Teaching Color Subtraction

As a physics teacher, I teach arts students about color subtraction. This topic lends itself well to teach (and test) hands on.

In what follows, I show you the test that my students have to perform to prove they understood the mechanisms of color subtraction.

A little bit about color subtraction:

White light consists of a mixture of red, green and blue wavelengths (these are the so called 'primary colors' of light). When white light shines upon an object that is not a light source, some of these wavelengths get absorbed by the object. Only the wavelengths/colors that pass through or get reflected will be interpreted as a color.

So one sees a red carpet as red, because the carpet absorbs the blue and the green colors of the white light that shines upon it. Only the red wavelengths remain.

With this in mind, one can use the three primary colors of 'color subtraction' to make other colors. The primary colors of color subtraction are:

Cyan: absorbs red

Magenta: absorbs green

Yellow: absorbs blue

Supplies

- transparant color sheets in the colors cyan, magenta and yellow (I used these ones: https://www.bol.com/nl/p/100-vellen-cellofaan-gekl...

- hobby knife

- cutting board

Step 1: Prepare and Plan the Picture

Draw a picture and plan which colors you want to use. (Use red, green, blue, cyan, magenta and yellow only).

In the next step, you determine which filters you need to use to end up with the desired color.

Keep in mind you start with white light (= R+G+B) . After passing through several filters, you are left with only the wavelengths you want.

Cyan (C) : absorbs red (R)
Magenta (M) : absorbs green (G)

Yellow (Y) : absorbs blue (B)

As an example: you want green, so you want the filters to absorb blue and red, so you need to use a yellow and a cyan filter.

Step 2: Cut Out the Filters

Using your plan, cut out the yellow, magenta and cyan transparent sheets with a hobby knife.

For example, take the yellow filter and make sure you cut out all pieces that don't need a yellow filter.

Do the same for the cyan and magenta.

Step 3: Final Testing

Now overlay the three filters and see the result! (order doesn't matter)