What has happened to this pencil? It was perfectly normal and intact until someone decided to pour water into the glass. What kind of water does that to a pencil? The water in this picture is actually not special at all: it is ordinary tap water. This pencil is still intact, but it appears distorted because of a property of light called refraction. Refraction causes light to bend when it passes from one substance into another, in this case from air to water. You can test this property with any object, not just a pencil. You may observe refraction affecting your own body when you are at the swimming pool. The parts of your body out of the water may not line up with the parts in the water. This distortion is all due to the bending of light.
The term "refraction" is not exclusive to light. Refraction refers to the turning or bending of any wave, such as light and sound, when it passes through a substance of differing density . Refraction does not occur when light passes from blue-colored water to red-colored water because both waters have the same densities. We most commonly see refraction occur when light passes between air, water and glass.
Why does Light Bend?
Light only bends when it passes from a substance of one density into a substance of a different density. The bending occurs because the speed in which light travels through a substance depends on the substance's density. In air light travels at about 186,000 miles per second but in water light travels only about 140,000 miles per second (see chart below). Since light travels slower in water than in air, water is said to have a greater optical density than air.
When light travels through air it travels at 186,000 miles per second. As the light encounters a water surface some of the light is reflected and some of the light passes into the water. Upon entering the water, however, the light slows down by 46,000 miles per second. This change in speed causes the direction of the light ray to change. When we look at a pencil in a glass of water the light from the pencil in the water is being refracted, so it appears in a different location. The result is the illusion of a broken pencil.
Laws of Refraction
Refraction does not occur at random angles; there are specific laws that describe how refraction works. The first two laws of refraction are:
These two laws tell us that if the pencil were to stand straight up on its lead in the glass that we would not see any distortion because refraction would not occur. When the pencil leaned against the inside of the glass, as we saw, it entered obliquely. Refraction occurred when the pencil was leaning and we saw the distortion that indicated so.
The third and fourth laws of refraction are demonstrated in figures 1 and 2 below.
These two laws explain the angles to which light is refracted when entering a substance. The diagrams show light passing from air to water and from water to air. In figure 1, light (green ray) hits the surface of water at angle A, called the angle of incidence. Since water has a higher optical density than air we know that the light will be refracted to a smaller angle (angle B) relative to the normal. This angle is called the angle of refraction.
In figure 2, light is being emitted from the bottom of the glass. The water travels out of the water at angle C and, following the fourth law, is refracted at angle D, which is larger than angle C. In this instance the angle of refraction is greater than the angle of incidence.