Worlds of David Darling > Children's
Encyclopedia of Science > Making Light Work > Chapter 3
MAKING LIGHT WORK:
The Science of Optics
a book in the eXperiment! series by David Darling
3. In the Hall of Mirrors
A visit to the hall of
mirrors at a fair will reveal two facts. First, not all mirrors are flat.
And second, curved mirrors can do very strange things to your appearance!
A mirror that curves in from the edge to the mirror is said to be CONCAVE.
One that bulges out from the edge to the middle is called CONVEX.
| The images of NASA technicians reflected
in the curved surface of the Hubble Space Telescope's main mirror
Reflections in a Spoon
You will need:
- A large, clean, unscratched spoon
- An ordinary flat mirror
- A 12” ruler
- The beam tank
- Several different concave and convex mirrors
What to do:
The inside surface of the spoon acts as a concave mirror. Hold the
spoon 12 inches in front of your face. Which way up is your reflection?
Measure the height of the image of you head. Now replace the spoon
with a flat mirror held in the same position. Measure the height of
your head as seen in the flat mirror. Which image is larger? Divide
the height of the image in the spoon by that in the flat mirror to
find by how much your reflection has been shrunk or enlarged in the
spoon. Finally, hold the spoon so that the concave side is almost
touching your eye. What do you see? Which way up is the image? Is
it smaller, larger, or the same as it would be in a flat mirror?
The outside surface of a spoon works as a convex mirror. Repeat all
the steps just described with this side of the spoon.
Taking it further:
Hold the spoon upright in the beam tank with the concave side facing
the light. Observe what happened to the beams, both from above and
from the side of the tank. Tilt the spoon in different directions.
How does this affect the reflected beams? Repeat these steps using
the convex side of the spoon.
If you can obtain proper concave and convex mirrors, experiment with
these in the beam tank. How do the results differ from those using
The concave side of a spoon will make light rays come together. But
it cannot them all meet exactly at a point. This is because the spoon
is curved by different amounts in different places. The center part
of the spoon is fairly flat, while parts nearer the edge curve more
sharply. Because of this the focal length varies from place to place.
By contrast, a specially made concave mirror has a regular curvature
all over so that it can focus all incoming light rays to a point.
Uses of Curved Mirrors
A concave mirror can produce two types of images. When an object is placed
quite far from the mirror, an image is formed that is upside down and reduced
in size. When the object is brought close up, however, the concave mirror
gives an image that is upright and magnified. In the case of a spoon, which
is highly curved, objects almost have to touch the spoon for the magnified
image to appear. But a mirror that curves more gently gives an upright,
magnified image at a greater distance – say, a foot or two away. This
makes it useful, for example, as a shaving mirror or a makeup mirror.
|The side-view mirror of a car is a convex
mirror, while a cosmetic mirror is concave
Concave mirrors are also used in large telescope (see "The Hubble Space
Telescope" below). They allow a lot of light to be collected from the direction
in which they are pointing, so that even very faint objects can be seen.
Convex mirrors, on the other hand, give only one type of image – upright
and reduced in size. Their advantage is that they can show objects spread
over a wide area. This makes them useful as side-view mirrors on cars and
trucks. A flat mirror would give a restricted view, but a concave mirror
provides a view of most of the road behind the drive.
The Hubble Space Telescope
One of the problems scientists face in trying to study the universe is that
the Earth’s atmosphere gets in the way. Moving currents of air and dust
blur the appearance of stars and other distant objects. As a result, telescopes
on the Earth’s surface are limited in the amount of detail they can show.
|The Hubble Space Telescope
orbiting the Earth
It was a great step forward, therefore, when the Hubble Space Telescope
was launched on April 24, 1990. This instrument now circles around the Earth
in an orbit 373 miles high, taking the clearest pictures of the universe
ever seen. The most important part of the new telescope is a concave mirror
measuring 98 inches across. This gathers the light from distant objects
and concentrates it onto a smaller flat mirror, ready to be magnified.
Shortly after it was launched, scientists discovered a serious error in
the spacing of the two mirrors inside the Hubble Space Telescope that affected
its ability to focus images. In 1993, this problem was fixed during a planned
maintenance visit by astronauts.