Why Is The Sky Blue?

"Why is the sky blue?" is a question which has often been asked, but never satisfactorily answered. Helmholtz offered an explanation which depended on the reflection of solar light by the air particles in the atmosphere. These particles being very minute would reflect preferably the shorter waves of light, i.e., blue waves, while they would allow the longer waves, corresponding to green and red light, to pass through them; just as a log of wood floating on the surface of still water would throw off the tiny wavelets caused by a falling drop in its neighborhood, while the same log in long ocean swells would be tossed to and fro without noticeably impeding the progress of the waves. Dr. E. L. Nichols (Philosophical Magazine, December) has propounded another view, which has much to recommend it. According to Young and Helmholtz's theory of color-impression, there are in the eye three sets of nerve-termini, one set chiefly influenced by the red, another by the green, the third by the violet rays. The impression of colors is the resultant of the intensities of these three rays. The impression upon the nerves is not directly proportional to the intensity of the ray, the different nerve-termini being subject to different laws. For very feeble rays the "violet" nerves are very sensitive, while the "green" and "red" nerves scarcely see at all. As the light increases in intensity the "red" and "green" nerves increase in activity, while the "violet" nerves become tired and dazzled. For rays of dazzling brilliancy, the "red" nerves are in their most sensitive condition. Thus, of the simple colors, as the brightness increases, red and green change to yellow, blue becomes white. Daylight, at ordinary intensities, affects the three sets of nerve-termini equally: the resultant impression is whiteness. Now, daylight is simply the light of the sun weakened by manifold diffuse reflections. The direct rays of the sun, as we let them fall upon any colorless object, appear also a white light; but, on attempting at noon, on a clear day, to gaze into the sun's face, the impression is of blinding yellow. It is not that the direct rays differ in composition from diffused daylight, but that the "violet" nerves cannot transmit the action of such strong light. The moon, with enormously less illuminating power than the sun, seems bright, and is far brighter than the open sky. In passing from the intensity of the moon's rays to those reaching us from a corresponding bit of the open sky, we may, perhaps, take a step as great as that between the brightness of the sun and moon. In general, white light will appear bluer and bluer as its intensity diminishes, and this law will apply to the skies; as the light they reflect becomes fainter and fainter, they will increase in blueness, even though the light by the process of reflection suffer no change in composition.

—London Academy.