redgreen

Red-Green Afterimage

Adaptive Optics

Look at a bright white piece of paper while covering one eye with a green filter and the other with a red.

Introduction

Look through red and green filters at a bright light, then look at the world around you. The colors in the world will change.

Material

Color pictures or a colorful world around you.
Transparent red and green filters.
A bright light source, a fluorescent light panel, white paper in a floodlight, white paper in sunlight.

To Do and Notice

Hold the red filter over one eye and the green filter over the other.

Look at the bright light through these filers for 30 seconds.

Remove the filters.

Look at the color pictures or the world around you first with one eye and then with the other.

Notice how the colors around you appear different through each eye.

Notice that this color difference lasts for minutes.

After the world has returned to a normal color in both eyes, look at the brightly lit piece of paper through a red filter covering both eyes. Look for 30 seconds.

Remove the filter and look at the color picture or the world.

Notice how the world at first looks greenish, and that after tens of seconds it returns to its normal appearance.

What's Going On?

When you look at the world through a red filter your red cones fire rapidly while your green cones do not. After 30 seconds your red cones will respond to red light by firing less often, the green cones respond to green light by firing rapidly. This gives the world a greenish appearance.

When both eyes are covered by the red filter the eye and brain adapt quickly to the greenish appearance of the world. It is as if the world were illuminated by a greenish light and in the presence of greenish illumination the brain can adapt its perception to give a true view of colors. This is called color constancy.

However if you adapt one eye to red light and the other to green light then switch back and forth between the two eyes as you view the world the brain does not adapt and you can see that the effects of exposure to a bright light actually last for minutes.

Activity written by Paul Doherty after an activity from Richard.