Dr. Gadget's Science Machine
Two light waves can produce darkness

By Joe Laszlo

Dr. Gadget (Joe Laszlo), center, helps Billy Dyer and Ashley Hirashima of Moanalua Middle School conduct an experiment on light interference. Jeff Widener • The Honolulu Advertiser

Aloha! I want to discuss light and some ways to test it. First, try this: Get a small sealed box or an old tennis ball. Carefully, make a small hole in whichever one you use. Look into the hole. What do you see? Blackness? Why?

Look at your eye in a mirror. In the center is a black spot, called the pupil. Now close your eyes. What do you see? Blackness! Why did things turn black? Because no light could enter your eye. Enter your eye? Is there a "window," a hole in your eye? Yes! The pupil is a hole. Why is it black?

The pupil and the hole in the box or tennis ball were black because you were looking into a "room" that had no lights inside of it. It was dark. Light must enter through the pupil for you to see.

To see any thing, the thing must either produce its own light or reflect light from another source. Light must enter through the pupil. The pupil's size is controlled by the iris, the colored part of your eye. At night, your pupil is quite large, to let more light in. In bright daylight, the iris makes the pupil smaller.

Light has two forms. It can be a particle or a wave.

You can prove that light travels in waves with a simple experiment. Double your left hand into a fist and place it onto your right cheek. Place your right hand so that it is against your left fist. Look between your thumb and first finger. Slowly close your fingers. Do you notice some black lines? What are these?

These are places in the space between your fingers where two light waves have canceled each other out. A physicist will say the waves have "destructively interfered" with each other. When one light wave cancels out another, you have no light. You have blackness.

Is it possible to prove that light can be a particle? Yes, but it requires some complicated equipment and a strong gravitational field to attract and bend the beam of light. This was done in 1919 by two astronomical expeditions going to view a solar eclipse — one in northeastern Brazil, the other on Principe Island in West Africa.

During the eclipse, when the sun is blocked by the moon, it is possible to see stars in space past the edge of the sun because the sky is black. If gravity can bend light, a star that has "set" below the edge of the sun could still be visible. This is exactly what happened.

These observations also helped to prove Einstein's General Theory of Relativity. This explains why collapsed stars that have turned into black holes are black. Their gravity is so strong that light can not escape from them!

Think about that. Light is energy that travels in waves, but it is also particles. It almost sounds like science fiction! We'll talk more about light next time. Until then ... a hui hou!

Do you have a question or suggestion for Dr. Gadget? Mail your letter with your name, age, school and telephone number to: Dr. Gadget, c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to ohana@honoluluadvertiser.com only if you're 13 or older.