fuckyeahphysica: Try bringing two of your fingers closer in the…

fuckyeahphysica:

Try bringing two of your fingers closer in the back drop of a light source and you would observe this:

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Long before your fingers actually touch, the edges magically seem to touch each other. How is this even possible?

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Transit of Venus

When scientists were observing the transit of Venus from Earth i.e when the planet Venus passes directly between the Sun and Earth,they faced a similar problem. 

At the moment when Venus should
nearly touch the edge of the sun, the circular planet began to elongate.

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                                                PC: NASA

And they noticed the same phenomenon for Mercury as well (which has no atmosphere).


What is causing this optical phenomenon?

The physics behind this beautifully bizarre optical phenomenon will be revealed tomorrow on FYP!.

But since this is something that you can all try at home, we strongly encourage you to play around with this and get a feel for it. It requires only your hands and a source of light.

Once you do, try to hypothesize  a solution for this behavior.

Have fun!

The black drop effect

Let’s take a closer look at this optical phenomenon by projecting your hand on to a screen and bringing the fingers closer together.

Observe the behavior of the penumbral region of the shadow i.e the less darker region:

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Ahhaa… Notice that even though my fingers are not touching each other in the last image, if you see the shadow it seems as though they are!

This is because of when two penumbral regions of the shadow overlap, you get a much darker region in the middle. Here’s an illustration:

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Yes in reality, you do observe gradients of darkness in between the two objects like so:

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BUT our eyes are not that great at handling such fine contrasts in darkness; It clips off the less darker regions between the two shadows and replaces it with the surrounding darker region.

A similar response is rendered by the camera’s noise suppression algorithm too. That’s why you get that bulge connecting the two umbras irrespective whether you view it through your eye or through the camera.

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In the case of the camera, this can be rectified using the appropriate tools and processing, whereas in the case of the eye you are stuck with it.

The case for Venus Transit

You can observe the same clipping phenomenon (called Black drop effect) that we talked about if we project the image onto a camera instead of a screen:

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If the light source were the sun, the object were Venus instead of your fingers, and the screen were your eyes, you get this fuzzy shadow behavior commonly observed during the transit of Venus.

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This is not a droplet, but the black drop effect observed during the  transit of Venus

The case for Diffraction (OPEN DISCUSSION):

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When I posed this question to a lot of my friends, their first response was Diffraction but honestly I couldn’t visualize this phenomenon with Diffraction.

I spent a lot of time playing with a laser module trying to figure out how Diffraction would fit into this explained but I am unable to come up with a reasonable argument for it.

If you have a valid explanation of this using Diffraction(or other), please enlighten me and the rest of the community. I personally would really love to know.

Thanks and have fun!

** If this fascinated you, check out this stackexchange post for more