Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

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Teacher's Lab
The Science of Light

IntroductionLight in ColorLaws of Light
  what's important

Although mixing light seems simple on the surface, it can be really confusing. Some people dispute whether the things you see on the screen really represent shadows or not. Whatever you call them, this is what happens.

Suppose you have a red light and a green light. You see this (without the letters!):

It looks as if the green light casts a red shadow and the red one casts a green shadow. What's going on?

Try to think about it one region at a time. For example, both the red and the green lights are shining on region D (the yellow part). It may seem odd, but in light, red and green mix together to make yellow.

Neither light can get to region C, so it shows up black.

This leaves regions A and B. In region A, the red light is shining but the green is blocked by the card. Here's another way to think of this: From region A, you can see the red light bulb but not the green one. Just the opposite is true for region B.

About mixing, pigment is different from light.

You are probably used to mixing colors as pigments—like paints or food coloring. In such situations, colors subtract. The more colors you put in, the less color you get; that is, you get closer to black, the absence of color. In pigments, for example, blue and yellow make green.

Lights are different. When you put more light in, the colors add. The more colors you put in, the closer you get to white, the mixture of all colors. With light, though it seems weird, red and green make yellow. The other two combinations are more intuitive: Red and blue make MAGENTA; blue and green make CYAN.

The reason for the colored shadows may be too hard for very young children, but they can still reason about mixing colors and see the patterns that emerge.

Older children can ask even more sophisticated questions and design experiments or demonstrations to answer them, for example:

  • What happens if there are three different-colored lamps instead of two?

  • How can it be that light and pigment have different primary colors?

  • What geometrical shapes do the shadows make?

The National Science Education Standards (1996) state that "as a result of the activities in grades K–4, all students should develop an understanding of light...." (p. 123). The Standards state specifically that "By experimenting with light..., students begin to understand that phenomena can be observed, measured, and controlled in various ways" (p. 126).

The Exploratorium has more material on colored shadows.

Back to Colored Shadows


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