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Affected · Posted: Sat Apr 16, 2005 4:02 am

I've noticed on several occasions that sometimes a solid, opaque object will seem to refract light around it's edges. This becomes apparent when it casts a shadow far away. A light fringe may then be seen around the edge of the shadow, and beyond optical illusion, which I don't think it is, the only reason for this I can think of is that the light is being refracted. But how? I'm guessing the phenomenon is linked to the way a pinhole can act as a lens in a camera obscura. Does anyone have any info on what is happening here? Can it be called refraction? What causes it?

eyewoo · Posted: Sat Apr 16, 2005 5:37 am

it's caused by the angels... Shocked

Sorry... I have no idea, but I am feeling frisky this morning.
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eskay · junior member Member # Joined: 06 Apr 2005 Posts: 4

I think that what you're referring to is closer to diffraction then refraction...Diffraction is bending of waves (light) around a corner (as in a camera obscura like you mentioned), refraction is bending as light enters a different medium (the "broken pencil" effect in water).

I don't know how that works in the case of coloured edges around shadows, but if you have an opaque object blocking light then the light is going to bend a little inwards, towards the shadow. As for the colour, this is really just a guess, but maybe at the surface of the object if its thin enough it will be translucent? Hmm, that would be more for plastic or other things...

I hope that was a little help, at least. Maybe google has more answers.

Impaler · Posted: Sat Apr 16, 2005 2:45 pm

eskay's right, it's diffraction. Light bends around a corner (like water) and spreads out. If you have enough tiny edges next to each other, you'll get a prism. A good example of this is the back of a CD.
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Affected · Posted: Sat Apr 16, 2005 11:52 pm

and the reason for this is...? That the edges of any given object aren't actually exactly "sharp" but rather "feather" out just a tiny bit? Or perhaps it's more like the edges aren't smooth on a molecular level.

Thansk for the answers.

Ragnarok · Posted: Sun Apr 17, 2005 2:27 am

The difraction is caused by the wave part of light (you know, light can be described as a wave or as particles). It's the same as when you have some water waves and they meet a hard edge, some "new waves" generate in the border, that's what happens with light. However wavelength in light is so small the effect isn't very noticeable except in some particular cases, like the one you just mentioned.
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matter · member Member # Joined: 10 Aug 2004 Posts: 82 Location: ny

this may be unrelated (except as another odd illusion), but while we're on it.. i've also noticed sometimes on clear nights, the moon produces a wide halo, very far away from its edges... as if the reflected light of the sun is somehow diffracting on our atmosphere? my guess?... any opinions, give now!

-matt
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Impaler · Posted: Fri Apr 29, 2005 9:59 pm

Those are ice crystals in the sky refracting (since they're transparent) the moon's light. Usually a good indicator of a storm coming in.

http://antwrp.gsfc.nasa.gov/apod/ap000515.html
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jfrancis · Posted: Sat Apr 30, 2005 8:58 am

And when a nearby galaxy acts as a gravitational lens and refracts the light of a distant galaxy, you get an Einstein Ring

http://www.universetoday.com/am/publish/perfect_einstein_ring.html

Here's a recently discovered "nearly perfect" one

Pfft · junior member Member # Joined: 13 May 2005 Posts: 6

There is indeed a phenomenon that lets waves diffract around an opaque object -- this can be observed in water waves, for example at the opening to a harbour. However, it is only significant when the size of the object is on the same scale as the wavelength, so for lightwaves is is only relevant on literally microscopic scales.

From the decription in the original post, I think what is going on here is was is known in the 3D graphics world as simply "soft shadows". This is not at all mysterious:

Imagine a football lying in sunlight and casting a shadow on the ground. Now imagine being a small bug crawling across the edge of the shadow and looking up towards the sun. At first you are in direct sunlight, and the entire disc of the sun is visible. As you start crawling in under the ball, it obscures more and more of the sun. After a while only half the disc is visible (and the other half is hidden behind the ball). Then the spot you are at is only getting half as much sunlight, and the ground will only be half as bright. When you crawl still further, the entire disk becomes obscured and you are in deep shade.

Thus the edge of the shadow will have an area where it smoothly transitions from sunlight to shade. (This "grey area" is known as the penumbra, by the way). How pronounced the effect is depends on the apparent size of the lightsource: a small point source from far away will cast sharp shadows, and a big lightbulb close to the object will cast soft shadows.

In the extreme case, when the apparent size of the light source when viewed from the shadow is greater than the object casting the shadow, the shape of the shadow will depend on the lightsource rather than the object. Thus for example, the shadow on the ground of an airplane high in the air will be approximately a fuzzy circle, because the sun is circular. Similarly, if you lie under a tree and look at the shadow, you often see approximately circular areas of light on the ground. Of course, the gaps that let the light throuh are not exacly circular, but since they are so small, the shape of the shadow depends just an the circular sun. *This* situation is very much like a pin-hole camera, the airplane or the gap in the foliage being the pin-hole that foces the image of the sun on the ground.