HI Gerhard, I think you were seeing a specular reflection. When light hits a surface, some of it is scattered in many different directions (non-specular or diffuse reflection) and some of it is reflected in a beam which is emitted in a narrow range of angles. The angle of this beam is related to the angle of the incident light by the Law of Reflection. A perfectly smooth surface would have no diffuse component. Mirrors approximate this. As the angle of the incident light gets more shallow (i.e., nearer to parallel with the surface), the specular reflectivity coefficient becomes less dependent on frequency. In other words, light reflecting off a surface at a shallow angle is not very much affected by the color of the surface. I'm not sure about the specular reflection at near perpendicular angles. I think that it is much more strongly wavelength dependent, but probably less so than the diffuse reflection. Specular reflection is also the source of glare. When you want good color rendition in photography, you often use diffuse light sources to avoid any strong specular reflection from surfaces. This property is sometimes used to make mirrors for x-rays, since they are not strongly reflected by many materials. If you use materials placed at shallow angles to the x-ray beam, the material will then reflect much more of the x-rays than it would if they hit the object perpendicular to the surface. Sean On Fri, Nov 27, 2009 at 2:44 PM, Gerhard Fiedler wrote: > Herbert Graf wrote: > >> On Wed, 2009-11-18 at 18:44 -0700, YES NOPE9 wrote: >>> To be "white" is one required to include all wavelengths of light >>> from infrared to ultraviolet ? =A0I am not sure what that means. Or >>> does one just hit the high points of the cone cells ? >> >> That depends on how you define "white". >> >> To the human eye, justing hitting the high points usually looks >> pretty white. > > I had an interesting experience the other day (after this thread > started, or else I might not have stopped to notice it :) > > I was in the attic, and sunlight shined at an angle through the window > at a standard cardboard-brown shipping box. The part that was > illuminated appeared white. When I blocked the light to the box, I saw > it as brown. But I couldn't make my eyes see it brown when the light hit > it, not clear brown, not even a hint of brown, just white... and the > light wasn't that high intensity. Strange. > > Gerhard > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist