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How do I calculate the size of a shadow, how??????? how??????? how??????

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High School Dude	Posted: Aug 13 2006, 01:37 PM Quote
Newbie Group: Members Posts: 20 Joined: 29-June 06 Positive Feedback: 0% Feedback Score: -1	For my science homework, I have to calculate the size of a shadow on a screen. The object is a vertical pencil 15cm high which is 30cm away from a small spotlight which is almost level with the pencil. The screen is 1m away from the spotlight. Please, I need help. Is there some sort of formula for calulating the size of a shadow formed by a certain object aginst a screen. -------------------- "It is always better to know what you know, rather than just to know what you know." Eli Simic-Prosic
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rpenner

Posted: Aug 13 2006, 07:14 PM

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Think about triangles. Light spreads in straight lines, so the source of the light is in a straight line with all of shadows of the top of the pencil. Also, the source of the light is in a straight light with all of the shadows of the bottom of the pencil. So if the screen and the pencil are parallel, we have a situation of similar triangles. Therefore the problem reduces to one of units and ratios.

CODE

+---------+ X / 1 m (at screen)
| /
| /
| /
| /
+----+ 15 cm / 30cm (at pencil)
| /
| /
| /
|/
+ source

X = 1 m * 15 cm / 30 cm = 100 cm * 1/2 = 50 cm.

Image by photoshop, copyright 2006 by Triangle Technologies Inc.

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