Physics Toys Tuesday: Colored shadows

by Stephanie Chasteen on May 27, 2009

I’m not actually committing to posting a physics toy every Tuesday, but I’ll start small.

One of my favorite places to watch people back at the Exploratorium was the colored shadows exhibit.  This one’s always a winner.
Images from

This is an example of color addition.  Remember this from grade school? I only remember it because I had to teach it.  Color subtraction is what happens when you mix together pigments.  Red pigment absorbs all light but red (which is reflected to your eye).  Blue pigment absorbs all light but blue.  So mix red and blue and you’ve subtracted all colors, getting black.

Light’s weird, though.  You mix together all colors of light and you get white.  The primary colors of light are red, green, and blue.  You have receptors in your eyes for each of those colors.  If your eye senses both red and green light at the same place, your brain says “cyan” (sort of blue-green).  The really weird one is that red and green light together make yellow.  So, that’s why the shadows are colored.  The white light has all colors (R+G+B).  If you block just one of the lights, (say, the blue one) then you get (R+G+B) minus (B) which equals (R+G), or yellow.   Block the blue and the green lights and you get (R+G+B) minus (B) and minus (G), or Red.  Block all three, and you get a normally colored black shadow.

Of course, even if your receptors get the same amount of light as someone standing next to you, your brain might interpret that color differently, so people will often disagree if something is orange or yellow, for example.

Arbor Scientific has a version (Color Addition Spotlights) of this that you can buy for your classroom or, hey, if you’ve got a dorm room and some extra cash, wow, this would be a really cool party trick.  It’s actually not that expensive, considering.  But if it’s too much for you, they’ve got a Spectrum Demo kit that teaches some of the same stuff using your overhead projector (better spectrum than a wimpy little prism demo).

You can make this on the cheap from the Exploratorium’s Science Snacks website (which also has a good explanation of the science behind it) and a more detailed lesson on Paul Doherty’s teacher institute page.  And here’s a link to the Teacher’s Lab with some step by step science explanations, and how to use the science of light and color in the classroom.  And Science Buddies gives some detailed inquiry lessons using colored shadows, and a video of students doing the activity.


Matt Kriebel May 29, 2009 at 3:32 am

You can get colored outdoor spotlights in Red Green & Blue if you look around (colored floodlight are easier). Get those, a few sockets, mount them to a board and you’ve got a budget version of this toy!

sciencegeekgirl May 29, 2009 at 5:45 am

Yup, that’s pretty much what the Exploratorium Science Snack link will help you put together — cheaper, though it’s likely to not be quite as bright.

Try Rocking Horses September 17, 2010 at 7:03 am

I wonder id you could do something to a lesser degree with multiple projectors (lite-pro) , probably easier to beg / borrow / steal if you are in an educational establishment

How about an article on momentum using Rocking horses

Gough June 5, 2011 at 12:56 am

As the special bulbs called for in the “Colored Shadows” in the Exploratorium Cookbook have gotten harder and harder to find, we’ve been looking at alternatives. We purchased a set of the Arbor Scientifics spotlights, and found them to be less than satisfactory. We finally settled on a three-LED set from Sparkfun, hooked up to a “wall wart”. It works great, and the total cost was about $60. Since then, we’ve found a nice three-LED color mixing device from American Science and Surplus, called Light Demo, for $75. That one has the advantage of controls to adjust the levels of the three LEDs.

Comments on this entry are closed.

Previous post:

Next post: