MadSci Network: Physics

Re: Why does my shadow follow me?

Date: Tue Mar 13 10:26:43 2001
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Physics
ID: 983807504.Ph

Dear Deb, 

You ask a very good question.  A shadow is, indeed, a very strange creature.  
It moves with you, it only appears sometimes, it appears differently at 
different times, and it does not have any thickness to it.  Why is that?

Well, shadows are actually caused by a lack of light.  Your shadow is caused 
by your body blocking the light from the sun or from a light bulb.  When 
your body moves, your shadow moves with it because your body blocks the 
light in the same way, no matter where you stand.  Sometimes you can see 
several shadows.  This occurs when several sources of light from different 
directions are being blocked by your body.  Try this experiment.  Turn on 
several lights at different places in a room.  Then try to find the shadows 
that are cast by your body from each light.  This tells you something about 
how shadows form.  If you imagine a line from the shadow to your body and 
continue following that line, it will lead you to the source of the light.  
Find a shadow of your hand.  Now imagine a line from the shadow of your 
hand, through your hand and continue imagining the line and it will lead you 
to the light that is causing the shadow.

It was a pleasure speaking with you today, Deb.  Have fun with your shadow 
and try out that little experiment.

Todd Jamison
Chief Scientist, Observera, Inc.

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