Some Technical Parameters of the Optical Microscope
Numerical aperture, abbreviated as NA, is a key technical parameter for objective lenses and condensers. It serves as an important indicator for evaluating their performance, especially for objective lenses. Its value is marked on the outer housing of the objective lens and condenser respectively.
The numerical aperture (NA) is defined as the product of the refractive index (n) of the medium between the front lens of the objective and the specimen, and the sine of half the aperture angle (u). It is expressed by the formula: NA = n*sin(u/2)
The aperture angle, also known as the angular aperture, is the angle formed between an object point on the optical axis of the objective and the effective diameter of the objective front lens. A larger aperture angle allows more light to enter the objective. It is proportional to the effective diameter of the objective and inversely proportional to the focal length.
2. Resolution
The resolution of a microscope refers to the minimum distance between two object points that can be clearly distinguished by the microscope, also called resolving power. Its calculation formula is: σ = λ / NA
where σ is the minimum resolvable distance; λ is the wavelength of light; NA is the numerical aperture of the objective lens. It can be seen that the resolution of a microscope is determined by the NA of the objective and the wavelength of the illumination light. The larger the NA and the shorter the illumination wavelength, the smaller the value of σ, and the higher the resolution.
3. Magnification and Useful Magnification
Since magnification is achieved twice by the objective lens and eyepiece, the total magnification Γ of the microscope is the product of the objective magnification β and the eyepiece magnification Γ₁: Γ = β*Γ₁
Obviously, compared with a magnifying glass, a microscope can achieve much higher magnification. The magnification can be easily adjusted by switching objective lenses and eyepieces with different magnifications.
Magnification is also an important parameter of the microscope, but higher magnification is not always better. The limit of the microscope magnification is defined as the useful magnification.
4. Depth of Focus
Depth of focus is the abbreviation for focal depth. When focusing on an object with a microscope, not only the points on the focused plane can be seen clearly, but also a certain thickness range above and below this plane remains sharp. The thickness of this clear zone is the depth of focus. A large depth of focus allows observation of the entire layer of the specimen, while a small depth of focus only enables observation of a thin slice of the specimen. The depth of focus has the following relationships with other parameters: (1) The depth of focus is inversely proportional to the total magnification and the numerical aperture of the objective lens. (2) A greater depth of focus corresponds to lower resolution.
Low-magnification objectives generally have a large depth of focus, which brings difficulties for photomicrography. This will be described in detail in the section on photomicrography.
