{"id":35931,"date":"2014-11-24T16:19:08","date_gmt":"2014-11-24T16:19:08","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=35931"},"modified":"2014-11-24T16:19:08","modified_gmt":"2014-11-24T16:19:08","slug":"color-and-light-in-nature","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=35931","title":{"rendered":"Color and Light in Nature"},"content":{"rendered":"<p id=\"top\" \/>by Craig Bohren<\/p>\n<p>&nbsp;<\/p>\n<h3><b>Shadows<\/b><\/h3>\n<p>Believe it or not, a transparent object can cast a shadow. Look at the shadow from your eyeglasses or a drinking glass. This shadow is not due to blockage of light as with an opaque object, but rather to refraction. Refraction redirects the light and prevents it from falling where it would if it was traveling in a straight line. P.4 <i>How are te caustic and shadow of a transparent object paired together?<\/i><\/p>\n<p>The opposition effect is present over all types of landscape but is especially noticeable in uniform vegetation. It would be harder to see if it wasn\u2019t moving. Only over water is the opposition effect absent. P.6 <i>Thus the hot-spot is due to a view-dependent lack of shadowing!<\/i><\/p>\n<p>The color of the bright spot differs from the average landscape scene because it contains none of the blue component from reflected skylight that is found in shadows. Thus opposition effects tend to be slightly yellower (i.e. less blue) in tone than their immediate surroundings; dark green forests appear light green in the bright spot, light green grass goes to yellow-green. The shape of the opposition effect depends on the geometry of the shadowers. Above forested and bushy areas it is nearly circular. Over grass or cultivated fields where tall, slim plants grow parallel to one another and cast thin shadows. The opposition effect is noticeably oval with a vertical elongation. P.7 <i>Thus the hot spot should be both anisotropic and colored.<\/i><\/p>\n<p>&nbsp;<\/p>\n<h3><b>Clear air <\/b><\/h3>\n<p>The brightness of the horizon sky is found to be about as bright as the sky can be. The addition of further air would not brighten it further. This is because when there are enough air molecules in the line-of- sight, the sky becomes almost opaque due to multiple scattering, i.e. scattering of light more than once and possibly many times. P. 24<\/p>\n<p>The whiteness of the horizon sky may be tinted by light reflected from the landscape. Over water the low sky is dark, over vegetation it becomes slightly greenish, and in the desert it is brownish-yellow. P.23<\/p>\n<p>&nbsp;<\/p>\n<h3><b>Water and Light<\/b><\/h3>\n<p>Light from water comes from three places; the top, middle and bottom. This light maybe: (a) reflected from the water\u2019s surface, (b) refracted through the top and scattered from the volume of water itself, then refracted though the surface, or (c) refracted through the top, transmitted by the water before being reflected from the bottom, then refracted a second time through the air-water interface. P.71<\/p>\n<p>When waves travel in a preferred direction, glitter may curve away from the observer. This condition arises because the wave lopes are not uniformly distributed in azimuth and have a systematic tendency to be steeper on one side than on the other. Parallel waves also tend to distort glitter. P. 84 <i>Thus with strongly anisotropic waves, we should obtain curved anisotropic highlights!<\/i><\/p>\n<p>Time exposures of moon circles reveal their remarkable behavior. At a dark place on the surface, atiny bright point of light appears, apparently out of nowhere (it is actually a small distorted image of the moon). It pops into view then immediately splits in two. The two points of light glide away from each other and then merge, disappearing a fraction of a second after rejoining. Each point may divide in two several times and each pair quickly reunites. Therefore there is always an even number of points. P. 88<\/p>\n<p>Taken together, the optical relation between skypools and landpools is somewhat complementary. P.89 <i>How exactly do reflected and refracted patterns correlate here?<\/i><\/p>\n<p>Foaming bubble-filled water is white primarily for the same reason that clouds are white: scattering of light by spheres. In fact, clouds and foam are opposites of each other \u2013 the former being water drops surrounded by the air. The latter are air droplets surrounded by water. They are remarkably similar in the way they scatter light. Foam contains a wide range of spherical bubbles, ranging from less than the wavelength of light to several millimeters. Although each bubble scatters light with a well-defined and even colorful manner, when taken together with other bubbles the result is an achromatic sum. i.e. white. The same thing is true when colored material is ground into powder: regardless of its intrinsic color, many finely divided surfaces appear white. P. 99<\/p>\n<p>There seem to be two explanations for the darkness of a wet spot, depending on whether water penetrates the underlying material or not. The simplest case is when a thin layer of water overlays a non-porous substance, for example a piece of concrete. Light passes through the water and strikes the concrete below where some of it is absorbed. The rest is scattered diffusely in all directions. Some of the scattered light strikes the air-water interface from below at large enough angles of incidence to be totally reflected back downward. This internally reflected light again strikes the concrete where more of it is absorbed. In this manner, light entering through the film of water is repeatedly scattered and absorbed. When the light finally leaves the wet surface it is dimmer than light scattered by the surrounding dry surfaces and therefore appears dark by contrast. P.100 <i>Here the main cause of attenuation is TIR.<\/i><\/p>\n<p>On porous substances like sand or fabric, a different mechanism is at work. Viewed under a microscope, almost everything shows a great deal of surface structure, much of which is close to the size of the wavelength of light; sand is gritty and fabric fuzzy. Such features scatter and diffract light efficiently regardless of the material\u2019s intrinsic color. For these reasons there are always two components of light coming from every dry source: reflected light characteristic of the material\u2019s color and scattered light from its surface texture. It is this additional scattered light that lightens the surface and dilutes the object\u2019s true color. When the surface is moistened, the scattering structures become coated with a thin layer of water. This coating reduces the amount of light scattered from the surface, thereby making it appear darker and loser to its true (reflected) color. P. 100 <i>Here te main cause of attenuation is multiple scattering.<\/i><\/p>\n<h3><\/h3>\n<h3><b>Water drops<\/b><\/h3>\n<p>Figure 4.16, P.137<\/p>\n<p>What gives a cloud such definite form? Why are they not more diffuse in outline? The explanation involves the air\u2019s strictly limited ability to hold water vapor (an invisible gas). A state described by the relative humidity. Inside a cloud the relative humidity exceeds 100%. Only at the edge, where the humidity may be less than 100%, can a water cloud dissipate by evaporation. Ice clouds disappear by sublimation, meaning solid water changes directly to vapor without first going through a liquid phase, and this is a slower process. Clouds have their distinct shapes because volumes of air tend to be lofted as discrete units. These parcels maintain their identities including their boundaries which are relatively sharp. P.140<\/p>\n<p>Few objects in nature are white, yet clouds almost define the color. Why? The simple answer is that they are white because practically no light is absorbed and all colors are scattered equally. Although water itself has a faint bluish color in transmission, this hue plays no role in a cloud because light traverses such relatively small distances in traveling through individual drops or ice crystals. How light is scattered by jagged or irregular ice crystals is more complicated than for spherical water drops, but the end result is the same: ice clouds are white too. P.140<\/p>\n<p>Two factors are largely responsible for the variation in cloud brightness: (I) shadows and (2) cloud thickness or transparency. For thick clouds where multiple scattering is important and where no background light shines directly through the cloud, the cloud\u2019s brightness is determined primarily by how much light falls on it. In full sunlight it is intensely white. In the shadow of another cloud, it appears gray in comparison. Dark clouds are not made of dirty water; they are simply dark in relation to the surrounding, more brightly lit clouds. A thin cloud, on the other hand, will transmit some background light of the blue sky and much of the direct sunlight that asses through is lost to our eyes. Since nothing in the landscape ; as bright as a white cloud, transmitted background light is :limmer and so the thin cloud appears darker. An exception to this relation between thick and thin clouds happens when the cloud is back lit. Here a thick cloud appears de because of self-shadowing. P.140<\/p>\n<p>Clouds being intrinsically white, any coloration must be external in cause: (l) they may simply be illuminated by colored light; 2) they may be translucent and allow colored background light through; or (3) they may be seen at a long distance through air which absorbs certain wavelengths and adds others.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>by Craig Bohren &nbsp; Shadows Believe it or not, a transparent object can cast a shadow. Look at the shadow from your eyeglasses or a drinking glass. This shadow is not due to blockage of light as with an opaque object, but rather to refraction. Refraction redirects the light and prevents it from falling where &#8230; <a title=\"Color and Light in Nature\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=35931\" aria-label=\"Read more about Color and Light in Nature\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-35931","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/35931","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=35931"}],"version-history":[{"count":1,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/35931\/revisions"}],"predecessor-version":[{"id":35941,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/35931\/revisions\/35941"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=35931"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=35931"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=35931"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}