High School Dude
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.
rpenner
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.
PhysOrg scientific forums are totally dedicated to science, physics, and technology. Besides topical forums such as nanotechnology, quantum physics, silicon and III-V technology, applied physics, materials, space and others, you can also join our news and publications discussions. We also provide an off-topic forum category. If you need specific help on a scientific problem or have a question related to physics or technology, visit the PhysOrg Forums. Here you’ll find experts from various fields online every day.