The Colour and polarisation of Blue Sky Light 1

Abstract

THE theory of the colour of the sky has been of slow growth. One of the first explanations that we find in scientific literature—almost barbarous in its crudity and unsupported by fact or theory—is the speculation of Leonardo da Vinci that the blue of the sky is due to the mixing of the white sunlight, reflected from the upper layers of the air, with the intense blackness of space. This corresponds to the speculative stage of science, the age of the philosophers. In the next step analogy comes into play; this is a most valuable and effective tool for the man of science endowed with a vivid scientific imagination and with a keen, clear insight into nature, but for others a most dangerous weapon. In this case it is wielded by no less an intellect than that of Sir Isaac Newton. In his optical investigations, about 1675, he had been led to a study of the colours produced when light is reflected from thin films of transparent substances; these he found to depend upon the thickness of the film. When it is very thin it appears black; as the thickness gradually increases it becomes blue, then white, yellow, red, &c. This blue which first appears, and which may be seen surrounding the black spot on soap bubbles, Newton termed the “blue of the first order,” and he thought it was of the same tint as the blue of the sky. Analogy now steps in and suggests that the colour of the sky is due to the reflection of sunlight from transparent bodies of such a size that the reflected light is the blue of the first order. This was Newton's belief, and he thought that the reflecting particles were small drops of water.