After Image

Visitors explore why a flash of light can produce a colorful, lingering image in the eye. Visitors are asked to keep both eyes open, look into a long box and activate a sudden, brilliant flash of light illuminating a cross hairs pattern. Visitors are asked to close their eyes and cover them with their hands and observe what happens to the image. The ghostly shapes seen after a bright flash of light are called after images. These images are made by the light-sensitive cells on the retina inside the eye. These cells send signals to the brain whenever they are stimulated by light. When you see a very strong flash of light, the cells are stimulated so strongly that they go on sending signals to your brain for a short time after the flash is over.

Benham's Disk

At this exhibit, visitors control the speed and direction of a spinning black and white disc to see imaginary colors. Different people see different amounts of yellow, red, green, violet, and blue on this spinning disk. Why people see color is not fully understood, but the illusion probably involves the color vision cells in your eye. These cells come in three varieties. Some are sensitive to red light, some to green light, and some to blue light. These different types of color sensors respond at different rates. When visitors gaze at one place on the spinning disk, they are looking at alternating flashes of black and white. White is seen only when all three color sensors respond to a flash of light. If one type of color sensor responds at a different time than the others you see the illusion of color.

Blind Spot

At the Blind Spot exhibit, visitors experience how each eye has a small spot where no visual image can be formed. Visitors are shown a white card with an empty circle on the left and a circled triangle on the right. By holding the card at arm's length in the right hand, covering the left eye and looking only at the empty circle, the circled triangle can be made to disappear. Nerve fibers carry messages from the eye to the brain. These fibers pass through one spot on the light-sensitive lining of the eye, creating a blind spot. When the card is held in just the right place, the image of the circled triangle falls on the blind spot and it can't be seen.

Blood Vessels of the Eye

Visitors are provided with a penlight to be held against the corner of a closed eyelid. If it is moved about, the shadows of blood vessels can be seen as a dark network on a red field. If the light movement is stopped, the network fades quickly. Tiny veins and arteries lie in front of the retina, the layer of light-sensitive cells at the back of your eye. They normally do not interfere with vision because they are small and thin. One is not aware of them because the brain ignores any constant signal. But when the light is held against the eyelid the veins cast a shadow on the retina. The shadow moves when the light is moved. Movement makes the brain aware of the blood vessels.

Color Reversal

The eye's sensitivity to a color decreases the longer the eye is stimulated by a color. This exhibit contains a picture of a red tree in a green circle. A large black disk with an open window and a white screen is free to rotate in front of the picture. A white light illuminates the image. By flipping the switch to rotate the disk forward, visitors can observe the open window, followed by the white screen, passing in front of the picture. Upon rotation, the picture appears to reverse colors. The red tree appears green and the green circle appears red. Because the eyes are seeing a red tree followed by a white screen, the red-sensitive cells in the retina become exhausted and allow the complement of red to take over.

Macula

As light enters the eye, the lens focuses the input onto the back of the eye. Human vision is the most precise at one point where the focus is the most intense—the Fovea. The Fovea has a small yellow filter over it, though, called the Macula. Because of its constant presence, the brain "gets used" to the Macula and people never see it. This exhibit lets viewers see their Macula. A large light sits behind a window and the light inside slowly cycles from blue to violet. When the light cycles back to blue, the Macula becomes visible when the yellow filter blocks the blue light- leaving a dark shadow. What the Macula looks like varies as a smudge or a circle.

Motion Detector

The Motion Detector exhibit enables visitors to explore peripheral vision. While staring straight ahead, visitors are asked to move a white cylinder so that it is not visible. Pushing a button causes a diagonal line on the cylinder to spin. Visitors notice that they can detect the movement of the diagonal line. Even when the diagonal line is outside their field of vision, it can be seen when it moves. When the line is seen in the corner of the eye its image focuses near the edge of the retina. Since light-sensitive cells are less abundant on the periphery of the retina than they are in the center, an incomplete picture lacking color and detail is seen. What's more, the arrangement of the light-sensitive cells near the edge of the retina makes them particularly sensitive to movement, letting people notice something is moving even when details cannot be seen.

Peripheral Vision

A visitor sits in the center of the table and looks straight ahead. A triangular block is moved around the protractor starting at zero degrees. Visitors are asked to notice when they first catch sight of the block out of the corner of their eye. Movement of the block will be noticed before detail or color. When something is seen out of the corner of the eye, peripheral vision is being used. The image is focused toward the edge of the retina, where light-sensitive cells are sparse. Rather than sending individual signals to the brain, these light-sensitive cells are grouped together in large blocks and provide only general information about an object. As a result, details cannot be seen. Since there are no color sensitive cells near the edge of the retina, color cannot be seen either.

Pupil

The colored part of the eye, the iris, is a circular muscle that controls the size of the pupil, the hole that lets light into the eye. In dim light the iris expands causing the pupil to dilate, letting in more light. In bright light the iris contracts causing the pupil to shrink, limiting the amount of light that enters the eye. When a person steps into a dark room, the eye responds rapidly to the change in the light by adjusting the size of the pupil. This station lets viewers see their iris in action by staring at an enclosed mirror and getting an up close view of the expansion and contraction of the iris as the visitor slowly adjusts the brightness of a light source.

Rainbow Edges

Light passing through one clear material, like glass, and into another always bends. How much it bends depends partly on its color—blue light bends more than green, and green more than red. The result, called chromatic aberration, looks like a rainbow bleeding out from the edge of each color. The human eye has a lens that bends light, but humans don't see rainbow edges because photoreceptors—the cells that react to each light color- sit at different depths and "refocus" the image. At this station, two activities demonstrate chromatic aberation. Visitors look at an illuminated "H" through a series of colored filters—red, blue, and violet. Each filter results in the removal of some color, leaving distinct effects. The second activity has a small screen of point lights and two different lenses to view it through. One lens is color corrected to work with the human eye. The other leaves visitors seeing rainbow edges.

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