The propagation of light can be described using the electromagnetic wave theory. The distance separating one crest of a light wave from the next is the wavelength λ.

The range of optical radiation (100 nm - 1 mm) covers ultraviolet (UV), visible (VIS) and infrared (IR) light. The wavelengths of visible light lie between 380 and 780 nm. To describe a certain color of visible light, the wavelength is specified in air.

The refractive index n of a lens specifies the ratio of the velocity of light in air to the velocity of light in the lens.

Due to a reduction in its velocity in the lens, the light undergoes a change of direction if it is obliquely incident on the lens surface. This process is known as refraction. The higher the refractive index of the material, the greater the reduction in the light’s velocity and the greater its refraction. Light is more strongly refracted by lenses with a high refractive index.

Every color of light, characterized by its wavelength in air propagates at a different velocity in the lens. The shorter the wavelength, the lower the velocity of light in the lens. It is for this reason that shortwave blue light is more strongly refracted than long-wave red light. Different refractive indices can therefore be given for red, green and blue light.

If white light is refracted at a lens, it is split up into its various color components, as each color is refracted differently. This phenomenon is known as dispersion.

The Abbe number is used to describe the dispersion properties of a lens. It is the ratio of the angle of deflection δ_{e} to the mean dispersion angle δ_{F’C’} .

A low Abbe number indicates a high level of dispersion. The Abbe number should not be lower than 30 to ensure that color fringes do not impair peripheral vision.

The equivalent power F is the reciprocal of the focal length measured in meters. Like the equivalent power of an optically effective surface, the equivalent power of a spectacle lens is given in diopters (D).

The surface power is determined by the ratio of the difference between the refractive indices of two media to the radius of curvature of this surface. The two surface powers F1 and F2 yield the equivalent power F of a lens, taking into account the center thickness t.

The image on the retina of an eye corrected with spectacles is different in size to the retinal image of an emmetropic (normal) eye of the same length. This difference in image size depends on the shape factor of the lens as well as on other factors.

The shape factor is the ratio of the back vertex power to the equivalent power. In a lens of finite thickness, the equivalent power and the back vertex power differ (F ≠F’n). The shape magnification S is then greater than 1 (S > 1). An imaginary infinitely thin lens has a shape factor of 1 (S = 1), i. e. F’n = F is only the case for an infinitely thin lens.