What You Need to Know
Visible light is light that the human eye can see. The visible spectrum includes a list of seven light colors called hues. White light is light made of all the colors in the visible spectrum. A filter is a transparent material that allows only certain hues of light to pass through it.
How Does a Filter Work?
The seven hues of the visible spectrum in order from the least to most energy are: red, orange, yellow, green, blue, indigo, and violet. When combined, the seven hues of light produce white light. The color of an object depends on how the chemicals making up the object's surface absorb or reflect different hues. A filter used to separate light absorbs some hues and allows others to pass through or be reflected. For example, in the diagram on the left, when white light hits the red filter, red, orange, and yellow light pass through it. The filter absorbs the other colors in the white light. The combination of the red, orange, and yellow light is perceived by your eyes as a shade of red.
What Does This Have to Do with Using Colored Filters to Send Secret Messages?
A red filter that allows red, orange, and yellow light to pass through it could be used to make words or images appear or disappear. For example, words written in yellow ink on white paper would disappear if a red filter is placed over the writing. The yellow light from the ink blends in with other yellow light passing through the filter. What you see when you look through the filter is a red sheet of paper. The yellow ink seems to have disappeared. Remove the filter and the yellow words appear again. Would the filter cause any shade of red, yellow, or orange ink to disappear?
Fun Fact
The order of the hues in the visible spectrum is the order of the colors in a rainbow. To remember this order, memorize the name ROY G BIV, which is spelled using the first letter of each light color.
Real-Life Science Challenge
Colorblindness is the inability to see some or all colors. While there is no cure for colorblindness, scientists have discovered that colored filters can help with the problem. A red-tinted contact lens can help some colorblind people see certain colors. Recently, tinted prescription eyeglasses that are coated with a colored filter, usually magenta or orange, have become available to people who have trouble distinguishing shades of red and green from other colors. When wearing these glasses, a colorblind person who usually sees a muddy-brown leaf will see a green leaf.
Experiment
Now, start experimenting with filters to send secret messages. Remember that a secret message should be visible under special conditions. The message could be in plain view but camouflaged by its surroundings.
Hints
- Transparent, plastic, colored report folders make good filters.
- Light-colored felt-tip pens work better for writing secret messages.
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