What we perceive as light is a narrow band of electromagnetic energy, ranging from approximately 380 nanometers (nm) to 760 nm. Only wavelengths in this range stimulate receptors in the eye that permit vision (figure 1.1). These wavelengths are called visible energy even though we cannot directly see them.
visible light
Figure 1.1 Visible light is a narrow region of the total electromagnetic spectrum, which includes radio waves, infrared, ultraviolet, and x-rays. The physical difference is purely the wavelength of the radiation, but the effects are very different. Within the narrow band to which the eye is sensitive, different wavelengths give different colors.

In a perfect vacuum, light travels at approximately 186,000 miles per second. When light travels through glass or water or another transparent substance, it is slowed down to a velocity that depends on the density of the medium through which it is transmitted (figure 1.2). This slowing down of light is what causes prisms to bend light and lenses to form images.
visible light 2 Figure 1.2 The law of refraction (Snell’s law) states that when light passes from medium A into medium B the sine of the angle of incidence (i) bears a constant ratio to the sine of the angle of refraction (r).
When light is bent by a prism, each wavelength is refracted at a different angle so the emergent beam emanates from the prism as a fan of light, yielding all of the spectral colors.
All electromagnetic radiation is similar. The physical difference between radio waves, infrared, visible light, ultraviolet, and x-rays is their wavelength. A spectral color is light of a specific wavelength; it exhibits deep chromatic saturation. Hue is the attribute of color perception denoted by what we call red, orange, yellow, green, blue, and violet.
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