Animated image with red, blue and no filter applied.

Activity Notes


Using affordable supplies, demonstrate how light is reflected or absorbed when passed through color filters.



  • If you’re short on materials, projecting using a document camera can help students see the effect.
  • For students with colorblindness, consider adding a legend to your colored beads using numbers (i.e., red = 1, blue = 2, etc.) and adding these numbers to your images so that all students can follow along. This can be easily accomplished by writing the numbers and legend on a white piece of paper and putting the container of beads on top.
  • For younger students, it may be helpful to display a picture or screenshot of the beads without a filter to help them compare and follow along. Instruct them to pick a bead to focus on as you change the filter to help them see the changes.

Tips for Remote Instruction

  • If teaching online, the beads can be placed on a string and worn as a bracelet for ease of displaying on a computer camera. You can also cut out squares or stars from colored construction paper and attach them to a piece of white paper.


In astronomy, we look at stars and distant galaxies, studying the light that reaches our telescopes on the ground and in space. The color of the light we see tells us a lot about what we’re looking at, how far away it is, and even how fast it’s moving! Sometimes, we use filters – materials that allow different types of colors to pass through them – to better study specific parts of the light that is detected.

Andromeda galaxy in different wavelengths

The Andromeda galaxy is shown in different wavelengths of light, revealing some features and hiding others. By viewing stars and galaxies in different wavelengths, astronomers can study phenomena that aren't apparent in the visible light. Image credit: NASA, ESA | + Expand image

When we look at an object in normal light, we’re really looking at how it reflects or absorbs white light. For example, we think of plants as green because they absorb red light and bounce the green light back to our eyes. When we use a filter, we allow only certain wavelengths of light to reflect back to our eye. Depending on the filter, this can make objects appear lighter or darker and can even reveal objects that are undetectable in other wavelengths.


  1. Prepare a cup full of colorful beads or aquarium gravel, or cut out shapes, such as stars, from construction paper, and place them on a piece of white paper.

    If you’re doing this activity online, or without an overhead camera, a cup of beads may be difficult to angle toward your computer camera. Consider stringing the beads into a bracelet or necklace to keep your hands free. Wearing a light shirt or holding up a white background may help your students better see the effect.
  2. Place a red piece of cellophane over your paper or beads. You can also place the cellophane directly over your computer’s camera.
  3. Ask students to describe how the color of certain beads has changed from before and after the cellophane was used as a color filter. Students should note that red and orange beads continue to show through the filter, while blue and green beads now appear black.
  4. Repeat with a piece of blue cellophane. Students should now note that the blues and greens still come through, but the red beads are now much darker.
  5. Have students investigate this effect further by trying it with their own set of beads and scraps of cellophane.
  6. Ask students to share why they think the beads appear to change color when the filters are applied. Explain to students that when we see a color, we’re seeing only the color that is being reflected back to our eyes; like a mirror that absorbs part of our reflection and only returns some of it back to us. When a filter is applied, we change which colors are absorbed (appear dark) and which are reflected (appear bright).
  7. Combine filter colors over your beads or shape cutouts. What can you predict about the colors we see through multiple filters?
  8. Consider taking a laser pointer and shining it on your cellophane filters. As always, use caution when using a laser pointer, and be sure not to point it at your students or your camera. Ask students to predict which laser color they expect to be able to shine through the filters. What would they expect to happen if a green laser was pointed at a red or orange filter? Or a red laser through a blue or green filter?


  • Consider your student artists, who may have seen this effect when mixing paint. Pigments in paint or ink balance what light is absorbed and what light is reflected to result in the colors we see. By mixing different colors of paint or ink, we get entirely new colors.
  • Remind students that the colors we see are in the visible spectrum, a tiny sliver of the electromagnetic spectrum. Filters also work on portions of the electromagnetic spectrum that we can't see, such as in infrared and ultraviolet. See extension below.


To really impress your students, extend the demonstration using a standard TV remote control and the front facing camera of your phone or the camera on your computer.

  1. If demonstrating in class, have your students confirm that when looking at the bulb of a remote control with their naked eye, they don’t see any light when pushing the buttons.If teaching remotely, this can be accomplished simply by asking students if they normally see any light beam when they use their remote controls.
  2. Now point the remote control at your computer camera or front facing phone camera and press any button. You should see the light bulb illuminate as it passes through your camera’s filter. Be sure to let your students try this at home!

Note: This is best performed on a phone's front-facing camera or older phone cameras. Newer phones have more selective color filters that no longer transmit infrared light.

Explore More