To Do and Notice

Bring a group of 8 or more people to the exhibit, Macula.
Give them each a 3x5 card and a pen or pencil.

Have them look at the exhibit and ask, "What do you see?"

At first they say, "I see a screen that flashes bluish then purplish light."

Then someone will note,"I see a dark smudge in the center of the screen."

Important. Not everyone will see the smudge.

Suggest that people look with just one eye, then try the other eye. Sometimes this helps them see the smudge.
Have them draw their left eye view and right eye view of the splotches on the card.
Then show their sketches to other people.

Some people also see a dimmer bright smudge of the same shape as the dark one when the screen flashes purple.

What's Going On?

There is a region including the fovea of each retina called the macula. When you look at something it is imaged on your fovea. For many people this region is less sensitive to blue light than the rest of their retina. When a blue light flashes onto this region of reduced sensitivity, a dark smear is seen.

I Don't Know

Scientists do not agree on why this happens.

Going Further

Ask for questions and suggested experiments.

Some questions are: Why can't everyone see what I see?

What is the smudge? (It has a name "the macula" but scientists still argue about how it is made.)

Look at the 3 o'clock position on the screen, note that the smudge appears exactly where you are looking.
Why is that?

Try changing your distance from the screen, notice that the smudge changes size.
Notice further that when you get close to the screen the smudge gets small and when you get far the smudge gets big. That's surprising. Can you figure out how that works?

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Scientific Explorations with Paul Doherty

© 2002

18 September 2002