Investigation 6-2

How Filters Work

Based on GEMS Color Analyzers activities and work by Elizabeth E. Roettger [], Vivian Hoette, and Kevin McCarron.

Use colored filters to decode secret messages, look at rainbows, and learn how astronomers can decode information from the sky.


  • Filters - Roscolux: Medium Red #27, Kelly Green #94 or Dark Yellow Green #90, and Primary Blue #80
  • Astronomy images:  posters, slides, e.g. from ASP Splendors of the Universe slide set, or Kevin McCarron’s HOU Color Astronomy Page -
  • White and colored paper, colored objects/fabric/clothing

5.18. Secret codes. Look at Kevin McCarron’s Visible Spectrum image page [] and look through red, green and blue filters to view it. Which filter shows the “secret message that McCarron hid in the words?” Then  look at the rainbow/spectrum at the top of the page through different filters. See if you can figure out why one color filter works best to reveal this secret message. Then, once  you think you’ve figured it out, try making your own secret message will colored pens, crayons, or computer tools to see if your theory is correct.

5.19. What is an object’s color? What happens when you look at a colored object through different color filters? Objects reflect certain colors of light, and that’s why they look that color.

5.20. Color Astronomy. Look at images of astronomical objects (posters, slides or Kevin McCarron’s HOU Color Page) through the different filters. What are we seeing?