twistor wrote:Look at your shadow on the ground... it's a very sharp cutoff between the shadow and sunny part of the sidewalk. If there were a penumbra present you would see a gradation in your shadow. Since you can't see it, it must not be the effect he's referring to since his claim is that the effect is observable by the human eye.
Your shadow isn't fuzzy? Mine definitely is. Go outside and hold your hand about an inch off of the sidewalk. The outline should look sharp. Now raise it up a foot or two, and unless the sun acts very differently wherever you are, the outline will get fuzzy.
grae313 wrote:dumb question - is the penumbra just a fancy name for the effects of diffraction, or are the phenomena distinct physically?
As far as I know the phenomena are distinct. The umbra is the part of the shadow in which the light source is totally obscured, and the penumbra is the part where the light source is only partially obscured. No diffraction needed. Although, as twistor mentioned, the idea can be easily generalized. Wikipedia has a good picture of the effect in relation to eclipses: http://en.wikipedia.org/wiki/Umbra