# What is this called?

1. May 24, 2007

### algaidaman

My question is dealing with shadows.

I noticed this the other day while standing outside. The shadow that the building cast was on the ground in front of me and was very close to my shadow. I swayed slightly and noticed that both shadows were attracted to one another to the point where they moved, independently of my own movement, and touched. The shadows sppeared "warped"; the straight linear shadow of the building was now slightly bent.

Just being curious I thought this would be the best place to ask. What is this called?

2. May 24, 2007

### Staff: Mentor

That would probably be diffraction. Light gets bent slightly when going around obstructions or through orifices (such as between you and the building).

Also be aware that your shadow and moreso the building's won't be completely sharp due to the size of the sun.

3. May 24, 2007

### KingNothing

Light moving 'past' an object tends to diffract more in all directions than if there were no object at all. So the sunlight that gets by the edge of the building hits the edge of your body and diffracts around it, conforming slightly to the shape of your body. I have attached a diagram. I don't know what this effect is specifically called.

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4. May 24, 2007

### DaveC426913

Another effect that you might be seeing:

The edge of the building's shadow (and your own ahadow) may actually be somewhat wider than you perceive it to be - the fainter part of the shadow may not be noticeable to you in the bright sunlight. But when the two faint edges of the two shadows overlap, the sum of the shadows crosses a threshold where your eye can see it.

5. May 24, 2007

### Staff: Mentor

Yes, that's what I think it is. The fainter "edge" of the shadow is called the penumbra. There's a simplified diagram here:

http://www.schorsch.com/kbase/glossary/penumbra.html

I first encountered the "umbra" and "penumbra" in descriptions of solar and lunar eclipses, but they appear in shadows cast by any extended light source.

6. May 25, 2007

### cesiumfrog

Do you understand which effect the OP is describing? Actually look at a fairly sharp shadow of a small gap between two digits of your hands, and study this as you very slowly move your hands to nearly close the gap.

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7. May 25, 2007

### DaveC426913

The attached picture has a flat object and a round object with their penumbrae in close proximity to each other.

The upper set of images is viewing the objects with ideal resolution and sensitivity to subtlety.

The lower set of objects is identical except that the sensitivity to gradients has a threshold. Below the threshold the areas appear consistently bright whereas above the threshold the areas appear consistently dark.

It should be noted that I did not "fake" the lower images in any way to get that merging effect. They are an exact duplicate the upper set of images except that I applied a threshold of 50% to them.

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• ###### diffraction.gif
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Last edited: May 25, 2007
8. May 25, 2007

### cesiumfrog

That's interesting.