Visual Purple

The Experiment
The back of your eyeball gives residence to two different kinds of light-sensing receptors: rods and cones.  The cones see color and the rods see black, white, and shades of gray.  Our eyes adapt to darkness by producing a chemical called visual purple, which increases the ability of the rods to collect and transmit images in low-light situations. To increase the amount of light that enters the eyes, the diameter of the pupil (the black area in the middle of your iris) increases automatically.  The increased amount of light and the production of visual purple makes it easier for us to see in darker places.

    1    Dark Room   
    1    Light Switch
    1    Lab Partner with Pupil  Reflex
    1    Pencil   

1. Darken the room and allow your eyes to acclimate to the dark for five minutes.  Over the course of this experiment, as you  read the text on this page,  it should get noticeably easier each time you look at the page.

2. When your eyes have adjusted to the low light for a couple of minutes, draw a picture of your lab partner's eye in the left box  in the data section.

3. Flip the lights on and observe what happens to the size of  your partner's  pupil as it is flooded with light.

4. Once the light is on, draw a second picture of what your lab partner's eye looks like, paying special attention to the pupil.

How Come, Huh?
As the light enters the eye, the brain recognizes that there is a tremendous amount of light flooding the cones and rods. It doesn't like this, so it closes the size of the opening to restrict the amount of light. If you go outside on a bright sunny day after being in a dark house, you may notice that there is so much light that it actually hurts your eyes, and you have to squint and look down at the ground for a minute until your eyes adjust.

Science Fair Extensions
1. Photographers use a red light in the developing room.  Experiment with measuring the size a pupil dilates and the speed that it recovers in red light versus a darkened room.