Imagine that we stand on any ordinary seaside pier, and watch the waves striking against the iron columns of the pier. Large waves pay very little attention to the columns—they divide right and left and re-unite after passing each column. But the short waves find the columns of the pier a much more formidable obstacle. When the short waves impinge on the columns, they are reflected back and spread as new ripples in all directions. To use the technical term, they are “scattered.” The columns hardly affect the long waves at all, but scatter the short ripples.
We have been watching a working model of the way in which sunlight struggles through the earth’s atmosphere. The atmosphere interposes obstacles in the form of molecules of air, tiny droplets of water, and small particles of dust, which are represented by the columns.
The waves of the sea represent the sunlight. We know that sunlight is a blend of lights of many colors—as Nature demonstrates to us when she passes it through the raindrops of a summer shower and produces a rainbow. We also know that light consists of waves, and that the different colors of light are produced by waves of different lengths, red light by long waves and blue light by short waves. The mixture of waves which constitutes sunlight has to struggle through the obstacles it meets in the atmosphere, just as the mixture of waves at the seaside has to struggle past the columns of the pier. And these obstacles treat the light waves much as the columns of the pier treat the sea-waves. The long waves which constitute red light are hardly affected, but the short waves which constitute blue light are scattered in all directions.
Thus, the different constituents of sunlight are treated in different ways as they struggle through the earth’s atmosphere. A wave of blue light may be scattered by a dust particle, and turned out of its course. After a time a second dust particle again turns it out of its course, and so on, until finally it enters our eyes by a path as zigzag as that of a flash of lightning. Consequently, the blue waves of the sunlight enter our eyes from all directions. And that is why the sky looks blue.A scientist makes new observations and learns that water waves of shorter wavelengths spread in all directions not only because they scatter off piers but also because they interact with previously scattered short water waves. Drawing upon the analogy between water waves and light waves, we might hypothesize which of the following?