Color Mixing 

Springer, Ellen                      Nazareth Academy
                                     (312)354-0061
                           

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.