Color Mixing 

Springer, Ellen Nazareth Academy

Objectives: 1. To distinguish mixing of colored pigments (inks, dyes) and mixing of colored beams of light. 2. List the three primary colored light beams, and recognize that these are the colors used in colored TV. 3. List the three primary pigments and recognize that these are used in color printing. 4. By use of the color wheel predict the color produced when either colored beams or pigments are mixed. 5. Demonstrate that shadows are produced by blocking (subtracting) light, and that the bright side of a shadow can be observed by using a plane mirror to block the beam. 6. Explain the formation of the colored shadows formed in overlapping colored light beams. Apparatus: Colored filters (red, blue, green; magenta, yellow, cyan), 3 filmstrip projectors, overhead projector, white paper screen, plane mirror, food packages, simple magnifiers, darkened or semi-darkened room. Recommended Strategy: 1. Have students recall previous experience with color mixing. Ask what will happen when red and green crayons are mixed; what will happen when red and green filters are overlapped on an overhead projector. Do these; note similarity in results. 2. Ask what will happen when beams of red and green light are overlapped on a screen. The intensity of the light beams can be adjusted by blocking part of the lens. Students should realize that mixing colored pigments and mixing colored beams gives different results. 3. Have students predict the results of mixing red and blue beams, and mixing blue and green beams as above. Do it. Ask what will happen when all three colors are mixed. Do it. From the results draw the color wheel on the board. 4. Students are familiar with mixing crayons. Ask how to make green when no green crayon is available. Point out that the red (magenta), blue (cyan), and yellow on the color wheel are the three pigments they are used to mixing. Show mixing of combinations of magenta, cyan and yellow filters on the overhead. From end flap of a food carton find the color of the inks used. Use a magnifying glass to observe that the picture is formed from the overlapping of colored dots. 5. This next part has to do with the formation of shadows formed by blocking the light from overlapping colored light beams. Overlap red and green beams; have a student hold a hand or some other opaque object in front of the screen. Try varying the distance from the screen. Ask students to explain how the various colored shadows are formed. If a mirror is used to block the light, the color of the light beams(s) that are being blocked can be shown. Try forming shadows when all three beams are overlapped. 6. Interesting shadows can be formed in overlapping red and white beams. The resulting green shadow is most evident if the beams overlap entirely and the intensity of the white beam is reduced. Try combinations of green and white, and blue and white beams. Reference: Phys. Teach. 22, 419 (1986) has a discussion of yellow light produced from overlapping red and green beams.
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