What makes a surface red?

Suppose you shine white light on a surface and the surface looks red. White light is a mixture of photons with many wavelengths, but the red light you see coming from the surface is mostly the “red” wavelength. What happens in the surface to change the color to red?

In the NetLogo model below, arrowheads representing photons are created and directed at an opaque surface. Initially they are a random mixture of red, green, and blue, using the RGB system. When they hit the surface, you can choose which ones are absorbed. The other photons not absorbed are re-emitted in random directions.

The circle to the right displays the color of the average RGB values of all the photons that are reflected. If you looked at the surface, this is what you would see.

Click on setup and go to run the model.

• Try absorbing different colors with the three switches. Does the reflected color match your expectations?
• Try absorbing all three colors. What do you expect the result will be?
• Follow the fate of individual photons. Use the slider above the model to slow it down and reduce the “light-intensity” slider to 1.
• Take a snapshot that shows a red object. Explain in the caption what settings gave this.
• Take a snapshot that shows a blue-green object and explain the in caption why it appears that color.

To observe more closely what is going on, slow down the model with the slider above the graphics window. Depending on the settings, what happens to each incoming color when it hits the surface?

Note: this model uses the RGB system to give the incoming photons colors.
For example, RGB 1 0 0 is red; RGB 0 1 0 is green; RGB 0 0 1 is blue. RGB 1 1 0 is yellow — a mixture of red and green, and no blue. If the gas absorbs red, it changes the first number to 0. That is RGB 1 0 0 becomes RGB 0 0 0, which is black. Knowing these rules, can you figure out what the surface does to each color?

Summarize what you have discovered about how photon absorption and color are related.

Is it true that a red surface absorbs red light?
Explain your answer.

Leaves that are responsible for photosynthesis, which is a process that uses light energy to produce chemical energy for plants, are usually green. Based on that fact, what wavelengths are needed for photosynthesis?