a physic quiz
Sherrysherry
chapter8_light1_1.pptxChapter 8: Light
What is Light? How is it made? How can it travel through empty space?
What causes Light to have Color and how do we perceive Color?
What is Color-blindness?
How do Cameras work?
How do 3D movies work?
10/19/2016
Prof. Michael Opferman | Phys 0847 | Temple University
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1
Chapter 8A: Properties of Visible Light
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Prof. Michael Opferman | Phys 0847 | Temple University
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What is Light?
Like Sound, Light is a Wave
Since it is a Wave, it undergoes many of the same behaviors we saw in the last chapter such as Refraction, Interference, and the Doppler Effect
But unlike Sound, Light is not carried by air molecules.
So if the air molecules are not moving like in Sound, what is doing the “waving”?
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Prof. Michael Opferman | Phys 0847 | Temple University
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What is Light?
The Electric interaction between charges can be said to be carried by an abstract “Field” called the Electric Field
The Magnetic interaction between currents can be said to be carried by an abstract “Field” called the Magnetic Field
These Fields are not made of real matter (like air). They are just sort of an abstract, ethereal thing that can be around us, even in empty space
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Prof. Michael Opferman | Phys 0847 | Temple University
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What is Light?
Light is carried by a Wave in the Electric and Magnetic Fields
The strength of these Fields goes up and down in the same way that the air pressure goes up and down for Sound
http://www.physics.smu.edu/jcotton/ph1311/ch02a.htm
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Note: These Fields can travel even in empty space, where there is no matter
Example: Light can reach us from the Sun
Where Does Light Come From?
If you shake the air, a Sound Wave is emitted
If you shake the ground, an Earthquake Wave is emitted
If you shake an Electron, a Light Wave is emitted
http://www.modelofreality.org/Nyquist_Static2.gif
Red: Electron
Blue: Emitted Light
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But where does Light actually come from?
If you shake the water, a Water Wave is emitted
Properties of Light Wave
The Wavelength (size) of a Visible Light Wave is around 500 nanometers
About 1% the width of a hair
The size of about 1000 atoms
For comparison: Sound Waves have a Wavelength of a few feet
The Frequency (number of shakes per second) of a Visible Light Wave is around 100,000,000,000,000 (=1015)
For comparison: Sound shakes around 100-1,000 times per second
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Prof. Michael Opferman | Phys 0847 | Temple University
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Speed of Light
The speed at which a Light Wave travels (in empty space) is usually quoted as 3 x 108 m/s. This is about 670,000,000 mph.
For comparison: Sound travels in air at about 750 mph
Since this number is so hard to wrap our heads around, a more useful speed to remember is that Light travels at around 1 foot per nanosecond.
A 1 GHz processor takes about one 1 nanosecond to do a single operation
Light can cross a 10 ft long room in about 10 nanoseconds
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Prof. Michael Opferman | Phys 0847 | Temple University
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Speed of Light
The speed at which a Light Wave travels (in empty space) is usually quoted as 3 x 108 m/s. This is about 670,000,000 mph.
For comparison: Sound travels in air at about 750 mph
This is why you see lightning before you hear thunder.
Light can travel 1 mile in about 5280 nanoseconds (since 1 mile is 5280 ft), which is about 5 microseconds
Sound can travel 1 mile in about 5 seconds
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Speed of Light
If Light is traveling through a material, it travels slower.
In air: Light travels 99.97% as fast
In water: Light travels 75% as fast
In glass: Light travels 67% as fast
In diamond: Light travels 41% as fast
The ratio of the speed of Light in empty space to the speed of Light in a material is called the material’s “Index of Refraction”
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Bending Light
Recall: Waves bend toward areas where they move slower.
This is also true of Light. It bends when it crosses from air to water or glass, for example.
Like Sound, Light travels a little faster in hot air than cold
https://www.ec.gc.ca/foudre-lightning/default.asp?lang=En&n=4EFD3A52-1
So when the ground is very hot, what would you see when you look down?
Recall from last chapter:
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Mirages
The Light from the (blue) sky is visible on very hot ground. That’s why people see Mirages of (blue) water in the desert
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http://www.planet-science.com/categories/under-11s/our-world/2012/01/what-is-a-mirage.aspx
https://www.flickr.com/photos/12587661@N06/2671845245
Fiber Optics
“Wires” that carry Light are made of thin fibers of glass, and are simply called Fibers. A fiber optic cable contains many fibers.
http://electronics.howstuffworks.com/question402.htm
Fibers work on the same principle as the Sound Channel from Ch. 7
Light travels the slowest at the center of the Fiber, so it tends to turn back toward the center, and it becomes trapped inside.
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Fiber Optics
If you are sending information from one place to another (like in a computer), the speed that you can send it is limited by Speed (and Frequency) of the Wave
This makes “Fiber Optics” very fast
Optics = Light
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Fibers are gradually being phased in to replace wires in many applications, such as Verizon Fios and Google Fiber
https://static1.squarespace.com/static/530c2376e4b07902c646e31e/53a22fa9e4b01756877cf3c5/53c5baffe4b09b3f53ea337d/1432068725749/VZ-Banner-Ad.png?format=500w
Fiber Optics
Although Light travels very fast down Fibers, their high Index of Refraction means that the Light would travel even faster through air
For some very high speed applications, this loss in speed is not considered to be acceptable
For a discussion of how the Speed of Light is a limiting factor in High Frequency Stock Trading, see http://www.radiolab.org/story/267195-million-dollar-microsecond/
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Properties of Light: Summary
A Visible Light Wave is about a million times smaller than an audible Sound Wave
A Visible Light Wave shakes about a trillion times faster than an audible Sound Wave
A Visible Light Wave travels about a million times faster than an audible Sound Wave
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Prof. Michael Opferman | Phys 0847 | Temple University
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Chapter 8B: Color
What is color and how do we perceive it?
Do all animals see the same colors when they look at something?
What is colorblindness?
How do monitors and printers create colors?
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Prof. Michael Opferman | Phys 0847 | Temple University
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Color
Different Frequencies (or Wavelengths) of Sound Waves are perceived to our ears as different pitches or notes
Likewise, different Frequencies (or Wavelengths) of Light Waves are perceived to our eyes as different colors
http://deserthighlandspr.com/wp-content/uploads/2013/02/Visible-spectrum.jpg
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Color
Red light has the highest Wavelength and lowest Frequency of the Visible colors, and Violet has the lowest Wavelength and highest Frequency
Just as not all Sound Waves are audible to our ears, not all Light Waves are visible to our eyes. We will discuss invisible Frequencies of Light in the next chapter
http://deserthighlandspr.com/wp-content/uploads/2013/02/Visible-spectrum.jpg
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Color of Soap Bubbles
Soap Bubbles and Oil Slicks get their Color from the fact that they are very thin—about as thin as a Wavelength of Light.
Light Waves that bounce off the front surface of the Soap film and the ones from the back surface create an Interference Pattern
https://commons.wikimedia.org/wiki/File:Thin_film_interference_phase_1.svg
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Prof. Michael Opferman | Phys 0847 | Temple University
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Color of Soap Bubbles
Since Color depends on Wavelength, this means that different spots on the bubble where the Soap thickness is different will show different Colors due to different Interference patterns.
http://soapbubble.wikia.com/wiki/Color_and_Film_Thickness
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Seeing in Color
Our eyes have four kinds of Light sensors
Rods sense the brightness of Light (but not Color)
Red, Green, and Blue Cones can differentiate Colors
Each of the three Cone types can sense a range of colors with different levels of sensitivity
Your brain compares the strength of the three Cone signals to deduce the Color
http://cat.ocw.uci.edu/oo/getOCWPage.php?course=OC0811004&lesson=005&topic=001&page=20
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Seeing in Color
Examples: Your brain receives the following signal from each Cone. Which Color are you seeing?
Blue: None
Green: Strong
Red: Strong
Answer: Yellow
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Seeing in Color
Examples: Your brain receives the following signal from each Cone. Which Color are you seeing?
Blue: Strong
Green: Strong
Red: Weak
Answer: Blue-Green
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Seeing in Color
Examples: Your brain receives the following signal from each Cone. Which Color are you seeing?
Blue: None
Green: Medium
Red: Strong
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Seeing in Color
Although this is an ingenious system that allows us to see many Colors with only three kinds of Cones, it is possible to exploit the “bugs” in this system to trick the brain.
http://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response
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Seeing in Color
The nature of the Light is measurably different in these two cases, but our eye cannot distinguish between them because our eye assumes something is yellow if both the red and green sensors are triggered equally, even though it might actually be
http://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response
a mixture of red and green
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Seeing in Color
White Light is made of an assortment of all the Frequencies (Colors) mixed together.
The brain assumes Light is “White” if it gets Strong signals from all three Cones (Red, Green, Blue).
But really the Light could be a mixture of only those three colors. The eye cannot check to see if the others are present.
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Prof. Michael Opferman | Phys 0847 | Temple University
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RGB
Computer and TV screens are only capable of showing three Colors—Red, Green, and Blue. By showing these Colors in different combinations, the brain can be tricked into seeing nearly any Color.
In the example exercises from a moment ago, how do you know the difference between seeing Yellow Light, and a mixture of Red and Green? Either one would give the same brain signal from the Cones, so no need to actually be able to display yellow light.
https://bpiinc.wordpress.com/tag/rgb/
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RGB
What your computer monitor would really look like up close would be something
like the picture shown. Each pixel contains a tiny Red, Green, and Blue light. The relative strength of the three lights determines which Color you see, just like in our exercise from a few minutes ago.
http://i01.i.aliimg.com/img/pb/941/321/845/845321941_606.jpg
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RGB
The combination of red, green, and blue displayed on a screen is often specified using a system known as “hexadecimal color”
In this system, each color is specified by three (two digit) numbers. These three numbers tell you the relative amounts of red, green, and blue necessary to create a color to the human eye.
Example: To create this shade of purple, set the three RGB lights to
Red = 66, Green = 14, Blue = 52
In other words, we would say that this color is mostly red with almost as much blue, and just a little green.
For details: www.w3schools.com/tags/ref_colorpicker.asp
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Printers
Printers work using a similar principle, but ink absorbs Light instead of emitting it,
so it uses a different three colors:
Cyan (G+B), Magenta (R+B), and Yellow (R+G)
http://www.spielberg-ocr.com/printing/color-halftoning.png
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Painting
A similar trick is used in Impressionist paintings, especially in Pointillism. In Pointillism, the impression of Color
is created by painting small, distinct dots of Color. When the dots blur together, the brain interprets them as other Colors.
From Ferris Bueller’s Day Off (1986).
The painting is Georges Seurat’s A Sunday Afternoon on the Island of La Grand Jatte (1888).
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Painting
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Color-Blindness
What if you were missing one of the three Cones? With only two signals instead of three, it is more difficult to distinguish Colors. We call this Color Blindness.
Blue: None
Red: Strong
Answer: Orange? Yellow?
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Prof. Michael Opferman | Phys 0847 | Temple University
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Color-Blindness
A color-blind person simply has an eye which is more easily tricked into confusing colors—the same way that all of us can be confused by tricks like the RGB monitor
Blue: None
Red: Strong
Answer: Orange? Yellow?
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Prof. Michael Opferman | Phys 0847 | Temple University
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Cones in Other Animals
Just as some animals can hear Sound Wave Frequencies outside of human hearing, some animals can see Light Wave Frequencies outside of human vision
Just as Ultrasound contains “notes” we can’t hear, Ultraviolet (and Infrared) contain “colors” we can’t see
http://ecx.images-amazon.com/images/I/81-kBqbrjYL._SL1500_.jpg
http://ccpethospital.com/uv-lighting-for-birds-is-it-that-important
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Cones in Other Animals
Animals may have the ability to see invisible Colors outside the range of human sight as well as the ability to see extra shades of color by having extra cones
Some have only two Cones and are, by human standards, Colorblind (e.g. dog)
Some have three Cones like us, but see invisible Colors (e.g. bee)
Bee has a Cone for UV Light.
Some have four Cones and can distinguish more shades of Color (e.g. hummingbird)
https://fieldguidetohummingbirds.wordpress.com/2008/11/11/do-we-see-what-bees-see/
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Prof. Michael Opferman | Phys 0847 | Temple University
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Cones in Other Animals
The animal with the best Color vision is the Mantis Shrimp
The Mantis Shrimp has 16 Cones!!!
Mantis Shrimp. From http://www.radiolab.org/story/211178-rip-rainbow/
What would these animals see? Learn more from http://www.radiolab.org/story/211178-rip-rainbow/
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The Opposite of Color-Blindness
What if a person had four Cones instead of three?
Actually, perhaps a few people do. Learn more by listening to
http://www.radiolab.org/story/211193-perfect-yellow/
Then they’d be able to distinguish even more shades of Color from one another, and their eyes would be more difficult to trick.
For instance, you’d need four colors instead of just RGB in monitors and printers
We can build cameras that have this ability--called Multi-Spectral Cameras.
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Prof. Michael Opferman | Phys 0847 | Temple University
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Chapter 8C: Applications of Light
How do cameras work? Mirrors? Eyeglasses?
Why are diamonds and rainbows colorful?
What causes the red eye effect in photographs?
How do 3D movies work?
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Vision
Imagine Light being made of tiny Rays which travel in straight lines.
The eye uses these light Rays to form an “Image” of what you are seeing on the back of the eye.
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html
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Pinhole Camera
We can build a very simple device that does the same thing.
https://en.wikipedia.org/wiki/Pinhole_camera
Poke a small pinhole in a box, and an Image can be created on the back of the box.
Place some photographic film there to record the Image, and you have a camera!
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Film Photography
Traditionally, the film is covered with a chemical that hardens when exposed to light.
Wash away the parts that were not exposed to light, and you have a permanent record of the Light that hit the film—a photograph.
http://rogers99.users.sonic.net/rls86g.jpg
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Digital Photography
In a digital camera, the principle is the same, but replace the film with an array of light sensors and make a computer record of where the Light hit.
http://www.nickchillphotography.com/2014/04/lens-camera-whats-more-important/
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Human Photography?
In the eye, the Pupil acts like the pinhole
The Retina (where the rods and cones are) acts like the film/senor array.
The results of each sensor are transmitted to the brain for interpretation and storage like in a digital camera.
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html
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Mirrors
Mirrors reflect Light Rays
Since the eye has no way of knowing that the Light Rays bounced off of a Mirror, our brains assume the Light came straight from the source
Result: Our brain is tricked into seeing an object behind the Mirror
https://en.wikipedia.org/wiki/Plane_mirror
Actual Location of object
Perceived Location of object
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Retroreflectors
In a Retroreflector, Light Rays are reflected back to their original source
This is the same effect used technologically in many safety reflectors
http://drstyle.me/bicycles-lunar-lasers-cube-corner-retroreflectors/
https://ca.wikipedia.org/wiki/Retroreflector
http://www.core77.com/posts/24028/A-Scintillating-Retroreflective-City-Cycle-by-Bike-Safe-Bostons-Joshua-Zisson
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Retroreflectors
The Red Eye effect in photographs occurs when light from the camera’s flash bounces off of the “retroreflector” inside of the eye and is directed by the eye’s lenses back to its original source: the camera
http://www.bikexprt.com/bicycle/reflectors/reflwrk.htm
https://en.wikipedia.org/wiki/Red-eye_effect
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Retroreflectors
Some photographers use a flash far from the camera’s lens to avoid this issue
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http://www.digital-slr-guide.com/digital-slr-flash.html
http://www7.pcmag.com/media/images/225878-panasonic-lumix-dmc-g10-external-flash.jpg
http://images.anandtech.com/reviews/cameras/2008/nikond700/d700-popup.jpg
Retroreflectors
The same effect causes a cat’s eyes to seem to glow in the dark.
You are actually seeing the light from behind you reflecting off of the back of the cat’s eye through its unusually large pupil
https://i.ytimg.com/vi/91CFfcb3tk4/hqdefault.jpg
http://kids.britannica.com/comptons/art-53057/The-pupils-of-a-cats-eyes-expand-or-contract-in
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Dispersion
The Index of Refraction (Speed of Light) in materials is a little different for different colors of Light.
Since the amount that Light bends depends on its speed, the different colors sometimes bend differently and become separated (dispersed)
This is called “Dispersion” in physics or “Fire” in the gem business.
https://upload.wikimedia.org/wikipedia/commons/f/f5/Light_dispersion_conceptual_waves.gif
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Diamonds
The amount of Dispersion is high in diamonds. This is what gives them their distinctive appearance—the fact that you see a rainbow of changing colors depending on how you look at them.
http://www.jewelry-secrets.com/Diamonds/Diamond-Fire-Dispersion/What-Is-Diamond-Fire-Dispersion.html
http://www.modernjeweler.com/publication/article.jsp?id=9&pageNum=6
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Cubic Zirconia
Cubic Zirconia (left in picture below) are man-made gems similar to diamonds.
They have even more Dispersion (Fire) than diamonds.
Ironically, people think too much Dispersion makes a gem look cheap or fake even though this is the property that made diamonds special in the first place!
http://i.ytimg.com/vi/vQFAfaSWZ-Y/maxresdefault.jpg
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Rainbows
Rainbows occur when Dispersion happens in small water droplets in the air.
Rainbows occur when you look away from the Sun because it is the light that is reflected back out from the water in which the Light is dispersed into a Rainbow.
http://faculty.salisbury.edu/~jwhoward/physics123/html/ch24.htm
https://en.wikipedia.org/wiki/Rainbow
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Refraction
The bending of Light due to a change in the Index of Refraction is called “Refraction”
The most familiar examples of Refraction occur when we look underwater.
Since Light bends when it hits the surface of the water, objects appear to be in the “wrong” location when you look into the water.
https://plus.maths.org/content/light-bends-wrong-way
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Refraction
If you are trying to spear a fish, aim below it because the fish is not where it appears to be
http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_CPworkbook_Refraction.xml
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http://file2.answcdn.com/answ-cld/image/upload/w_760,c_fill,g_faces:center,fl_lossy,q_60/v1401366713/jc4cflstuchmhsyqh42e.gif
Lenses
A lens is a piece of glass that is shaped in such a way that it uses Refraction in glass to bend Light in a desired direction
Many lenses are designed to focus light to a point as shown
In the eye, the Lens and the Cornea both act as lenses to focus Light onto the Retina
https://prezi.com/egvc9cbjkhek/convex-and-concave-lenses/
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Eyesight
If the Cornea is too strong of a lens, then distant objects will be focused too much, in front of the Retina, and you will be near-sighted
If the Cornea is too weak of a lens, then nearby objects will not be focused enough, behind the Retina, and you will be far-sighted
The Cornea can be re-shaped surgically to fix this problem—this is Lasik
http://philschatz.com/physics-book/contents/m42484.html
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Eyesight
Eyeglasses add another lens to the system, which is made to correct for problems with your Cornea and Lens
Bifocals have two lenses in them to solve both problems
The lens becomes less effective at focusing as we age. This is why older people are far-sighted and need reading glasses.
http://www.apsubiology.org/anatomy/2010/2010_Exam_Reviews/Exam_4_Review/CH_15_Vision_Pathology.htm
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Telescopes and Microscopes
Lenses can also be used to create an Image that have desirable properties
Lenses can create a magnified image of a nearby object (microscopes and
magnifying glasses)
Lenses can create a small image of a distant object (telescope)
http://science.howstuffworks.com/telescope1.htm
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Polarization
Recall: Light is made of an Electric Field which is oscillating up and down.
But why up and down? Why not sideways, at an angle, or even in a corkscrew fashion?
http://www.physics.smu.edu/jcotton/ph1311/ch02a.htm
All of these are possible, and this is called the Light’s Polarization
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Polarization
Example: The two Light Waves below have Polarizations that are opposite
Left: Vertically Polarized
Right: Horizontally Polarized
http://ispex.nl/en/educatie/wat-is-polarisatie/
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Polarization
Ordinary Light is typically a mixture of all possible types of Polarization (called “Unpolarized”)
But we can make materials that allow only one kind of Polarization through (called “Polarizers” or “Polaroids”)
Polarizers are like microscopic picket fences. Only Light that fits through the slats can pass.
http://www.olympusmicro.com/primer/lightandcolor/polarization.html
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Polarized Sunglasses
When Light bounces off horizontal (non-metal) surfaces like water or a road, it tends to become Horizontally Polarized.
“Polarized Sunglasses” use a Polarizer to block out this reflected Light (glare) without blocking out all of the Unpolarized Light coming from the Sun.
https://www.microscopyu.com/articles/polarized/polarizedlightintro.html
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LCDs
LCD stands for Liquid Crystal Display
Liquid Crystals are Polarizers that can be turned on and off with Electricity.
In an LCD, the lights behind the screen have their brightness adjusted by rotating Liquid Crystal Polarizers
http://repairpc.co.il/en/the-most-important-components-of-lcd-monitors/
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https://i.ytimg.com/vi/eX60HCOwmy4/hqdefault.jpg
3D Movies
One way to display an Image that is perceived by our brains in 3D is to project two Images—one for each eye—and make the Images slightly different to trick our brains into seeing depth
In the old days, this was done by projecting each eye’s Image with a different Color of Light. That’s why old 3D glasses have color filters as shown.
http://www.aliexpress.com/w/wholesale-paper-red-cyan-3d-glasses.html
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3D Movies
Now, we use different Polarization of Light instead.
The two lenses of the 3D glasses are Polarizers with a different direction of Polarization
http://www.physics.org/article-questions.asp?id=56
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Chapter 8: Summary
Light is made of oscillating Electric and Magnetic Fields
Light is not carried by atoms, so it can even travel in empty space
Light is generated when Electrons are shaken
The size of a Light Wave (Wavelength) determines its Color.
Each Wavelength is associated with a different Frequency (rate at which the Electric and Magnetic Fields oscillate)
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Prof. Michael Opferman | Phys 0847 | Temple University
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Chapter 8: Summary
The rapid oscillations (high Frequency) of Light make it good for carrying information
“Light Wires” are made of thin Fibers of glass (Fiber Optics)
The Fibers work by bending Light toward the center of the Fiber, where it travels the slowest
Our eyes have only three types of Color sensors. Our brain guesses Light’s true Color based on just three signals
This allows us to use just three lights (RGB) to simulate any Color in monitors, TVs, and printers
10/19/2016
Prof. Michael Opferman | Phys 0847 | Temple University
70
Chapter 8: Summary
Color-blind people may have only two instead of three sensors, so they have a harder time guessing the correct Color
In a Camera, a pinhole admits only certain Rays of Light, and these Rays are recorded using either a Light-sensitive chemical or a computer
Mirrors reflect Light Rays, which tricks are eyes into thinking that the Rays came from a different location
Different Colors of Light become Dispersed because they travel at different speeds
10/19/2016
Prof. Michael Opferman | Phys 0847 | Temple University
71
Chapter 8: Summary
In empty space, the Speed of Light is about 1 foot per nanosecond
In materials, it travels slower. Its speed in a particular material can be looked up via its Index of Refraction
Mirages are caused by Light bending toward cooler air, where it travels slower
Refraction occurs when Light bends at a surface due to a difference in speed in the new material
Refraction causes illusions when we look into water
Lenses use Refraction to focus Light
10/19/2016
Prof. Michael Opferman | Phys 0847 | Temple University
72
Chapter 8: Summary
Eyes, Eyeglasses, Microscopes, and Telescopes all use Lenses to manipulate Light
The Polarization of Light refers to the direction in which the Electric Field is oscillating
Polarized Sunglasses, LCD TVs, and 3D glasses all make use of Polarizers to block or admit certain Polarizations of Light
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Prof. Michael Opferman | Phys 0847 | Temple University
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