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Light

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Zachary McAllister

on 29 April 2014

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Transcript of Light

Light
Light
Every time you "see", you are using light. You can't see anything in complete darkness! Whether you are looking at a light bulb, car, or this presentation, light brings information to your eyes. In fact, seeing means receiving light and forming images in your mind from the light received by your eyes.
What is light?
Light, like sound and heat, is a form of energy. Our understanding of light starts with what light does and what its properties are. Here are a few properties you all are probably familiar with: Light travels extremely fast and it carries energy & information; it travels in straight lines, but bounces and bends when it comes into contact with objects. It also has color and different intensities. We will discuss these properties as we develop our understanding of light.
Seeing with light
What happens when you read a book? Light in the room reflects off the page and into your eyes. The reflected light carries information about the page that your brain uses to make a mental picture of the page. You see because the light in the room reflects from the page into your eyes. If you were sitting in a perfectly dark room with not light, you would not be able to see the pages because the pages do not give off their own light. We see most of the world by reflected light.
Where does light come from?
For most of human history, people relied on the Sun, the Moon, and fire to provide light. Thomas Edison's electric light bulb (1879) changed our dependence on fire and daylight forever. The electric light is one of the most important inventions in the progress of human development.
Thomas Edison
1847-1931
Light & Atoms
Whether in an electric bulb or in the Sun, light is mostly produced by atoms. Atoms release energy by giving off light. It's similar to when you stretch a rubber band. You give the rubber band elastic energy and can use this energy to launch an object. The stored energy is released as kinetic energy of the flying object.
Heat and Light
In order to get light out of an atom, you must first put some energy into the atom. One way to do this is with heat. Making light with heat is called incandescence. Incandescent bulbs use electric current to heat a thin wire, or filament. Atoms in the filament convert electric energy to heat and then to light. Only a fraction of the electricity is converted into light. Most of the energy becomes heat.
Fluorescent Bulbs
Fluorescent bulbs are often used in schools and businesses because they use a more efficient method for creating light. Fluorescent bulbs use high-voltage electricity to energize atoms of gas in the bulb. These atoms release the electrical energy directly as light, in a process called fluorescence. The atoms give off high-energy UV light, the kind that causes sunburns. The atoms in the white coating on the inside of the bulb absorb the UV light and re-emits the light as white light that we can see.
White Light
When all the colors of the rainbow are combined, we see light without any color. We call the combination of all colors white light. The light that is all around us most of the time is white light. The light from the Sun and the light from most electric lights is white light.
Color
Not all light has the same energy. Color is how we perceive the energy of light. This definition was proposed by Albert Einstein. All of the colors in the rainbow are light of different energies. Red light has the lowest energy we can see, and violet light has the highest energy.
Color & Energy
If you have ever sat around a campfire, you may have noticed color differences between parts of the fire. You may have seen a stove that had blue flames like the picture below. Different colors possess different amounts of energy. For example, the gas flame produced by a gas stove is has more energy than a yellow or red flame found in a fire.
Color & Energy
What do we mean when we talk about the energy of light? Just as matter is made of atoms, light energy comes in tiny wave-bundles called photons. A photon is the smallest possible amount of light, in the form of a wave-bundle. Each photon has its own color (energy), no matter how you mix them up.
The Speed of Light!!!
When you shine a flashlight on a wall that is far away, you don't see a time delay as the light leaves the flashlight, travels to the wall, bounces off, and comes back to your eyes. This is exactly what happens, but it happens so fast that you don't notice it. Suppose the wall was 170 meters away (over 500 feet away), the light travels there and back in about one millionth of a second! If you shouted at the wall, your echo wouldn't return to you until nearly a full second later because sound is nearly a million times slower than light!
The Speed of Light
The speed at which light travels through air is about 300 million meters per second. Light is so fast, it can travel around the entire Earth 7.5 times in 1 second. The speed of light is so important in physics that it is given its own symbol, the lower case c. The speed of light is one of the variables in Einstein's famous energy equation: E = mc .
2
Traveling at the Speed
of Light?!?
Wavelength of Light
The wavelength of visible light is very small. Waves of orange light have a length of only 600 nanometers. Remember, nano (10 ) is a prefix used to measure very small things. 600 nanometers is only 0.0000006 meters! This means that thousands of wavelengths of red light would fit in the width of a single hair on your head.
-9
Frequency of Light
Wavelengths of visible light are very small, so the frequency of light waves is very high. For example, red light has a frequency of 460 trillion, or 460,000,000,000,000 cycles per second. To manage these large numbers, scientists use units of terahertz (THz) to measure light waves. As with other waves, the wavelength and frequency are inversely related.
Wavelength, Frequency, Color,
& Energy
As you can see from the table below, energy and frequency are directly related. The higher the frequency, the higher the energy. Since color is related to energy, the table also shows the relationship between color, frequency, and wavelength.
What Kind of Wave is Light?
A sound wave is an oscillation of air. A water wave is an oscillation of the surface of water. What is oscillating in a light wave? Electricity and Magnetism. Anything that creates an oscillation of electricity and magnetism also creates electromagnetic waves. An electromagnetic wave is a traveling oscillation in the electric and magnetic field. Any change in the electric or magnetic field travels at the speed of light.
The Electromagnetic
Spectrum
The entire range of electromagnetic waves, including all possible frequencies, is called the electromagnetic spectrum. It includes radio waves, microwaves, infrared light, ultraviolet light, X-rays and gamma rays. We use the electromagnetic spectrum for all kinds of human technologies.
How do X-ray films work?
X-rays are high-energy electromagnetic waves used in medicine and industry. When you get a medical X-ray, the film darkens where bones are because calcium and other elements in your bones absorb the X-rays before they reach the film. X-rays show the extent of an injury such as a broken bone.
Color and Vision
We know the energy of light explains how different colors are physically different. But it doesn't explain how we see colors. In the previous chapter we learned how the ear "hears" sound, now we will learn how the eye "sees" color.
The Human Eye
The human eye is yet another amazing component of the human body. Light enters the eye through the lens then lands on the retina. On the surface of the retina are light-sensitive cells called photoreceptors. When light hits a photoreceptor cell, the cell releases a chemical signal that travels along the optic nerve to the brain. In the brain, the signal is translated into a perception of color.
Rods and Cones
Our eyes have two kinds of photoreceptors, called cones and rods. Cones respond to color. There are three types of cone cells. One type responds best to low-energy (red) light. Another responds best to medium-energy (green) light. The third type responds best to higher-energy (blue) light.
Rods and Cones
The second type of photoreceptor cells are called rods. Rods respond to differences in light intensity, but not to color. Rod cells "see" black, white, and shades of gray. However, rods are much more sensitive than cone cells. At night, colors seem washed out because there is not enough light for cone cells to work. When there is little light, you see "black and white" images from you rod cells.
It looks better in......
black-and-white!?!
A human eye contains about 130 million rods and 7 million cones. Each "cell" contributes a "dot" to the image assembled by your brain. Because there are more rods, things look sharpest when there is a big difference between light and dark. For example, read the sentence at the bottom and notice how much easier it is to read the first part compared to the second part.

Physical science is
my favorite class!!!!!
The Additive Process
Because there are three kinds of cone cells, our eyes work by adding three signals to "see" different colors. The color you "see" depends on how much energy is received by each of the three different types of cone cells. The brain thinks "green" when there is a strong signal from the green cones but no signal from the blue or red cone cells.
The Additive Process
What color would you see if light creates signals from both the green and red cones? Yellow. We see yellow when the brain sees yellow light or when it gets an equally strong signal from both the red and green cone cells at the same time. Whether the light is actually yellow or a combination of red and green, the cones respond the same way and we perceive the color yellow. The additive process is also called RGB color model for the 3 primary colors red, blue, and green.
What about the animals!?!?
To the best of our knowledge, primates are the only animals with three-color vision similar to that of humans. Dogs, cats, and some squirrels are thought to have only two color photoreceptors. Although both octopi and squid can change their body color better than any other animal, it is believed that they cannot detect their own eyes.
What makes a blue shirt blue?
Your eye creates a sense of color by responding to red, green, and blue light. You don't see objects in their own light, you see them in reflected light! A blue shirt looks blue because it reflects blue light into your eyes. The shirt does not make the blue light; the color blue is not "in" the cloth. The reason you see blue is because the shirt absorbs every other color in the visible light spectrum except blue light.
The Subtractive Process
Colored fabrics and paints get color from a subtractive color process. Chemicals known as pigments in dyes and paints absorb some colors and reflect other colors. Pigments work by taking away colors from white light, which is a mixture of all the color.
The Subtractive Process
To make all colors by subtraction we need three primary pigments. We need one that absorbs blue (reflects green & red). This pigment is called yellow. We need another pigment that absorbs green (reflects red & blue). This is a pink-purple pigment we call magenta. The third pigment is cyan, which absorbs red. But, what two colors does cyan reflect? The subtractive process is often called CMYK for the four pigments it uses. The four letters stand for cyan, magenta, yellow, and black. K is used for black because B is already used for blue in the additive color process.
Additive vs Subtractive!
Why are most plants green?
Plants absorb energy from light and convert it to chemical energy in the form of sugar. This process is called photosynthesis. The plants absorb two colors of light and reflect a third.....can you guess which color is reflected?

The important molecule that absorbs light in a plant is called chlorophyll. There are several forms of chlorophyll. They absorb mostly blue and red light, and reflect green light. This is why most plants look green.
Why are most plants green?
Why don't plants absorb all colors of light? The reason is the same reason you wear light-colored clothes when it is hot outside. Like you, plants must reflect some light to avoid absorbing too much energy and overheating. Plants use visible light because the energy is just enough to change certain chemical bonds, but not enough to completely break them.
Why leaves change color
In Tennessee, we are blessed with very colorful autumns. During October and November, you can look a hillside in Cannon County and see a beautiful array of different colors where only a month before it was entirely green. The leaves of some plants, such as sugar maple trees and sassafras trees, turn brilliant red or gold in the fall. Chlorophyll masks other plant pigments during the spring and summer. In the fall, when photosynthesis slows down, chlorophyll breaks down and red, orange, and yellow pigments in the leaves are revealed.
Optics
Optics is the science and technology of light. Almost everyone has experience with optics. For example, trying on a new pair of glasses, checking your appearance in a mirror, or admiring the sparkle of a diamond ring all involve optics.
Lens
A lens bends light in a specific way. A converging lens bends light so that the light rays come together in a point. A diverging lens bends light so it spreads light apart instead of bringing it together. An object viewed through a diverging lens appears smaller than it would look without the lens.
Mirrors
A mirror reflects light and allows you to see yourself. Flat mirrors show a true-size image. Curved mirrors distort images. The curved surface of a fun house mirror can make you look thinner, wider, or even upside down!
Prisms
A prism is usually made of a solid piece of glass with flat polished surfaces. A common triangular prism is shown in the picture below. Prisms can both bend and/or reflect light. Telescopes, cameras, and supermarket laser scanners use prisms of different shapes to bend and reflect light in precise ways. A diamond is a prism with many flat, polished surfaces. Diamonds "sparkle" because light is reflected many times as it bounces around inside of a cut and polished diamond.
Four ways light is affected by matter
When light interacts with matter, such as glass, wood, water, or anything else, there are four things that can happen.

The light can go through almost unchanged (transparency).
The light can go through but gets scattered (translucency).
The light can bounce off (reflection).
The light can transfer its energy to the material (absorption).
Transparency & Translucency
Materials that allow light to pass through are called transparent. Polished glass is transparent, as are some kinds of plastic. Air is also transparent.

An object is translucent if some light can pass through but the light is scattered in many directions. Tissue paper is translucent, and so if frosted glass.
Reflection & Absorption
Almost all surfaces reflect some light. A mirror is a very good reflector but a sheet of white paper is also a good reflector. The difference is "how" they reflect. When light is absorbed, its energy is transferred. That is why a black road surface gets hot on a sunny day. A perfect absorber looks black because it reflects no light at all.
All at once!!!
All four interactions almost always happen together. A glass window is mostly transparent but also absorbs about 10% of light. Green colored paper absorbs some light, reflects some light, and is partly translucent. Can you tell which colors are absorbed and which are reflected?
Light Rays
When light moves through a material, it travels in straight lines. Diagrams that show how light travels use straight lines and arrows to represent light rays. These are much like the vector arrows we used to diagram forces.
Reflection
When you look directly into a mirror, your image appears to be the same distance from the other side of the mirror as you are on your side of the mirror. If you step back, so does your image. This is caused by the reflection of light. Reflection is the process of light rays bouncing off a surface.
Incidence Angle = Reflection Angle
Imagine a ray of light striking a mirror. The incident ray is the light ray that strikes the surface of the mirror. The reflected ray is the light ray that bounces off the surface of the mirror. The Law of Reflection states that the angle of incidence is equal to the angle of reflection. This means that a person who understands basic geometry can predict exactly where light will be reflected when it hits a mirror.
Specular or Scattered?
Both a mirror and a white piece of paper reflect all light. But why can't I see myself when I look at a piece of paper the same way I can when I use a mirror. Its partly due to the reflective coating of the mirror, but its largely due to the smoothness of the two surfaces. A piece of paper looks smooth, but its surface is actually quite bumpy. When a person uses a mirror, they are seeing specular reflection. Specular reflection is where each each incident ray produces only one reflected ray and they possess the same angle. Scattered reflection, what you get when you have a rougher surface, is where each incident ray produces only one reflected ray
Refraction
Refraction occurs when light bends while crossing a surface or moving through a material. Eye glasses, telescopes, and binoculars are a few inventions that use refraction to change the direction of light rays. Different materials have different abilities to bend light. The index of refraction is a number that measures how much a material is able to bend light. Materials with a higher index of refraction bend light by a greater angle.
How will it bend?
When light goes from air into glass (A), it bends towards the normal line because glass has a higher index of refraction than air. When light goes from glass into air again (B), it bends away from the normal line. Coming out of glass, the light ray is going into air with a lower index of refraction than glass.
Concave vs Convex
An ordinary lens is a polished, transparent disc, usually made of glass. The surfaces are curved to refract light in a specific way. The shape of the lens determines how strongly and in what way the lens will bend light. We learned about converging and diverging lenses earlier. A converging lens is also called a convex lens due to its shape (bulges in the middle). A diverging lens is called a concave lens because it bends inward or looks like a cave opening.
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