Observation and Measurement of Lenses Using Digital Microscopes

Optical lenses, such as telephoto lenses and zoom lenses, are created by combining multiple convex and concave lenses. This section explains the mechanism of lenses and introduces examples of their observation and measurement using digital microscopes.

A : Incident light B : Reflected light
C : Reflecting surface D : Refracted light

A refractive index is expressed with a value derived by dividing the speed of light in air by the speed of light in a substance.
As the speed of light varies according to the substance and the wavelength of light, the refractive index also varies according to the substance and wavelength of light. The direction in which light deflects can be calculated using Snell's law.
Snell's law (n₁sinα = n₂sinβ)
Refractive index 1 (e.g., air) : n₁
Refractive index 2 (e.g., water, glass) : n₂
Incident angle : α
Refraction angle : β

A : White light

When white light passes through a prism, the spectrum of light appears.
This phenomenon is called the dispersion of light. It occurs because the refractive index differs according to the wavelength of light.

Note : n_d is the refractive index of the D line emitted by sodium atoms.

The dispersion of an optical glass is expressed with a value called the Abbe number (v).

A : Light beam B : Focal point
C : Differences in the focal distance

As shown in the following figure, the focal position of light with short wavelengths is close to the lens and that of light with long wavelengths is further away from the lens due to the dispersion of light. When light with a specific wavelength is brought into focus, light with other wavelengths becomes out of focus, which blurs the colors of the image. This phenomenon is known as chromatic aberration.

A : Light

Chromatic aberration occurs due to differences in the wavelengths of light. Aberration, however, occurs even with a single color, called monochromatic aberration. A typical example is spherical aberration. A convex lens surface is in the shape of part of a sphere. Therefore, the closer a point is to the edge of a lens, the larger the incident angle, which then increases the refraction angle. The phenomenon in which the focal position differs between points close to the lens center and points further away from the lens center is called spherical aberration.

A : Light

Spherical aberration can be offset by, for example, combining a convex lens with a concave lens, which has aberration in the opposite direction, or combining lenses that have different refractive indices.

A : Light

Another method to offset spherical aberration is to use aspheric lenses. The spherical surface at the lens edge is processed to be a curved surface, instead of combining multiple lenses, which can correct the focal position without increasing the number of lenses.