AC5.2. 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 
  • Images of astronomical objects: observatory galleries such as  Hubble Space Telescope images,  EwellRogelio Bernal Andreo, National Optical Astronomy Observatory (NOAO), or posters/slides. Students may do their own web searches to find telescope images.
  • Color Analyzers web page
  • White and colored paper, colored objects/fabric/clothing

5.18. Secret codes. Look at the Color Analyzers page (from the PASS Colors from Space planetarium show) and look through red, green and blue filters to view the pictures linked on that page. Which filter shows the “secret messages” hidden in the pictures? Then  look at the rainbow/spectrum at the bottom of this page through different filters. See if you can figure out why one color filter works best to reveal secret messages. 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 through the different filters. What are we seeing?

The electromagnetic spectrum

The electromagnetic spectrum