In ophthalmic optics, the term light generally refers to visible light, which is just a very narrow portion of a long scale, analogous to a musical scale, in which there are “notes” both higher and lower than we can perceive (Fig 2-1). The visual perception of these notes—specified by frequency (ν) and wavelength (λ) along the “scale” of light (the electromagnetic spectrum) and related to each other via the “speed of light” (c = ν λ ≈ 3 × 108 m/s in a vacuum)—is what we call (spectral) color.
Although the visible spectrum is normally considered to run from 400 nm to 700 nm (Table 2-1), the boundaries are not precise. Under certain conditions, such as with sufficiently intense light, the eye’s sensitivity extends into the infrared and ultraviolet (UV) regions. As another example, in aphakia, without the UV absorption of the natural lens, the retina can detect wavelengths well below 400 nm.
Figure 2-1 The electromagnetic (EM) spectrum. The spectrum is divided into several regions according to the wavelength or frequency of light. Note that visible light is just a very narrow portion of the entire EM spectrum.
Table 2-1 Visible Spectrum and Approximate Color Associations
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.