8.1

Why does the color of a piece of dyed fabric look more saturated when it is wet?

8.2

Why does the sky look red during sunset?

8.3

The BRDF for a perfect Lambertian reflector is f _r = d L _r/d E _i = 1/ ?. A Lambertian sphere when illuminated by a collimated light source (all rays are parallel) does not appear as a uniform disk because different areas on the sphere are oriented at different angles from the light source and the surface irradiance E _i falls off as cos ? _i, where ? _i is the angle between the surface normal and the incident light beam. A full moon looks like a uniform bright disk (neglecting the somewhat random dark spots for the moment). Therefore, the moon surface is not a Lambertian reflector. Since the distance from the moon to the sun is much larger than the radius of the moon, we can assume that the moon is illuminated by parallel light rays coming from the sun. Let us further assume that the moon is a uniform, smooth spherical surface which reflects light as a function of the vertical angle, ? _r, but independent of the horizontal (azimuth) angle, _r. (The vertical angle, ? _r, is the angle between the reflected ray and the surface normal.) If the reflectance of the lunar surface is ?, what is its BRDF, f _r