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Figure 14-1. Radiant-energy spectrum.

to frequency because the energy travels at the speed of light). The average wavelength of cosmic rays, the shortest cycles of radiant energy known, is in the range 10-12 meters (million millionths of a meter). The wavelength of electric power waves is on the order of 106 (million) meters (50-60Hz, or cycles per second). The radiant energy visible to humans comprises a narrow portion of this spectrum, ranging from 380 to 760 nm (nanometers or billionths of a meter). Other organisms can see somewhat more, or less, of this same range. Snakes, for example, have infrared (heat) receptors, and bees and birds have receptors for ultraviolet light.

Visible light is detected by photoreceptors. These can take many forms, and we can only describe some of them. For example, the entire cell of some single-celled organisms, such as amoebas, may be sensitive to light, moving toward or away from it. They use a membrane-bound photoreceptor that induces the release of cAMP, which in turn induces changes in the beating of their cilia and cellular motion. Some worms have photoreceptive cells, or eyespots, scattered throughout the epithelium on the surface of their bodies. In the earthworm, these serve to orient the organism directionally, as they prefer to live underground in darkness.

Direct sunlight includes energy in the ultraviolet (UV) part of the spectrum, but reflected light is dimmer and retains little UV, and many species are sensitive to both. Vertebrates have evolved two basic kinds of photoreceptor cells, the rod and cone cells, which are embedded in the retina at the back of the eye and attached to the axons of neurons in the optic nerve (Figure 14-2A shows the structure of the human eye and retina, and 14-2B depicts the structure of rods and cones with a retinal molecule bound to a 7TMR photoreceptor protein embedded in the surface of a rhodopsin disc). Based on its properties, a photoreceptor molecule responds to light of particular energy and frequency. Rods perceive light and dark (i.e., black, white, and shades of gray), and organisms use these mainly for perception in dim light; they are maximally sensitive in the middle of the visual spectrum and able to respond to a very small amount of light. Cones are more specialized for color perception and acuity of vision and are used to detect form and motion but do require more light than rods to be activated, hence do not function well in dim light (humans lose color iris pupil lens blood vessels optic nerve blood vessels optic nerve iris pupil lens

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to brain axons of ganglionic cells optic ganglionic bipolar rod and cone nerve cells cells layer

Figure 14-2. Parts of vertebrate visual system (human). (A) Structure of the eye and retina; (B) cellular structure of rods and cones, including a diagram of an opsin and its chromophore.

to brain axons of ganglionic cells optic ganglionic bipolar rod and cone nerve cells cells layer

Figure 14-2. Parts of vertebrate visual system (human). (A) Structure of the eye and retina; (B) cellular structure of rods and cones, including a diagram of an opsin and its chromophore.

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