Why are the edges of the shadow so bright?

Question

Today I noticed something while playing with my specs (having diverging lenses).

Have a look at it :

This is the shadow of my specs on the wall in sunlight. The region of the lens appears darker and the edges appear extremely brighter (brighter than even the part of the wall outside the shadow). This leads me to two main questions:

Why is the region of the lens appearing dark ?
And why are the edges the brightest part of the wall ?

Made a small edit near the end, feel free to roll back if you didn't like it. — tryst with freedom, Dec 08 '20 at 16:52
You are short sighted. Long sighted people get a black ring instead of a bright ring. — cmaster - reinstate monica, Dec 08 '20 at 21:29
Btw, you are seeing stuff twice along the edge of your glasses. Once through the glasses, and once blurry outside of your glasses' frame. — cmaster - reinstate monica, Dec 08 '20 at 21:31

score 19 · Accepted Answer · edited Dec 20 '20 at 16:12

19

Look at it (pun intended) this way: your diverging lenses are making the central area darker because the irradiance, i.e. energy per unit area, is reduced. But that energy doesn't disappear: it has to go somewhere! In this case, at least some of it is observed to arrive in that "border" region which is thus brighter than the rest of the scene. This area gets both the direct light and some of the diverged light.

edited Dec 20 '20 at 16:12

Ankit

8,306

answered Dec 08 '20 at 14:03

Carl Witthoft

10,979

3

hmm if the total sunlight into our eyes is less, do we see stuff with less brightness less when using specs? – tryst with freedom Dec 08 '20 at 16:46
6

@Buraian if you are significantly nearsighted, yes, that is true. Given the "dynamic range" of our retinas and pupil diameters. Example: my correction was about -7.5 diopters, and when I first switched to contact lenses, it took a while for my eyes to adjust to the new brightness of direct sunlight outdoors. – Carl Witthoft Dec 08 '20 at 17:39
Damn that's interesting. I'm around -3.2 btw :D – tryst with freedom Dec 08 '20 at 17:49
3

@CarlWitthoft and Buraian, I was about -8.5 diopters before Lasik, and approximately 20/20 now (20/20 right eye, a bit undercorrected in the left eye). If you have the option and the money, I highly recommend Lasik. – David White Dec 09 '20 at 20:12

score 6 · Answer 2 · edited Dec 14 '20 at 14:19

6

Why is the region of the lens appearing dark (although I see things clearly when I wear them) ?

The lenses you are wearing are for short sighted vision because they are diverging the light that is why the light is not inside the region of the frame. You see clearly because they are made specifically for you :)

And why are the edges the brightest part of the wall ?

actually it is not necessary for edges to be the brightest. It is only observable at a certain distance from the wall and the brightest part coincides with your edges of the frame. :)

Hope it helps

edited Dec 14 '20 at 14:19

Ankit

8,306

answered Dec 08 '20 at 12:41

please confirm and tell me because i am always confused between these two terms – Dec 09 '20 at 13:20
@cmaster-reinstatemonica i would edit it then – Dec 09 '20 at 13:20
@cmaster-reinstatemonica thanks – Dec 09 '20 at 18:22
Thanks for the edit. One last comment: Whether the edges are the brightest part of the wall depends on a) the width of the frame (= the real shadow) b) that the glasses are for a shortsighted person. If the glasses were for a far sighted person, the lenses would focus the sunlight, leading to a very dark circular shadow surrounding two bright spots of concentrated light (the brightest on the wall). At least on short distance. If you move far-sight glasses further from the wall, the bright spot first shrinks to an image of the sun, then it grows again until it becomes wider than the shadow. – cmaster - reinstate monica Dec 09 '20 at 19:28

Why are the edges of the shadow so bright?

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