2008-10-17 / Columnists

Drawing On Science For Kids

WHY WE SEE
Commentary By Stephen S. Yaeger

Plants and animals have developed many ways to use light energy for their survival. The EMS is composed of wavelengths of energy from low radio waves to high gamma waves. Light is a very small part of the Electromagnetic Spectrum (EMS). Light energy is visible to animals having photoreceptors (photo=light, receptor=receiver). Light is also used by specialized cells in green plants.

In unicellular (uni=one) organisms such as Euglena, special regions in the cell are able to detect light to which the organism responds. The organism will either move toward the light or away from the light depending on its preference. Green plants such as Maple trees have chloroplasts containing the light-sensitive chemical, chlorophyll, which uses the light energy for the production of food. Multicellular (multi=many) organisms have evolved two types of photoreceptors called eyes: compound eyes and camera eyes.

Insects have compound eyes consisting of many lenses and light-sensitive cells. Compound eyes can detect light and movement. That cute insect called a dragonfly has eyes which consist of some 30,000 lenses and light sensitive cells. Dragonflies can see 360o around them, they can distinguish color, and can spot a mosquito flying many feet away. You and your mammal friends, on the other hand, have camera type eyes. A camera basically has a lens, a diaphragm controlling an opening to allow light in, and film on which an image is captured (Digital cameras do not have film. Instead the image is captured by a charge coupled-device or CCD. A CCD is a light-sensitive thing that captures an image electronically.)

Your eye is globe-shaped with a protective outer covering called the sclera. The front part of the sclera, called the cornea, is transparent and is slightly bulged. Just inside the sclera is a dark-colored tissue, the choroid. This tissue supplies blood to the eye. The choroid has a thickened region called the ciliary body and a ring of tissue, the iris, in front of it. In the center of the iris is the pupil. Just behind the pupil and attached to the ciliary body is the lens.

The ciliary body and the lens divide the eye into two chambers. The forward chamber is filled with a watery substance called the aqueous humor. The rear chamber is filled with a jelly-like substance called the vitreous humor. Covering the inner surface of the choroid in the back of the eye is the retina. The retina is composed of two types of cells: rod cells and cone cells. Rods are found along the outer rim of the retina and are very sensitive to light, but not to color. They produce blurry images. Cones are found in the middle of the retina. Cones are sensitive to color and produce a clearer, sharper image than rods. Rods and cones combine to form the optic nerve, which leads into your brain's visual centers.

A camera's diaphragm controls the amount of light that enters the camera by controlling the size of the opening. When an image passes through its lens the image is turned upside down, so the film (or CCD) receives the upside down image as is. Your eye works the same way. Your eye's iris controls the pupil's size. Contracting the pupil lets in less light while dilating (opening) the pupil allows in more light. Your retina also receives an image upside down. So how come you see things right side up? Well, when the image is transported though the optic nerve to your brain, your brain turns the image right-side up.

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