Light and Color - Mission LC9 Detailed Help

Three spotlights of equal intensity illuminate a 'white' screen with red, green and blue light. A person stands between the spotlights and the screen in such a manner as to cast three colored shadows upon the screen. The three shadows overlap, producing several colored regions. Indicate the color of each region.


Light and Shadows
Shadows are projected onto a surface when an object is positioned between the light source and the surface. In such instances, light is projected onto part of the surface but not onto all the surface. An outline of the object is formed on the part of the surface that light does not reach.

Many students of physics have seen a diagram similar to the one shown at the right. The diagram depicts three circles colored with the primary colors of light - red, green and blue. The primary colored circles overlap to produce other colors of light, known as the secondary colors of light: cyan (C), magenta (M) and yellow (Y). For instance, the red and green light combine to form yellow light. Green and blue light combine to form cyan light. Blue and red light combine to form magenta light.

Three shadows will be cast onto the screen - one for each light. That is, there will be one region on the screen - in the outline of the person - where red light does not reach. And there will a second and a third region on the screen - in the outline of the person - where green light does not reach and where blue light does not reach. Some logical thinking will help you determine which shadow is which. The shadow resulting from the blocking of blue light will be filled in with red and green light. This area will appear yellow since red and green light combine to form yellow. The appearance of the other two shadows can be determined using the same logic. (If necessary, use the color wheel in the Dig That Diagram section.)
There is some overlapping of shadows - in some cases two shadows and in one region three shadows overlapping. The region where all three shadows overlap (directly behind the person) is a region where none of the lights reach; it is a dark region that is not illuminated by light at all. Regions where two shadows overlap are regions where only one light will reach. Some logical thinking and some simple straight line geometry will help you determine the color of these overlapping regions.
Finally, outside of the three shadows, red, green and blue light will combine to produce a third color. If you know what red, green and blue light produce, then you will know the color of this region.