When the color mixer is completed, switch on all three LEDs. The colored lights are mixed and the ball glows in the resulting color shade, appearing approximately equal in brightness from all directions. Adjust the variable resistors until you obtain the best white (W) color (see Fig. 2). It is instructive to slightly change the current through one diode and show students how the human eye is sensitive to a minute change in color proportions when the resultant color is close to white.

Figure 2.

By switching on two LEDs at the same time in different color combinations, you can produce cyan (C), magenta (M), and yellow (Y) and check the basic rules for additive mixing of colors (Fig. 2). The white shading in the center of the cyan ball is photographic artifact.

Here comes the puzzle for the students. Insert a wire through the slit on the top of the light ball (an extended paper clip works well) all the way down into the ball. The colored shadows should appear on the ball wall, opposite to the LEDs [see photo in Fig. 3(a)]. Repeat the same with a paper strip (about 4 mm wide) and you will observe a different color pattern [photo in Fig. 3(b)]. Pull the paper strip half way out of the ball and notice a different color pattern. Ask students to explain the formation of the colors in all cases.

Figure 3.

In order to give the right explanation, the students should understand how the light emitted by the point-light source propagates, how it forms the shadows, and how the rules of additive color mixing operate. They should also take into account the arrangement of the LEDs in the ball [see inset photo in Fig. 1(a)]. The corresponding explanations of the observed colored patterns are given schematically below the photos. More pretty color patterns can be obtained by making a second slit on the side of the ball and inserting two (or more) objects into the ball (see cover photos).