Worlds of David Darling > Children's
Encyclopedia of Science > Making Light Work > Chapter 5
MAKING LIGHT WORK:
The Science of Optics
a book in the eXperiment! series by David Darling
5. Looking into Eyes
Everyone is born with two very important lenses – the lenses of our
eyes. Light enters the eye through a small round opening called the PUPIL.
This is the black dot in the middle of your eye. Surrounding the pupil is
a circle of muscle known as the IRIS. Colored chemicals in the iris give
it various shades of brown, blue, or green. By altering the size of the
pupil, the muscles of the iris control the amount of light that passes into
Just behind the pupil is the lens. It is flexible like rubber, convex in
shape, and small – about the size of your little fingernail. Light,
falling on the lens, is focused to form an image on a sort of screen at
the back of the eye called the RETINA. As you move your eyes from distant
to near objects, your lenses are pulled from a thin shape to a fatter one
by muscles that are attached to them. In this way, whatever you look at
can be brought into focus, even though the distance from the lens to the
retina stays the same.
You will need:
What to do:
Partly darken the room, wait about a minute, and look into the mirror.
Notice the size of your pupils. Shine the flashlight into one eye.
What happens to the pupil of this eye? Notice the movement of the
muscles in the iris around the pupil. Turn off the flashlight. How
long does it take for your pupil to return to its original size?
Close one eye and hold this page in front of your other eye. Slowly
bring the page closer and closer until you can only just see the words
clearly. Hold the book in this position for a few seconds, then quickly
look up at a distant object. What do you notice? Does the distant
object seem clear right away? If not, how long does it take to come
into focus? Try to explain the results in terms of what is happening
inside your eye. How do you think this affects the ability of your
eye to form clear images?
Caution: Eyes are easily damaged. Never press hard on them or stare
into bright lights. Never stare at the sun. Looking at the sun through
a magnifying glass, binoculars, or a telescope can cause permanent
blindness within a few seconds.
Cameras and Eyes
A camera works almost exactly like a human eye. It, too, uses a convex lens
to focus images onto a light-sensitive surface.
And, just as the eye has a pupil, so the camera has an opening called an
aperture that can be widened or narrowed to regulate the amount of light
that passes through.
A Model Eye
You will need:
- A lamp
- A cardboard box, such as a shoe box
- Black paint and brush
- A sheet of greaseproof paper or other translucent paper
- Sticky tape
- A straight pin
What to do:
Paint the inside of the box completely black. Cut a square opening
on one side and cover this with a piece of greaseproof paper held
in place by sticky tape. In the middle of the opposite side, make
a tiny hole with a pin. Darken the room except for a single bright
lamp. Point the pinhole at the lamp and look at the back of the greaseproof
paper. What do you see? Which way up is the image?
The device you have made is known as a pinhole camera. But you can
also think of it as being a simple model of an eye. The pinhole serves
as both a lens and a pupil. It forms a sharp image because it only
lets in light travelling in a single direction from any point on the
Incredible though it seems, the images that form on our retinas are upside
down. We actually see everything the wrong way up! Fortunately, though,
early in life, our brains learn to turn all the images right-side up again
to mach what we touch and feel. Newborn babies may see everything upside
down, but we do not know this for sure. Experiments have also been carried
out on people wearing special glasses that turn the images of things upside
down. After a while, the brains of these people learned to cancel out the
effects of the glasses so that the world seemed normal again. But when they
took the glasses off, everything looked upside down until their brains once
Help for Eyes
Many people's eyes do not work perfectly. One of the most common problems
is when the lens of the eye cannot focus objects clearly on the retina.
If the lens is a bit too convex, or highly curved, then it will form images
of distant objects that fall in front of the retina. People with this condition
are said to be nearsighted because they can see objects clearly only when
they are near. The problem is easily solved, however, by wearing glasses
with concave lenses. These bend the paths of incoming light rays outward
slightly to cancel out the overbending of light by the eye lenses. Their
exact shape is prescribed by an eye specialist, or optician, after testing
the patientís vision.
Another common eye disorder is farsightedness. This occurs when a person
can see faraway things clearly but not objects nearby. The solution then
is to wear glasses with convex lenses.