Light is all around us. It is the primary way we come to know the universe, and thus is very important to physicists. Until the middle of 1800’s, light was taken to be a stream of tiny particles. This was the stance advocated by Newton. However, by the late 1800’s the particle theory was replaced by the wave theory. This was because light exhibited certain properties that could only be explained by the wave theory.
One of the properties of light is that it reflects off surfaces. Among other things, this reflection allows us to see images in mirrors. We see the images in mirrors as apparently coming from behind the mirror because our eyes interpret it in this manner. But when we see ourselves reflected in the mirror and raise our left arm, the image apparently raises its right arm.
Another property is the speed of light, which is the fastest anything has been observed to move. In a vacuum, the speed is 300 million meters per second. At that speed, it takes light one ten thousandth of a second to travel around the earth. When light enters a material, it slows down. The amount depends on the material it enters and it’s density. For example, light travels about 30% slower in water than it does in a vacuum, while in diamonds, which is about the most dense material, it travels at about half the speed it does in a vacuum. This slowing down of light plays a role in another property, refraction.
Refraction means that light bends when it passes from one medium to another. When light enters a denser medium from one that is less dense, it bends toward a line normal to the boundary between the two media. The greater the density difference between the two media, the more the light bends. This property is used with respect to optical devices such as microscopes, corrective lenses for vision, magnifying lenses, and so on.
You may have noticed that, when you look into the surface of a lake or pond while fishing, the fish you catch seems larger when under the water than when you actually land it. This is due to refraction. Since the air is less dense than water, the light beds away from the normal as it emerges. Another common example is that your feel look larger and closer to the surface underwater than they really are.
Another property that combines both refraction and reflection is total internal reflection. This is an interesting concept. When light coming from the air strikes water, part is reflected and part is refracted. When the angle of incidence of the light striking the water is large enough, it gets totally reflected and in fact cannot leave the water. Fiber optics uses this property of light to keep light beams focused without significant loss, as long as the bending of the cable is not too sharp. TV and telephone cables use fiber optic cable more and more since it is much faster and more efficient than electrons in an electric current.
Dispersion is another property of light. This refers to the ability to break white light into its constituent colors. White light consists of all of the colors we are able to see. If white light enters a prism, what emerges from the other side is a spread out beam of multi-colored light. Blue light, with longer wavelengths, gets bent more by the different angles of the prism than red light, and the other colors are in between blue and red on the wave spectrum.
Rainbows are natural phenomena that exemplify all of the above properties of light. They use refraction, dispersion, and internal reflection to produce their amazing hues. White light enters raindrops from the sun and gets dispersed and refracted inside the raindrops. When the dispersed light hits the back of the raindrop, it gets internally reflected, and when it emerges it gets dispersed even more.
Because it refracts more, the blue is always sat the top of the rainbow and the red on the bottom. The color you see most vividly depends on the angle of your eye. Generally, you must look higher in the sky to see the red, and lower to see the blue. What you actually see is the red on the top and the blue on the bottom, with all of the other colors in between. The arc of the rainbow depends on the angle that your line of sight makes relative to the sun behind you.
Diffraction is yet another property of light. This term refers to the fact that light bends as it goes through an opening. While it is hard to give an everyday example of this, the closest would be when there is a light source shielded by a door such that only a limited amount of light can get through the opening. However, even the area shielded is a little brighter, reflecting some actual reflection and diffraction as well. An easier example is with another wave form, sound. When someone speaks from in front of an open door, a person standing way around the corner from the door will still hear the diffracted sound waves.
Interference is another property of light. It is a phenomenon that occurs when two beams of light meet. Depending on both the nature of the two beams and when they meet, they can either merge and enhance one another and give a brighter beam, or they might interfere in such a way as to make the merged beam less bright. The former is called constructive interference, and the latter is destructive interference.
It is not all that usual for us to encounter light interference in our everyday lives. One situation that is illustrative is where there is oil or gasoline floating on the surface of a puddle. Sometimes, you will see a brilliant pattern of colors given off by the oil or gas, even when the gas or oil is subjected to white light. What happens is that different potions of the film cause different colors in the white light to interfere constructively or destructively, depending on the thickness of the film. One region of the film might look red because the red light bouncing off the top of the film interferes constructively with red light passing through the film and is then reflected back off the water below it.
We can see this more clearly with sound. When you are in the back of an auditorium, sound can reach you in a great may ways. It can take a direct path, be reflected off a ceiling, or walls, or the floor. All of these will reach you at slightly different times, and sometimes not at all. They can actually cancel each other out and you hear nothing when you sit in one area (a so called dead zone), and sitting in another, you can hear an abnormally loud sound. These are examples of destructive and constructive interference and the reason that modern auditoriums use sound absorbing materials on ceilings, walls and floors.
One experiment used to demonstrate how light signals can interfere with one another is called “Young’s double slit experiment” after the physicist who used it for demonstrating the interference phenomenon. He set up a screen with two small slits and behind it set up another screen some distance away. When he subjected the first screen to a single light source, he found that there were alternate light and dark spots on the distance screen, corresponding to points where light rays coming from the two different slits underwent constructive and destructive interference. The light rays did not interact with one another in that one ray did not exert a force on the other. This is only possible when we think of light in terms of waves.
So, what you “see” as you look out over your favorite landscape is a combination of light being reflected, refracted, dispersed, internally reflected, and diffracted. The computer inside your head (your brain) interprets all of the signals it receives from your eyes and makes a “picture” that we interpret as “seeing” that landscape. When they say beauty is in the eye of the beholder, it is really true.