How to determine the optical microscope's eyepiece and objective lens magnification
An optical microscope's magnification is a combination of the objective lens's and the eyepiece's magnification. The magnification is 1010=100, for instance, if the objective lens is 10 and the eyepiece is 10.
One objective lens:
1. Classification of objective lenses:
According to the various conditions of usage, the objective lens may be separated into dry objective lenses and liquid immersion lenses; among these, the liquid immersion lenses can be divided into water immersion lenses and oil immersion lenses (commonly used magnification is 90-100 times).
It can be categorized into low magnification objective lenses (less than 10 times), medium magnification objective lenses (approximately 20 times), and high magnification objective lenses based on the various magnifications (40-65 times).
Achromatic objective lenses (typically used, the objective lens that can correct the chromatic aberration of two different types of color light in the spectrum) and apochromatic objectives lenses are split according to the aberration correction circumstances (the objective lens that can correct the chromatic aberration of three kinds of color light in the spectrum, which is expensive and rarely used).
2. The main parameters of the objective lens:
The magnification, numerical aperture, and working distance are the three primary objective lens characteristics.
The ratio of the size of the picture perceived by the eyes to the size of the comparable specimen is referred to as 1 magnification. Instead of the ratio of areas, it relates to the ratio of lengths. A specimen having a length of 1 m is referenced by the magnification factor of 100. The enlarged image measures 100 m in length. If it is calculated based on area, it is 10,000 times larger.
The sum of the microscope's objective and eyepiece magnifications is its overall magnification.
2. The abbreviation NA or A stands for numerical aperture, often known as aperture ratio. It is the primary variable of the objective lens and condenser, and it is inversely related to the microscope's resolution. The numerical aperture of dry objectives ranges from 0.05-0.95, while that of oil immersion objectives (cedar oil) is 1.25.
3. The term "working distance" describes the distance between the top of the specimen's cover glass and the bottom of the objective lens when the item being examined is the clearest. The focal length of the objective lens affects the working distance of the lens. The working distance and magnification of an objective lens increase with increasing focal length. Example: A 10x objective lens is marked with 10/0.25 and 160/0.17, where 10 is the lens's magnification, 0.25 is its numerical aperture, 160 is its length (in millimeters), and 0.17 is its typical thickness (in millimeters).The effective working distance of the 10x objective lens is 6.5mm, and the effective working distance of the 40x objective lens is 0.48mm.
3. The most crucial component that affects the resolution of the microscope is the objective lens, which has the job of initially magnifying the specimen.
Resolution and resolving power are other names for resolution. The value of the resolution distance represents the resolution's size (the minimum distance between two object points that can be resolved). Normal human eyes can distinguish between two object points that are 0.073mm apart at the photopic distance of 25 cm. The resolution distance of typical human eyes is 0.073 mm. The resolution and performance of the microscope improve as the resolution distance decreases.
Other names for resolution include resolving power and resolution. The resolution's size is indicated by the resolution distance's value (the minimum distance between two object points that can be resolved). At the photopic distance of 25 cm, two object points that are 0.073mm apart can be distinguished by normal human eyes. The typical human eye has a resolution distance of 0.073 mm. As the resolution distance reduces, the microscope's resolution and functionality increase.
D is the objective lens's resolution distance, expressed in nanometers, in the formula.
—In nm, the illuminating light's wavelength.
The objective lens's numerical aperture is abbreviated NA.
For instance, the visible light spectrum ranges from 400 to 700 nm, and the numerical aperture of the oil immersion objective lens is 1.25. d=270 nm, or roughly half the wavelength of the illuminating light, if the average wavelength is 550 nm. Typically, visible light-illuminated microscopes have a resolution limit of 0.2 m.
(2), eyepiece
Because it is close to the observer's eyes, it is also called the eyepiece. Installed on the upper end of the lens barrel.
1. Structure of the eyepiece
Usually the eyepiece is composed of upper and lower sets of lenses, the upper lens is called the eye lens, and the lower lens is called the converging lens or field lens. There is a diaphragm between the upper and lower lenses or under the field mirror (its size determines the size of the field of view), because the specimen is just imaged on the diaphragm surface, a small piece of hair can be glued on this diaphragm as a pointer to indicate target of a certain characteristic. An eyepiece micrometer can also be placed on it to measure the size of the observed specimen.
The shorter the length of the eyepiece, the greater the magnification (because the magnification of the eyepiece is inversely proportional to the focal length of the eyepiece).
2. The role of the eyepiece
It is to further magnify the clearly resolved real image that has been magnified by the objective lens to the extent that the human eye can easily distinguish it clearly. The magnification of commonly used eyepieces is 5-16 times.
3. Relationship between eyepiece and objective lens
The fine structure that has been clearly resolved by the objective lens, if it is not re-magnified by the eyepiece, and cannot reach the size that the human eye can distinguish, then it will not be clear; but the fine structure that the objective lens cannot distinguish, although it is re-magnified by the high-power eyepiece, It is still not clear, so the eyepiece can only magnify, and will not improve the resolution of the microscope. Sometimes although the objective lens can distinguish two very close object points, it is still impossible to see clearly because the distance between the images of these two object points is smaller than the resolution distance of the eyes. Therefore, the eyepiece and the objective lens are not only related to each other, but also restrict each other.
