Darrelene Jones, age 13, of Lebanon, Ohio, for her question:
In what direction does light travel?
A beam of light aims to travel on and on in a straight line. But along its endless route, it usually bashes into various obstacles in its path. What happens depends upon the type of collision. The original traveling direction may be changed. But rarely is the original speed reduced more than a trifle.
Late in 1972, scientists announced a revised figure for the speed of light. It is 299,792,456.2 meters per second which comes to 186,282.2 miles per second. This is slightly faster than the previous, less precise figure and is thought to be accurate to within 1.1 meters per second. This speed is somewhat slower when light streams down through the air and much slower when its angles through water.
Light sets forth from a source of energy, which must be some substance that is hot enough to flow. It fans out in straight lines, spreading through larger and larger spheres as it goes. The light of a star may zoom through vast oceans of space with very few changes in its original travel plans.
Most of the light shed by our starry sun is destined to travel on and on in straight lines. Avery small portion of its radiant energy falls upon our small planet, where different destinies await it. Some will strike mirrors and various other smooth, flat, shiny surfaces. These are head on collisions that force the light rays to reverse their directions. They bounce backward and show us reflected images.
Several changes occur when light strikes a lake. Water is a denser medium than the air, which forces a speed reduction of more than 40,000 miles per second. If a beam strikes the surface of the water from 90 degrees overhead, it may be allowed to pass straight down without changing direction. If it strikes at any angle, it must make a sharp turn and continue down in a different direction. These highly complex changes are called refraction.
Far more complicated changes occur when the various features of a light beam separate. This is called dispersion and it occurs when different wavelengths separate to display the rainbow colored spectrum. What happens becomes visible when light strikes a glass prism. Its assortment of longer and shorter wavelengths are bent at different angles. This dispenses and sends them off in different directions to display their bands of color.
Collisions with other substances cause many other changes in the direction of a light beam. Interference occurs when two light beams tangle together. If the troughs and crests of their wavelengths coincide, they reinforce each other. When the troughs and crests are out of harmony, they cancel each other out. The colors on a soap bubble are caused by interference that angles wavelengths in different directions.