Hands-on Science Carnival 2010 Activity Stations: Light & Optics

Coloring in 3-D

[Shopping List: Paper or plastic ChromaDepth 3-D glasses; glass or plastic prisms; Rainbow glasses; black mat; colored letters, numbers and other shapes; white boards with dry-erase markers; black chalk board and colored chalk; white paper and crayons; rainbow glasses and peepholes]

1. Put on a pair of 3-D glasses and look at several brightly colored objects placed on the black mat. Some colors should appear floating above the mat, while others will appear to be below the mat, giving a 3-D effect.
2. Look at the same colors on a white background, using the white board or a large piece of white paper. Notice that while you still see the 3-D effect, the locations of the various colors in 3-D space have switched.
3. Try making your own 3-D art- all that matters are the colors you choose. You can use chalk, crayon, paint, cloth, marker, etc. Any colored artwork will appear to be 3-dimensional as long as you are wearing the special glasses.
4. To help understand how the glasses are producing this effect, look through the glasses at a thick solid black line on a white background, closing one eye at a time. With your right eye, you should see a red edge on the right side of the line, and a blue edge on the left side of the line. With the left eye, the edge colors will be reversed. Also take off the glasses and hold them in front of the line (at least several inches away) like a magnifying glass, and look through one lens at a time. Notice that not only does the black line have colored edges as before, but that the whole line is shifted- to the left in the right lens, and to the right in the left lens.

The glasses are actually available in Crayola 3-D Sidewalk Chalk and other coloring kits which can be purchased at most stores that sell other Crayola products. They lead you to believe that there is something special about the chalk (so you’ll buy more “3-D” chalk from them), but it’s the glasses that produce the effect. Anything colored will work.

What’s Happening: We see the world in 3-D because we have two eyes, and we call this stereo vision. When an object is very close to us, our eyes must turn at different angles to look at it (imagine crossing your eyes), and see slightly different views of the object, while for objects very far away, both of our eyes are looking straight ahead and see almost exactly the same view. Try holding your finger about 1 foot away from your nose while looking at something very far away. Now close one eye, then open that eye and close the other, and keep switching back and forth a few times. Notice that your finger seems to jump back and forth each time you switch eyes, while everything far away stays the same. This is called the parallax effect, and it’s how your brain determines which objects are closer than others in 3-D space (old time cameras also used this effect to help you focus).

The Chromadepth glasses work by tricking your brain into thinking it’s seeing parallax that isn’t really there. Lenses and especially prisms can bend light rays so that your eyes would need to turn to follow them, so each of the lenses in the glasses have a layer that acts like a prism, and aligned oppositely (that’s why the right lens shifts the black line to the left, and vice-versa). But this is not enough to produce the 3-D effect. For that we need another property of lenses or prisms called dispersion, which just means that light of different colors bend at different angles as they pass through the glass. For visible light, violet bends the most, red bends the least, and the rest of the colors fall in between. This is how we say prisms separate white light into all of the visible colors of the rainbow (they were all there to begin with). While prisms disperse light pretty well, diffraction gratings can really spread the colors out. Try on a pair of the rainbow glasses, which have diffraction gratings as their “lenses”, to see how dramatic dispersion can be, These diffraction gratings are simply made by scratching very thin and closely spaced parallel lines in the plastic, but how they work requires a discussion of the wave nature of light which is a little beyond the scope of this article.

The Chromadepth glasses are really a combination of a prism layer with a diffraction grating layer (called binary optics) that causes the various colors of light to bend at different angles (and in different directions for the left and right eye), so that your brain thinks (using parallax) that some colors must be closer than others. When you look at a black background, red should appear the closest, then orange, yellow, green and finally blue and violet should appear the farthest away. More accurately, yellow appears to be at the about the same depth as the background, with red and orange appearing closer, and green and blue appearing farther away. [That’s the reason for using both a prism and a diffraction grating layer, and it just makes it a little more comfortable for your eyes to view.] When you look at a white background, colors appear at different depths determined by the complimentary colors. [The primary colors are red, green and blue, while the complimentary colors are cyan (green + blue, i.e. no red), magenta (red + blue) and yellow (red + green)] So on a white background, cyan appears the closest, magenta appears at about the same level as the white background, and yellow should appear farthest away. [Note that some people may see yellow as the closest color, that’s because some yellow crayons, paint etc. have a little green in them which actually puts them closer to cyan on the color scale than a pure yellow, or a yellow with a little bit of orange.]

Variations: Hold your hand up against the bright sky outside, or a bright light inside and spread your fingers. Can you see your bones? When you look at a dark object against a bright background, the bright light near the edges is diffracted or bent and appears to be coming from the edge of your hand, creating a bright halo effect. Your fingers are too wide for this halo to completely cover them however, leaving a darker region in the center which- especially with a little power of suggestion- you might easily assume must be your bones, as if you were looking at an X-ray (of course in a traditional X-ray, bones actually appear lighter, not darker than the surrounding tissue.) Of course this is just an optical illusion. This diffraction is caused by very narrow lines etched in the plastic lenses (a diffraction grating). Before such diffration gratings were common, glasses which produced this effect were made using translucent materials with very closely placed slits, like feathers or nylon stockings and sold as "X-ray Specs" in comic books and other places. A very fine screen can produce this same effect as well.

Another common example of a diffraction grating are "Rainbow Glasses." Like the ChromaDepth lenses, the diffraction gratings in Rainbow Glasses bend light of different colors by different amounts, producing a rainbow effect from every bright light source, especially point sources like light bulbs.