Vision

The eyes are composed of an optical portion, which focuses the visual image on the receptor cells, and a neural component, which transforms the visual image into a pattern of neural discharges.

Light

The receptors of the eye are sensitive only to that tiny portion of the vast spectrum of electromagnetic radiation that we call visible light (Figure 9-20). Radiant energy is described in terms of wavelengths and

Long-wave radio Broadcast bands HF radio

VHF radio UHF radio

SHF radio Radar

Microwaves

Extreme infrared

Near infrared Visible light Ultraviolet

X-rays

Gamma rays

104-

106-

108-

1014-

1016-

1018-

-104

-102

FIGURE 9-20

Electromagnetic spectrum.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

The Human Body Electromagnetic

FIGURE 9-21

Properties of a wave. The frequency of this wave is 2 Hz (cycles/s).

FIGURE 9-21

Properties of a wave. The frequency of this wave is 2 Hz (cycles/s).

Ciliary muscle

Lens

Cornea Iris

Pupil

Ciliary muscle

Lens

Cornea Iris

Pupil

Aqueous humor

Choroid frequencies. The wavelength is the distance between two successive wave peaks of the electromagnetic radiation (Figure 9-21). Wavelengths vary from several kilometers at the long-wave radio end of the spectrum to minute fractions of a millimeter at the gamma-ray end. The frequency (in hertz, the number of cycles per second) of the radiation wave varies inversely with wavelength. Those wavelengths capable of stimulating the receptors of the eye—the visible spectrum—are between 400 and 700 nm. Light of different wavelengths within this band is perceived as having different colors.

The Optics of Vision

The light wave can be represented by a line drawn in the direction in which the wave is traveling. Light waves are propagated in all directions from every point of a visible object. Before an accurate image of a point on the object is achieved, these divergent light waves must pass through an optical system that focuses them back into a point. In the eye, the image of the object being viewed is focused upon the retina, a thin layer of neural tissue lining the back of the eyeball (Figure 9-22). The retina contains the light-sensitive receptor cells, the rods and cones, as well as several types of neurons.

The lens and cornea of the eye are the optical systems that focus impinging light rays into an image upon the retina. At a boundary between two substances of different densities, such as the cornea and the air, light rays are bent so that they travel in a new direction. The cornea plays a larger quantitative role than the lens in focusing light rays because the rays are bent more in passing from air into the cornea than they are when passing into and out of the lens or any other transparent structure of the eye.

The surface of the cornea is curved so that light rays coming from a single point source hit the cornea at different angles and are bent different amounts,

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