Portable Assisted Study Sequence (PASS) Program

All animals need food to survive. One way animals find food is through their sight. Do animals see things as we do? The answer is no. Your dog, for example, sees little or no color. Its eyes do not focus as well as yours do. But, before you feel too superior, consider the eagle. The eagle can spot a peanut on the ground from the top of a 120 story building! A rabbit can see behind itself without turning its head. Let's look at the adaptive value of vision - both for the eagle and for the rabbit.

High in the sky, an eagle circles a field. In the field a rabbit sits motionless near his hollow log home. The eagle spots the rabbit from a thousand feet up. It swoops down. The rabbit can notice the movement, even though the eagle approaches from behind. The rabbit runs inside the log.

The rabbit, the eagle, and other animals have eyes that are especially adapted for their survival. For animals that are usually prey, it is important for them to be able to see all around. The rabbit's eyes are on opposite sides of its head. The placement of their eyes allows them to see a full circle, except for two blind areas at very close range. The rabbit sees two separate images with each eye. This is called monucular vision (muh-NOK-yuh-ler). The rabbit has a broad view but little or no depth. Your own eyes have mostly binocular vision. Binocular vision results from overlapping fields of vision. In the overlap area, objects are in sharp focus and have depth. In the areas that don't overlap, objects appear less focused and more flat. A human has a large blind area - beginning at about the ears and around the back of the head.

For more information
about the eye and vision visit:

Visit CNN to view The Human Eye and Its Parts. Click the link called "INTERACTIVE: The human eye" to see an interactive animation.

When an eagle dives at its prey from a thousand feet it has to strike in the right spot. It must be able to judge distances accurately. The eagle has binocular vision, as we do, but it sees better. Its eyes have four times as many light sensitive cells as ours, and its lens focuses light equally well onto large areas of the retina. The eagle sees images at a great distance as sharp, detailed, and colorful. They have the sharpest vision of all living creatures.

Vertebrates have one lens in each eye. The purpose of the lens is to focus an inverted image on the retina. Eyes of insects and some other creatures are compound. The eyes have many tiny lenses, bundled together - up to 30,000. Each lens takes in a small part of the scene; the brain translates the images into a pattern. Compound eyes cannot change focus as the lenses of vertebrates (including yours) can. Compound eyes can detect the slightest motion, an important feature for catching meals.

Compound eyes are on the sides of heads in insects and on movable stalks in crabs and lobsters. The stalks can aim forward or back, right or left. Either way, the creatures have a wide field of vision.

Compound eyes in bees have the additional function of serving as a compass. When one bee finds food, it returns to the hive and communicates the direction and distance of the food through a dance. Other bees are able to orient themselves so they see the sun on the same side and at the same angle.

There is another important way that bees and other insects see differently from us. Electromagnetic radiation from the sun contains visible light, ultraviolet, gamma and x-rays, infrared, and radio waves.

We see only visible, or ordinary light. In visible light we see the colors of the rainbow - violet, indigo, blue, green, yellow, orange and red. A bee doesn't see colors the same way. It can see ultraviolet light reflected from flowers and other objects. Ultraviolet rays help guide bees to the nectar in the flowers. But the bee cannot see the yellow-to-red part of the spectrum. To a bee, red appears as black. We take advantage of this difference in vision when we use yellow lights outdoors to avoid attracting insects.

Environmental Science