Biomedical Engineering Reference
In-Depth Information
White light
Red (650 nm)
Orange (600 nm)
Yellow (570 nm)
Green (505 nm)
Blue (480 nm)
Violet (450 nm)
Figure 15-13.
Because refractive index varies with the wavelength of light, a prism
separates white light into a spectrum. Shorter wavelengths are more refracted than
longer wavelengths. One nanometer (nm) is equal to 10
−9
m.
CHROMATIC ABERRATION
While the aberrations we've discussed so far can be produced with a single wave-
length of light, chromatic aberration occurs only with polychromatic light, a mix-
ture of different wavelengths. The transmission speed within a refractive medium
depends upon the wavelength, with shorter wavelengths travelling more slowly
than longer wavelengths. Consequently, the refractive index, which is the ratio of
the speed of light in a vacuum to that in a given medium, is different for each wave-
length. Shorter wavelengths, which have a higher index of refraction, are more
refracted than longer wavelengths.
Figure 15-13 shows white light—a combination of different wavelengths—
incident upon a prism. Because the index of refraction and, consequently, the
amount that light is refracted, depends on wavelength, the emergent light forms a
spectrum of colors.
11
This separation of white light into its component elements by
a prism (or other optical element) is referred to as chromatic
dispersion
.
Dispersive Power and Constringence
As a general rule, a single refractive index is specified for a refractive medium. For
instance, the refractive index for CR-39 is typically given as 1.498. This index is for
a specific wavelength, namely 589 nm. As we have just learned, however, the refrac-
tive index is different for other wavelengths, resulting in a spectrum of colors when
the incident light is white.
To quantify the amount of dispersion produced by a prism or lens, we use three
wavelengths: 486, 589, and 656 nm. The refractive indices for these wavelengths
11. The color of light is dependent on its wavelength. Wavelength is a physical property, and color is a
perception. See Schwartz (2010) for an introductory discussion of color perception.
