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Mirror, mirror

The study of reflection is a good learning tool

The study of reflection is a good learning tool

April 18, 2003|by LISA TEDRICK PREJEAN

Gazing at his spoon in a most pensive fashion, my son seemed oblivious to the food on his plate.

"Why is my reflection upside down in my spoon?"

And I thought he was daydreaming.

"It probably has something to do with that side of the spoon being concave," I said, thinking this would be something fun to explore together.

(When I shared this story with a friend, she said her daughter recently asked the same question. So, if there's a child in your life, you might need to know the answer at some point, too.)

First, review the law of reflection, recommends Jason Powell, assistant professor of chemistry and physics at Ferrum College in Ferrum, Va.

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"Everybody uses mirrors but we don't really think about how they work," Powell said.

Light is reflected from a plane (flat) mirror at the same angle that it hits the mirror.

It's similar to bouncing a ball against a wall. If you want the ball to come straight back to you, you need to hit the wall directly in front of you. If light hits a flat mirror at a 90-degree angle, it will be reflected at a 90-degree angle.

As light rays strike a curved surface, however, they spread or concentrate and are sent back at various angles. The result is a distorted image.

That's why reflections from fun house mirrors can be so bizarre. Light is being bounced back to your eyes at various angles.

The study of mirrors and reflection is an important building block, Powell said.

"It's really an introduction to optics and how they work," he said.

A spoon can act as a mirror because it has a smooth, shiny surface.

But why does a concave curve reflect an upside-down image?

When outside the focal length of a concave mirror, we see ourselves upside-down because light from low objects bounces off the spoon in such a way that it enters our eye from the top part of the spoon, and the light from high objects enters from the bottom part of the spoon. This makes the image appear upside-down, Powell said.

As you move the concave part of a spoon closer to your eye, you'll notice the reflection disappears just before it flips right-side-up. At the point where you can't see a reflection, your eye is at the focal point (focus) of the spoon. Focal length is the distance from the curved mirror to the point where light from a distance source would focus.

When inside the focal length of the concave mirror, we see ourselves right-side-up, because the light comes from the "correct" direction.

To see the right-side-up image in the concave side of a tablespoon, you may have to cover one eye and bring the tablespoon in very close to the uncovered eye. (That's the only way I was able to see a right-side-up image.)

The upside-down image you see in the spoon's concave side (where you scoop food) is an inverted real image, explains Lou Bloomfield, a professor of physics at the University of Virginia.

Real images are patterns of light in space that you can touch.

Virtual images are ones you cannot touch.

In a flat mirror, you see an object's virtual image, which is not on the surface of the mirror. It is on the mirror's far side, at a distance equal to the distance from the mirror to the actual object.

The inverted real image in the concave side of a spoon is a few centimeters in front of the spoon, where you can touch it with your finger or insert a small piece of paper into it, Bloomfield explains. A spoon forms a right-side-up virtual image of you when you look into the convex (bowed outward) side. This virtual image is a few centimeters behind the spoon.

Thomas Jefferson was so intrigued by concave mirrors that he had one in the entryway of Monticello.

He planned to use his mirrors with his microscopes, according to Susan Stein in "The Worlds of Thomas Jefferson at Monticello."

Plus, it must have been fun to see guests' reactions as they walked in the door.

Here's an experiment to try, from Bloomfield's Web site, howthingswork.virginia.edu: Put a magnifying glass on the surface of your newspaper. Slowly lift it off the page. As you do, the virtual image of the newspaper starts just behind the glass and slowly moves back away from the glass. As the distance between the magnifying glass and newspaper approach the focal length of the magnifying glass, the virtual image moves behind the glass. After that, there is no longer a virtual image. A real image begins to appear on the other side of the magnifying glass.

Check out the interactive tutorial on concave mirrors at www.mic-d.com/java/mirrors/concave.

It explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror. It also shows the effects of moving an object closer to the mirror, first between the center of curvature and the focal point (focus), and then between the focal point and the mirror surface (to form a virtual image).

Lisa Tedrick Prejean writes a weekly column for The Herald-Mail's Family page. Send e-mail to her at lisap@herald-mail.com.

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