# The Naked Scientists Forum

#### science_guy

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« on: 03/11/2006 08:23:50 »
I was walking on the school track today and I noticed that the shadow of my head and a bench I just walked by touched before the shadow of my head hit the bench.  I later experimented and found that the shadows tend to connect at half a centimeter from the bench's shadow.

What causes this?  I am very interested, and I dont think it could be an example of my gravity bending the light around me... Could it?

#### lightarrow

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« Reply #1 on: 03/11/2006 08:41:29 »
I was walking on the school track today and I noticed that the shadow of my head and a bench I just walked by touched before the shadow of my head hit the bench.  I later experimented and found that the shadows tend to connect at half a centimeter from the bench's shadow.

(Relax, I'm joking!).

This effect is due to light Diffraction. See the other thread "Two slits experiment":
http://www.thenakedscientists.com/forum/index.php?topic=5761.0
Look for "Single slit Diffraction". The shadow formed by an object is similar to it, just in reverse, with shadows replacing light spots. Can you figure it out? Your head shadow is made of a principal big shadow, then a bright ring, then another, little shadow ring... and so on.

By the way, if someone of you have nearsightedness in one eye, you can experiment light diffraction in this way: find an object that you can distinguish well with glasses but bad without them, then look at it through a very narrow hole, a fraction of millimeter, for example made with a pin in paper. If everything is made in the right way, you should see well again the object, almost as with glasses.

Explanation: light is spread out of the hole, as if it were bent from a glass lens.
« Last Edit: 03/11/2006 08:53:37 by lightarrow »

#### daveshorts

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« Reply #2 on: 03/11/2006 09:12:33 »
Do you mean that the edges of the shadows are slightly fuzzy? This could be because the sun isn't actually a point so each part of the sun will make a different shadow in a slightly different direction, so as you move away from the object the different shadows will go in different directions and so go fuzzy.

#### Soul Surfer

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« Reply #3 on: 03/11/2006 11:31:18 »
The latter explanation is the correct one  the sun has an angular width ov a bout half a degree so shadows have soft edges as the sun slowly goes behind the object.  The reason that the shadows appear to touch before the objects is that where they overlap it is slightly darker that the area where they dont overlap so a sott of dark area apears to grow between the two shadows as they approach each other.

#### syhprum

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« Reply #4 on: 03/11/2006 15:27:59 »
When I was in the Australian outback far from city lights in a dry semi desert area Venus was sufficiently bright to give a shadow and it was quite sharp unlike that thrown by the Sun or Moon

#### syhprum

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« Reply #5 on: 03/11/2006 15:40:01 »
"By the way, if someone of you have nearsightedness in one eye, you can experiment light diffraction in this way: find an object that you can distinguish well with glasses but bad without them, then look at it through a very narrow hole, a fraction of millimeter, for example made with a pin in paper. If everything is made in the right way, you should see well again the object, almost as with glasses.
Explanation: light is spread out of the hole, as if it were bent from a glass lens"

I often use this method to read the digital clock when I wake up, I have experimented with pin hole cameras and found the best "F" value to be about 300

#### science_guy

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« Reply #6 on: 03/11/2006 15:49:25 »
thankyou for the explanation.  That's one less thing I do not know

#### moonfire

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« Reply #7 on: 05/11/2006 14:11:18 »
Try walking backwards and seeing your shadow....hehe  Just kidding Sexy Science Man

#### Heliotrope

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« Reply #8 on: 12/11/2006 18:00:20 »
When I was in the Australian outback far from city lights in a dry semi desert area Venus was sufficiently bright to give a shadow and it was quite sharp unlike that thrown by the Sun or Moon

That's because Venus has almost no angular diameter as seen from Earth so it's basically a point source. Hence comnparatively sharp shadow.