Multicolored shadows

No, you aren’t seeing double!

15 minutes


  • Carefully review the general safety advice on the back of the box cover before starting the experiment.
  • Read the "Working with Batteries" section of the safety guidelines carefully before proceeding. Always disconnect the setup after finishing the experiment.
  • Disassemble the setup after the experiment.


  • Dispose of solid waste together with household garbage.
  • Dispose of used batteries in accordance with local regulations.

Scientific description

Earlier, you used optical  filters to subtract light beams and get new colors. Here, you mixed the colored rays back in to check that it actually worked!

You have tinted the light of the penlights using your filters . On the screen, in places where the light of only one penlight falls, the color  matches the color of the filter . And in places where the beams of two penlights hit, we see colors that are a result of the combination of two colored beams. When two rays with different colors blend , they form a beam with a new color . So, we get yellow  from red and green , magenta  from red and blue , and cyan  from blue and green . Where the rays of the three colors combine , we see white.

Light rays move in a straight line. When they hit an opaque object , they do not pass through it, and thus, an unilluminated area arises behind the object—this is how a shadow appears. In the experiment, the light comes from three different penlights . In select places, the object blocks all three colors, creating a shadow. But where the object blocks the light from only one penlight, rays from the other two reach the screen, creating a colored penumbra. The penumbra’s color will depend on the colors of the rays reaching the screen.