{"id":38561,"date":"2015-11-16T07:44:29","date_gmt":"2015-11-16T07:44:29","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=38561"},"modified":"2015-11-16T07:48:50","modified_gmt":"2015-11-16T07:48:50","slug":"38561","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=38561","title":{"rendered":"What light through yonder window breaks?"},"content":{"rendered":"<p id=\"top\" \/><em>By Craig Bohren<\/em><\/p>\n<h3>Interference patterns on garage door windows<\/h3>\n<p>Just as for the thinner film, the reflectance varies with wavelength, but there are several peaks in the reflection spectrum rather than only one. Interference certainly occurs in the thicker film, as indeed it should since my arguments did not hinge on any special film thickness. What is at issue is not whether there is interference in thick films but rather why we do not perceive colors in light reflected by them. p.19<\/p>\n<p>A particle on a window illuminated by a beam scatters light toward the back surface of the window, and part of this scattered light is reflected to the observer. But light from the incident beam also is reflected by the back surface and illuminates the particle, which scatters some of this reflected light to the observer. Interference between these two beams with different histories &#8211; scattered by the particle, then reflected by the glass; or reflected by the glass, then scattered by the particle &#8211; is the origin of the beautiful colored fringes I saw. p.22<\/p>\n<h3>Window watching and polarized light<\/h3>\n<p>A phase shift is introduced between two perpendicularly poladzed components of light transmitted by a retarder because the refractive index for each component is slightly different. This phase shift depends on the optical path difference. Here the optical path difference arises from a difference in refractive indices, whereas for the thin film it arose from a difference in physical paths. Even though a retarder may be physically thick (i.e., many wavelengths thick), the optical path difference for light polarized along the fast and slow axes may be comparable to that for the component beams reflected by thin films because of the small difference in refractive indices for light of these two polarizations. That is, a large physical distance can be compensated for by a small refractive index difference. p.33<\/p>\n<h3>Strange footprints in snow<\/h3>\n<p>The optical thickness of a cloud (or a snowpack) is unaffected by compaction because the number density of scatterers increases in the same proportion as the physical thickness decreases. And similarly for expansion, the density of scatterers decreases in the same proportion as the physical thickness increases.\u00a0Because the product of number density and physical thickness is constant, the brightness of an illuminated cloud should not change when compacted provided that its droplets are not brought so close together that they _coalesce into larger ones. And the brightness of a snowpack should not change when compacted provided that one stops well short of forming a slab of ice. p.142<\/p>\n<p>The transverse optical thickness of the physically thicker suspension is less than that of the thinner one. As a consequence, light diffusing laterally through the thicker suspension is more likely to reach boundaries where it is absorbed, thus not contributing to the brightness as seen from above. p.147<\/p>\n<p>To an observer, the effect of a diffusing screen is to surround a point source with a diffuse halo of light centered on it, the intensity of which decreases with distance from the center. The distance r at which the intensity falls to half the central value depends on the details of scattering (e.g., the angle a) and the distance from source to screen. A diffusing screen therefore effaces details of an object to an extent that depends on its d~stance from the screen. p.150<\/p>\n<p>The nude-in-the-shower is a single-scattering phenomenon, whereas the disappearing sun is a multiple-scattering phenomenon. But the two can be brought into rough congruence by expressing the one in the language of the other. We may ask, Under what conditions will the angle a (see Figure 13.9) be 90 degrees? When a has this value, its tangent is infinite, hence so is the radius of the halo around a point source seen through the scattering medium. This radius can be increased without limit in two ways, thereby obscuring all details of an object viewed through a scattering medium: move the object farther from the medium or increase its optical thickness. p.153<\/p>\n<h3>All that&#8217;s best of dark and bright<\/h3>\n<p>How can snow be brighter than its source of illumination? Objectively, it cannot; but subjectively, it often is. We compare snow on the ground with surrounding darker objects such as trees or the horizon sky, which often is not as bright as the zenith sky on an overcast day. To observe snow on the ground and the zenith sky simultaneously requires the help of a mirror. On many a cloudy winter day when I would have sworn that my snow-covered lawn was brighter than the overhead sky, a mirror set in the snow convinced me otherwise (see Figure 15.6) even though mirrors are not perfectly reflecting. p.174<\/p>\n<p>The brightness of an object does depend on the size of its retinal image, especially when the angular size of the object is small. Brightness also depends on pupil size and the state of adaption of the eye. We are all aware of the consequences of adaptation. p.177<\/p>\n","protected":false},"excerpt":{"rendered":"<p>By Craig Bohren Interference patterns on garage door windows Just as for the thinner film, the reflectance varies with wavelength, but there are several peaks in the reflection spectrum rather than only one. Interference certainly occurs in the thicker film, as indeed it should since my arguments did not hinge on any special film thickness. &#8230; <a title=\"What light through yonder window breaks?\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=38561\" aria-label=\"Read more about What light through yonder window breaks?\">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-38561","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\/38561","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=38561"}],"version-history":[{"count":6,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/38561\/revisions"}],"predecessor-version":[{"id":38621,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/38561\/revisions\/38621"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=38561"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=38561"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=38561"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}