by Marcel Minnaert

Reflection of light

Most people think that the reflection of a scene in calm water resembles the scene itself upside down. Nothing could be further from the truth. You only have to look at night how streetlights are reflected. The reflection of a bank sloping toward a river appears shortened. and even disappears when we look from high enough above the water. – p.11

The stalk acts as a capillary so that the surface tension of the water causes it to heap up around the stalk: the mound of water so formed reflects the sunlight so that it can be seen from a long way off. If one part of the pool reflects a nearby dark slope of a dune and the other the bright sky, you can see how near the diViding line, those tiny mounds of water show up light or dark dependtng on the direction in which you are looking. – p.21

First, if we suppose ourselves to be at the same height above the water as the light source, the angle subtended by the longer axis of the patch is at the same time the angle between the two steepest slopes of the wavelets. Relative to this, the transverse axis of the patch is smaller the more obliquely we look at the surface of the water. Second, if the Source is higher above the water than our eye, all the dimensions of the light patch become larger (in angular measure); they approach twice what they were originally if the source recedes to infinity. However, the ratio between the long and short axes remains about the same. – p.29

Refraction of light

The grayish aspect of the bedewed grass is caused by the reflection of the rays of light in all the tiny drops, inside as well as outside; a great many of the rays do not even touch the blade of grass. Large flattened drops have a beautiful silver sheen when see. at fairly large angles because the rays are then totally reflected at the back surface. – p.56

The curvature of light rays in the atmosphere

To fish, the sun twinkles just as stars do to us, with only the difference that fluctuations in the thickness of the water layers replace fluctuations in the density of the air layers. The latter are so much less effective that we can see the scintillation of only the sharpest point-like sources of light. – p.93

Judging shape and motion

There are a number of observations which show that the shape of the celestial vault and the apparent increase in size of the celestial bodies near the horizon depend on the direction of our gaze in relation to our body. Gauss therefore assumed that the experience of many generations has made us better adapted to the observation of those things that are in front of us than of those above us, and that this influences our estimation of distances and dimensions. – p. 180

Rainbows, halos and coronas

The modifications caused by diffraction consist in the colors being slightly different depending on the size of the drops and the fact that inside the bow supernumerary bows appear. – p.196

Closer investigation has shown that only the used, oxidized oll dripping from a car engine is capable of spreading on a wet surface. The more complete the oxidation of the oil, the thinner the layer becomes. – p.234

The coronas we see in the clouds are formed by diffraction of light by the drops of water in the clouds. The smaller the drops, the larger the coronas. In clouds where the drops are all of equal size, the coronas are well developed and their colors pure; in those clouds, however, in which drops of all sizes are mixed together, coronas of different sizes occur simultaneously, the one overlapping the other. This is why finely developed corona phenomena occur only in very definite kinds of cloud, where circumstances causing the condensation of the water vapor are sufficiently uniform; and for the same reason, a finer distinction in the sequence of the tints will depend on the number of drops of various sizes, on the thickness of the clouds, and so on. – p.241

Each of the dewdrops forms an image of the sun on the blade of grass supporting it, and rays are emitted from the image along almost the same path as the path of the incldent rays, that is, in the general direction of the sun. This would explain why the drops seem to emit light from within, in the same way as the eyes of a cat. It is also a good explanation of why one can see so much light coming from the grass in directions close to that of the antisolar point and why the intensity of the light diminishes rapidly when you look away from it. But why, then, is this light not green? In the dewdrops, total internal reflection is very likely to be the most Important factor, since the drops are distorted irregularly, especially on hairy, white woolly plants, and the light being totally reflected at various points is as fierce and white as when it arrived from the sun. This second group of reflected rays shows no decided preference for reflection in the direction of incidence. But the following ingenious observation has been made: only those blades of grass on which the sun’s light actually falls re-emit light, and these are naturally unscreened in the direction of the sun by other blades, whereas in most of the other directions there is no clear opening in front of them – p.255

Light and color of the sky

The smoke of a cigar or cigarette is blue when blown immediately into the air, but becomes white if it has been kept in the mouth first. The particles of smoke in the latter case are covered by a coat of water and become much larger. – p.260

The scattered blue light of cumulus clouds on a dark background is much more distinct than the yellow coloring of the bright parts. In the fonner case, darkness is replaced by a small quantity of light; and in the latter case, there is only a small change in an already considerable brightness; the relative difference is much smaller. – p.264

That the sky at the zenith remains blue at sunset and even afterward is a consequence of ozone absorption. – p.270

Light and color of the landscape

The color of the sea in the distance is therefore about the same as that of the sky at a height of 20-30° and therefore darker than the sky immediately above the horizon, and all the more so because only part of the light is reflected. – p.335

A local elevation at the bottom of the sea alters the swell of the waves and the rippling of the water above it and, correspondingly, more solid particles are stirred up there than where it is deeper, causing an increase in the scatterlng. So the bottom of the sea has indeed some effect but not a direct one.  – p.336

When, after a period of frost, thaw sets in, trees and walls get covered in myriads of tiny ice crystals: hoarfrost. These crystals disperse light in a special, almost unique way: when you look at right angles at the layer of ice, it is hardly perceptible; when you look more and more obliquely, the brighter the layer becomes. until. at very small angles, it is almost silvery white. Evidently, each crystal disperses the light in all directions like a miniature lamp. The more obliquely we look, the more of these sources of light come into our field of vision. – p.356  This is another instance of asperity scattering.

The leaves on the outside of the crown of the tree differ from those inside; they differ not only in size, thickness, and hairiness, but also in color. The color of the shoots at the foot of the tree, and on the trunk, is usually very light. – p.358 This is probably due to a lower concentration of chlorophilia at this point of the leaf growth.
Under the influence of sun or wind, a number of plants, such as aconite (monk’s hood), shine as if they were lacquered. This is caused by the swelling of the epidermal cells, which stretches the surface of the leaf until it is perfectly smooth. – p.358

There is a remarkable difference in color and structure to be seen in almost any landscape. depending on whether you look at it toward the sun or away from it. The entire aspect of the scenery changes. – p.361  Follows a detailed list of material effects that would be challenging to reproduce!