The correct interpretation of visual information depends on the eye's ability to focus incoming rays of light on the retina. An understanding of this process and how it is influenced by normal variations or ocular disease is essential to the successful use of any optical aid, for example, glasses, contact lenses, intraocular lenses, or low-vision aids. To achieve this understanding, it is necessary to master the concepts of geometric optics, which define the effect on light rays as they pass through different surfaces and media.

Speed, Frequency, & Wavelength of Light

Speed, frequency, and wavelength of light are related by the following expression: In different optical media, speed and wavelength of light change, but frequency is constant. Color depends on frequency, so that the color of a ray of light is not altered as it passes through optical media except by selective nontransmittance or fluorescence. The optical characteristics of a substance can only be defined with respect to clearly specified frequencies of light. A substance to be used for lenses to refract visible light is usually tested with the yellow sodium light (D line) and the blue (F line) and the red (C line) of a rarefied hydrogen discharge tube.

In a vacuum, the speed of all frequencies of light is the same, that is, 299,792.46 kilometers per second (186,282.40 statute miles per second). Since the frequency of the yellow D line is approximately 5.085 × 1014 Hz, the wavelength of this line in a vacuum is 0.5896 μm. Similarly, the wavelengths in a vacuum of the blue F and red C lines are 0.4861 and 0.6563 μm, respectively.

Index of Refraction

If the speed of a light ray is altered by a change in the optical medium, refraction of the ray will also occur (Figure 21–1). The effect of an optical substance on the speed of light is expressed as its index of refraction, n. The higher the index, the slower the speed and the greater the effect on refraction.

Figure 21-1. Refraction of light as it enters a transparent medium of higher refractive index n′.

In a vacuum, n has the value 1.00000. The absolute index of refraction of a substance is the ratio of the speed of light in a vacuum to the speed of light in the substance. The relative index of refraction of a substance is calculated with reference to the speed of light in air. The absolute index of refraction of air varies with the temperature, pressure, and humidity of the air and the frequency of the light, but it is about 1.00032. In optics, n is assumed to be relative to air unless specified as absolute.

Thermal Coefficient of Index of Refraction

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