The human eye is a marvelous and intricate organ. If we leave the biological details to biologists and focus on the eye’s optical properties, we find that it functions very much like a camera. Like a camera, the eye has refracting surfaces that focus incoming light rays, an adjustable iris to control the light intensity, and a light-sensitive detector.

FIGURE 35.6 shows the basic structure of the eye. It is roughly spherical, about 2.4 cm in diameter. The transparent cornea, which is somewhat more sharply curved, and the lens are the eye’s refractive elements. The eye is filled with a clear, jellylike fluid called the aqueous humor (in front of the lens) and the vitreous humor (behind the lens). The indices of refraction of the aqueous and vitreous humors are 1.34, only slightly different from the 1.33 of pure water. The lens, although not uniform, has an average index of 1.40. The pupil, a variable-diameter aperture in the iris, automatically opens and closes to control the light intensity. A fully dark-adapted eye can open to $\approx 8\text{mm,}$ and the pupil closes down to $\approx 1.5\text{mm}$ in bright sun. This corresponds to f-numbers from roughly F3 to F16, very similar to a camera.

FIGURE 35.6 The human eye.

The eye’s detector, the retina, consists of specialized light-sensitive cells called rods and cones. The rods, sensitive mostly to light and dark, are most important in very dim lighting. Color vision, which requires somewhat more light, is due to the cones, of which there are three types. FIGURE 35.7 shows the wavelength responses of the cones. They have overlapping ranges, especially the red- and green-sensitive cones, so two or even all three cones respond to light of any particular wavelength. The relative response of the different cones is interpreted by your brain as light of a particular color. Color is a perception, a response of our sensory and nervous systems, not something inherent in the light itself. Other animals, with slightly different retinal cells, can see ultraviolet or infrared wavelengths that we cannot see.

FIGURE 35.7 Wavelength sensitivity of the three types of cones in the human retina.