The Human Eye & Optical Instruments



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The Human Eye & Optical Instruments

(adapted from the Pasco manual for the Model Human Eye 8/2008)
In this part of the optics lab, you will learn about the anatomy and optics of the human eye and do experiments to understand how our eyes form images, and how eyeglasses correct refractive errors and astigmatism. In the final part, you will investigate the optics of microscopes and telescopes. You should read all of the background material, but you can choose to do only the eye experiments or only the optical instrument experiments in lab (if you have time you can do both.)
Anatomy of the Eye

The human eye achieves vision by forming an image that stimulates nerve endings, creating the sensation of sight. The eye consists of an aperture and lens system at the front, and a light-sensitive surface at the back. Light enters the eye through the aperture-lens system, and is focused on the back wall. The lens system consists of two lenses: the corneal lens on the front surface of the eye, and the crystalline lens inside



the eye. The space between the lenses is filled with a transparent fluid called the aqueous humor. Also between the lenses is the iris, an opaque, colored membrane. At the center of the iris is the pupil, a muscle-controlled, variable-diameter hole, or aperture, which

controls the amount of light that enters the eye. The interior of the eye behind the crystalline lens is filled with a colorless, transparent material called the vitreous humor. On the back wall of the eye is the retina, a membrane containing light-sensitive nerve cells known as rods and cones. Rods are very sensitive to low light levels, but provide us only with low-resolution black-and-white vision. Cones allow us to see in color at higher resolution, but they require higher light levels. The fovea, a small area near the center of the retina, contains only cones and is responsible for the most acute vision. Surrounding the fovea is the sensitive region called the macula, which is responsible for central vision. Signals from the rods and cones are carried by nerve fibers to the optic nerve, which leads to the brain. The optic nerve connects to the back of the eye; there are no light-sensitive cells at the point where it attaches, resulting in a blind spot.
Cameras share the same optical design as the human eye. Cameras of course have glass or quartz lenses that move mechanically back and forth rather than being fixed in place like the crystalline lens and cornea, and they sense light with electronic sensors or film rather than a retina, but the optical design issues you learn here will apply to cameras.
The Model Eye Apparatus

The PASCO Human Eye Model consists of a sealed plastic tank shaped roughly like a horizontal cross section of an eyeball. A permanently mounted, plano-convex, glass lens on the front of the eye model acts as the cornea. The tank is filled with water, which models the aqueous and vitreous humors. The crystalline lens of the eye is modeled by a changeable lens behind the cornea. A movable screen at the back of the model represents the retina. An optics caliper is provided for measuring images on the retina screen.



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