the time for light to make a round trip was the same amount of time it took the octagonal mirror 1/8 of a rotation.
 * He divided the 70 km round trip distance by the time. Michelson’s experimental value for speed of light was 299 920 km/s which we round to 300 000 km/s. Michelson received 1907 Nobel Prize in physics for experiment. He was the f irst American scientist to receive the prize.
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Speed of light in vacuum is a universal constant.
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Light so fast, it could make 7.5 trips around the earth in 1 second
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Light takes 8 minutes to travel from sun to earth and 4 years from next nearest star Alpha Centauri
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Light year-distance light travels in one year
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Alpha Centauri 4 light years away
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Our galaxy has diameter of 100 000 light years
 * <span style="font-family: 'Times New Roman', Times, serif; msolist: IgnoremsoList;">Some galaxies are 10 billion light years from Earth

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 * ==<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.3 Electromagnetic Waves ==
 * __Light__ is energy emitted by accelerating electric charges—often electrons in atoms.
 * **__Electromagnetic Wave__**-partly electric and partly magnetic
 * Radio waves, microwaves, and x rays all __electromagnetic waves__
 * __**Electromagnetic Spectrum**__-range of electromagnetic waves
 * Lowest frequency of light we can see is red (700 nm)
 * Highest frequencies are nearly 2x the frequency of red and appear violet (roughly 400 nm)
 * __**Infrared**__-electromagnetic waves of frequencies lower than red of visible light
 * Ex: heat lamps
 * __**Ultraviolet**__-Electromagnetic waves of frequencies higher than those of violet
 * Responsible for sun burns
 * ==<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.4 Light and Transparent Materials ==
 * <span style="font-family: 'Times New Roman', Times, serif;">Light is energy carried in an electromagnetic wave that’s generated by vibrating electric charges.
 * <span style="font-family: 'Times New Roman', Times, serif;">When light hits any piece of matter, the electrons in the matter vibrate
 * <span style="font-family: 'Times New Roman', Times, serif;">The electron vibrations in the light transfer to the electrons in the matter.
 * <span style="font-family: 'Times New Roman', Times, serif;">Similar to sound received by a receiver
 * <span style="font-family: 'Times New Roman', Times, serif;">How receiving material responds when light hits it depends on frequency of light and natural frequency of electrons in material
 * <span style="font-family: 'Times New Roman', Times, serif;">Visible light vibrates at very hight rate, more than 100 trillion times per second (10^14 hertz)
 * <span style="font-family: 'Times New Roman', Times, serif;">If charged object responds to these ultrafast vibrations, must have very little inertia
 * <span style="font-family: 'Times New Roman', Times, serif;">**__Transparent__**-materials that allow light to pass through in straight lines
 * <span style="font-family: 'Times New Roman', Times, serif;">All materials that are elastic respond differently to different vibrations.
 * <span style="font-family: 'Times New Roman', Times, serif;">Example :Bells and tuning forks have particular frequencies
 * <span style="font-family: 'Times New Roman', Times, serif;">Natural vibration frequencies of electrons depend on how strongly they’re attached to nearby nucleus.
 * <span style="font-family: 'Times New Roman', Times, serif;">Example: Electrons in glass have a natural vibration frequency in ultraviolet range
 * <span style="font-family: 'Times New Roman', Times, serif;">Energy received by atoms can be passed onto neighboring atoms by collisions, causing the light to be reemitted
 * <span style="font-family: 'Times New Roman', Times, serif;">If UV light interacts with an atom with the same natural frequency, the vibration amplitude becomes unusually large
 * <span style="font-family: 'Times New Roman', Times, serif;">Atoms hang onto this energy for long time and makes many collisions w/ other atoms and gives up its energy in form of heat
 * <span style="font-family: 'Times New Roman', Times, serif;">When electromagnetic wave has lower frequency than UV, electrons forced into vibration w/ smaller amplitudes.
 * <span style="font-family: 'Times New Roman', Times, serif;">Atom holds energy for less time, w/ less chance of collision w/ neighboring atoms, and less energy transferred into heat
 * <span style="font-family: 'Times New Roman', Times, serif;">Energy of vibrating electrons reemitted as transmitted light
 * <span style="font-family: 'Times New Roman', Times, serif;">Glass is transparent to all frequencies of visible light
 * <span style="font-family: 'Times New Roman', Times, serif;">Frequency of the reemitted light passed from atom to atom identical to that of the light that produced the vibration
 * <span style="font-family: 'Times New Roman', Times, serif;">Time delays result in lower average speed of light through transparent material
 * <span style="font-family: 'Times New Roman', Times, serif;">Light travels at different average speeds through different materials
 * <span style="font-family: 'Times New Roman', Times, serif;">Light travels very slightly slower than speed of light in atmosphere, but usually rounded off to speed of light (c)
 * <span style="font-family: 'Times New Roman', Times, serif;">In water, light travels at 75% off its speed in a vacuum or .75c
 * <span style="font-family: 'Times New Roman', Times, serif;">In glass light travels around about .67c, depending on type of glass
 * <span style="font-family: 'Times New Roman', Times, serif;">In diamond, light travels at .40c
 * <span style="font-family: 'Times New Roman', Times, serif;">[[image:http://micro.magnet.fsu.edu/primer/java/speedoflight/javalightspeedfigure1.jpg align="center"]]
 * <span style="font-family: 'Times New Roman', Times, serif;">[]
 * <span style="font-family: 'Times New Roman', Times, serif;">When light emerges from materials into air, travels at original the speed (c) again
 * <span style="font-family: 'Times New Roman', Times, serif;">Infrared waves (frequencies lower than that of visible light) vibrate not only the electrons, but also entire structure (like glass)
 * <span style="font-family: 'Times New Roman', Times, serif;">This vibration of the structure increases the internal energy of the glass and makes it warmer
 * <span style="font-family: 'Times New Roman', Times, serif;">Transparent materials successively absorb and reemit light by the atoms it strikes

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 * ==<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.5 Opaque Materials ==
 * __**Opaque**__-materials that absorb light without reemission and thus allowing no light through them
 * Ex: wood, stone, people
 * Any coordinated vibrations given by light to atoms and molecules turned into random kinetic energy—that is not internal energy. __Materials become slightly warmer.__
 * In metals, outer electrons of atoms not bound to any particular atom. Free to wander with very little restraint thought material. That’s why metal conducts electricity and heat so well
 * When light shines on metal and sets free electrons into vibration, energy does not ‘spring’ from atom to atom, but it’s reemitted as visible light which is seen as reflection which is why metals are shiney
 * Our atmosphere transparent to visible light and some infrared but almost opaque to UVs
 * UVs responsible for sunburn
 * Clouds semi transparent to UV which is why you can get sunburn on cloudy days
 * UV reflects from sand and water which is why you can sometimes get sunburn while in shade of beach umbrella


 * ==<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.6 Shadows ==
 * __Ray__-thin beam of light
 * Any beam of light is made up of bundle of rays
 * When light shines on object, some rays are stopped while others pass on in straight line path
 * __Shadow__- formed where light rays cannot reach
 * Sharp shadows are produced by a small light source nearby or larger source farther away
 * Usually a dark part on inside and lighter part around edges
 * __Umbra__**-total shadow (all rays blocked from reaching part of surface)**
 * __Penumbra__**-partial shadow (Only some rays blocked from reaching part of surface)**
 * Penumbra __appears__ where some of light is blocked, but other light fills in.
 * Penumbra __occurs__ where light from broad source only partially blocked
 * Pg 412 figure 27.11
 * Example: when moon passes between earth and sun during solar eclipse
 * Because sun so large, rays taper to provide umbra and surrounding penumbra
 * Moon’s shadow barely reaches earth
 * __**Lunar eclipse**__-shadow from earth extends into space and moon passes into it
 * Shadows will occur when light bent in passing through transparent material like water
 * Light travels at different speeds in warm and cold water
 * Difference bends light, just like layers of warm and cool air in night sky bend starlight and cause twinkling.
 * ==<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.7 Polarization ==
 * Light travels in waves
 * <span style="font-family: 'Times New Roman', Times, serif;">**__Polarization__**-aligning of vibrations in transverse wave, usually by filtering out of waves of other directions (light caused to vibrate in one direction)
 * <span style="font-family: 'Times New Roman', Times, serif;">Light traveling forward usually vibrates up and down, left and right, everything in between
 * Single vibrating electron emits electromagnetic wave that’s polarized:
 * Vertically vibrating electron emits light vertically polarized
 * Horizontally vibrating electron emits light horizontally polarized
 * Common light source, like incandescent, emits __NOT__ polarized light because vibrating electrons that produce the light vibrate in random directions
 * When the light from these sources shines on polarizing filter (like sunglasses) the light transmitted IS polarized
 * Filter is said to have polarizations axis that is in direction of vibrations of polarized light wave
 * All light will pass through pair of polarizing filters when polarization axis are aligned, but not when they’re perpendicular
 * Pg 414 figure 27.17
 * This behavior like filtering of vibrating rope that passes through pair of picket fences
 * <span style="font-size: 13px; line-height: 19px; font-family: 'Times New Roman', Times, serif;">All of the vertical waves will pass, but none of the horizontal ones.
 * [[image:http://whatis.techtarget.com/WhatIs/images/polariza.gif align="center"]]
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 * <span style="font-size: 13px; line-height: 19px; font-family: 'Times New Roman', Times, serif;">Light that reflects at glancing angles from nonmetallic surfaces (glass, water, roads) vibrates mainly in plane of reflecting surfaces
 * Glare from horizontal surface is horizontally polarized

<span style="display: block; font-family: Georgia, serif; text-align: center;"> 27.8 Polarized Light and 3D Viewing

 * Vision in 3D depends on the fact both eyes give impressions simultaneously each eye viewing a scene from slightly different angle
 * Combo of views in eye-brain system gives depth
 * Pair of photos/movie frames taken a short distance apart (about average eye spacing) can be seen in 3D when left eye sees only left view and right eye sees only right view
 * Slide shows/movies accomplish this by projecting pair of views through polarization filters onto a screen
 * Polarization axes are at right angles to eachother
 * Overlapping pics look blurry to naked eye
 * To see in 3D, viewer wears polarizing glasses w/ lens axes also at right angles.
 * [[image:http://www.3dglassesonline.com/how-do-3d-glasses-work/3-d-glasses-traditional.gif]]
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 * Each eye sees separate picture, just as in real life.
 * Brain interprets 2 pictures as single picture w/ feeling of depth
 * Depth also seen in compture-generated stereo-grams