Optics
Optics is the branch of physics that is concerned with light and its properties. Physicists who focus on optics study the properties of light. They also apply these properties to phenomena such as color, mirrors, and lenses.
Ancient Greek philosophers were the first to study light. They theorized that light was made up of tiny particles that could enter the eye, creating vision. The idea of the particulate nature of light was widely accepted even past Sir Isaac Newton's (1642-1727) time, although a few people, such as the Greek philosopher Empedocles (490-430 B.C.) and the Dutch scientist Christiaan Huygens (1629-1695), believed light was actually a wave. In the nineteenth century, the wave theory of light was accepted. In 1905, Albert Einstein (1879-1955) theorized that light behaves both as a particle and a wave. This dual nature of light is the theory scientists agree upon today.
We now know that light is a form of energy that travels in a wave with both electric and magnetic behavior. Light, therefore, is called an electromagnetic wave. Other electromagnetic waves include radio waves, microwaves, and x rays. Light waves are transverse waves, in other words, they oscillate perpendicular to their direction of travel. A wave that oscillates up and down is vertically polarized, and one that oscillates from side to side is horizontally polarized. Polarized light oscillates in one direction. The light from a common light source, such as a light bulb or the sun, is not polarized. Light waves originating from these sources can oscillate at any orientation. When the light passes through a polarizing filter, such as polarized sunglasses, it exits as polarized light. The filter only passes light waves that are oscillating in a certain direction.
Electromagnetic waves occur at different frequencies. The frequency of a wave is the number of wave crests that pass by a certain point in a given amount of time, usually expressed as waves per second. The frequency of a given light wave is directly related to color. Isaac Newton was the first scientist to study color. He passed sunlight through a prism and found that it could be separated into beams of light of different colors. He showed that visible light actually consists of red, orange, yellow, green, blue, and violet light. Each of these colors corresponds to a particular frequency of light. Newton passed the individual color bands produced by the prism through a second prism. This second prism re-combined the individual bands and the light exited the prism as white light. This showed that white light is actually the combination of all of the colors of the spectrum.
The color of an object is due to the frequencies of light absorbed by the object. Most objects absorb the majority of the frequencies of light. Any frequencies that are not absorbed by the object are reflected, giving the object a particular color. If an object absorbs all light except the frequencies found in the red region of the spectrum, the object appears red. Red light is reflected off of the object. White is actually not a color, but a combination of all colors, occurring when all frequencies of light are reflected. Likewise, black is actually the absence of reflected light, occurring when all frequencies of light are absorbed.
Interference patterns can impact light waves two different ways, constructively or destructively. Constructive interference occurs when two or more light waves meet in phase, meaning all of their crests (top of the wave) meet at the same time. This usually results in a more intense or bright resulting light. When the light waves meet out of phase--the crest of one wave meets at the trough (bottom of the wave) of another--destructive interference takes place. Because the two wavelengths are out of phase, they cancel each other and no light is visible.
The concept of interference is important for understanding the phenomena of diffraction. Thomas Young's double-slit interference experiment is a classic explanation for diffraction, which is the bending of light as it passes around an object. Young made two small slits relatively close to each other on a dark board. When he shined a light through the slits and observed the light on a screen, he noticed that the light did not pass directly though in two straight lines. Instead, there was a pattern of alternating bright and dark bands of light. This resulted from the light waves fanning out-diffracting-as they passed through the barrier slits, much like water ripples when it passes from a small opening into a larger body of water. Because light waves were passing through two slits, two fans were created that overlapped at certain points. Some of these points experienced destructive interference, while other were constructive, thus leading to the alternating bands of light. The dark bands occurred when light waves canceled each other out.
Other phenomena associated with light are called reflection and refraction. Light is reflected when the light waves bounce off of something and travel in a new direction. A surface that causes light to bounce back is called a reflective surface. A mirror is an example of a reflective surface. The angle an out going light ray makes with a reflective surface will be equal to the angle of the incoming light ray. To an observer, a reflected light ray will appear to come from behind the reflecting surface. For instance, when a person stands in front of a mirror, they will see an image of themselves that appears to be behind the mirror. Because the image appears to originate from an imaginary point, the image is called a virtual image. A virtual image created by a mirror is the same size as the original object.
Refraction can occur when light travels through one medium into another. The velocity of light is different for various materials. For instance, the velocity of light in air is slower than the its velocity in vacuum and slower still in glass or plastic. Under the right circumstances, the light ray will be refracted back into the original material. In a sense, the light ray reflects off the boundary. For example, if a waterproof flashlight is held in a bathtub of water at different angles, a particular angle can be found where the beam does not escape the water to shine light through the air above the water surface. The light is refracted at the surface of the water back into the water instead of being passed through the water and into the air. This angle is called the critical angle. Any angle beyond the critical angle will cause total internal reflection.
Optical fibers, also called light pipes, utilize this phenomenon. Light travels through the transparent fibers by a series of total internal reflections, much like a rubber ball would bounce through a pipe. Fiber optics have many different important uses today. Mechanics use optical fibers to shine light deep into engines. Surgeons use them to see inside a patient's body. Optical fibers are also used in communications because they are less bulky and more inexpensive than copper cables. Information in these fibers is carried by light instead of electrical current.
Another form of light that has become indispensable in society is the laser. The light in lasers results from photons emitted by highly excited atoms returning to their ground state. The photons are harnessed between two mirrors where they continue to collide until they collectively exit in one direction at a specific wavelength. Laser light is a very precise, specific wavelength that can be altered to match the absorption of almost any substance. The laser light will only damage materials whose absorption band matches the lasers wavelength. This controlled intensity makes the laser a handy tool for several applications ranging from surgery to reading compact disks.
This is the complete article, containing 1,285 words
(approx. 4 pages at 300 words per page).
