Optics

Other phenomena associated with light include reflection and refraction. Light is reflected when the light waves bounce off of an object to 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 (i.e., stream of photons or wave path) 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 example, 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 than 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.