Figure 8-13 The electromagnetic (EM) spectrum. The spectrum is divided for practicality into regions in which the radiation is generated and detected by similar techniques and equipment; thus, various EM regions partially overlap. Notice the relatively small visible portion of the EM spectrum (red bar) compared with other portions. (Illustration b y Edmond

H. Thall, MD.)

For instance, “light” generally refers to the very narrow portion of the electromagnetic spectrum that stimulates the retina. However, the limits of the visible spectrum are not well defined because the sensitivity of the retina approaches zero asymptotically. If the limits are taken (arbitrarily) as the points where the eye’s sensitivity is 1% of maximum, then the visible spectrum spans the frequencies between 7.0 × 1014 Hz (λ = 430 nm) and 4.3 × 1014 Hz (λ = 690 nm). However, the eye can detect radiation, if sufficiently intense, beyond these limits.

Frequency and Color

Despite the use of terms such as blue light or red light, color is not a property of light but rather a phenomenon of perception. “Spectral colors” are perceived when an area of the retina is stimulated by a single frequency. The colors of the rainbow are spectral colors. Other color sensations not seen in the rainbow (eg, cyan, magenta) are perceived when multiple frequencies stimulate the same region of the retina simultaneously.

Energy in an Electromagnetic Wave

Light is both a form of energy and a means of conveying energy from one point to another. In EM wave theory, the energy carried by the wave is proportional to the square of its amplitude and does not depend at all on the frequency of the wave.