Light Visible light is radiant energy of those particular wavelengths that can be detected by the human eye. The entire electromagnetic spectrum extends from extremely long radio waves to extremely short gamma rays. The wavelengths visible to us occupy a very narrow band within that spectrum. Wavelengths from about 7 x 10 cm down to about 4 x 10 cm are seen as light. Light reflected from surfaces into our eyes stimulates sensory cells of the retina at the back of the eye, our brain interprets the signals from the retina, and we perceive our visual world. The colors seen by us depend on the wavelengths of light reflected or absorbed by an object. Different wavelengths and combinations of wavelength are seen as different colors. White light is a combination of all of the wavelengths of the visual spectrum, and an object that reflects all colors is seen as white. If all of the light energy is absorbed by an object and none is reflected, the object is black. A totally black object is literally invisible; we see it in terms of its surrounding space. In nature, a rainbow is a common example of white light separated into its component colors. The spectrum of colors in white light can also be seen by passing it through a prism. The colors in a spectrum always appear in the order of longest visible wavelength (red) to shortest visible wavelength (violet). The name “Roy G. Biv” is a memory device that is sometimes used for the order of colors in the spectrum: red, orange, yellow, green, blue, indigo, violet. The propagation of light is influenced by several Chapter 3- Diving Physics factors: refraction, diffusion, absorption, and reflection. These occur when light passes through or interact with some medium. Passing through air or water affects the propagation of light. Light striking an object is affected as it is reflected or absorbed. The effects happen everywhere, but they are sometimes particularly pronounced and noticeable when we dive. Refraction refers to the bending of light rays as they pass from one medium to another. In a vacuum light travels at 299,792 kilometers per second (186,282 miles per second). It travels a few percent slower in air and about 25% slower in water. Whenever light waves cross the boundary from one transparent medium to another, they are bent, or refracted, by an amount that depends on their speed in the two mediums. Magnifiers, eyeglasses, and contact lenses all employ the principles of refraction to enhance vision or correct refractive problems in one’s eyes. The rainbow and the prism mentioned above are examples of refraction. Refraction is also affected by the wavelength. In passing through a prism, the different refractions of the various wavelengths in white light spread them apart (figure 3-12). Light is refracted whenever it passes from air into water or water into air. Our eyes have evolved to focus correctly in air. If our eyes are in the water, as in swimming, the refraction of light as it passes from the water Diving Physics 97 The spectrum of colors in white light can be seen by passing it through a prism glass prism white light spectral displacement FIGURE 3-11. THE SPECTRUM OF COLORS Air Reflection Diffusion Absorption Water Refraction FIGURE 3-12. REFRACTION, ABSORBTION, DIFFUSION, AND REFLECTION OF LIGHT IN WATER -5 -5
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