Lenses are an essential component of a microscope. The power of lenses determines the worth of a microscope. These lenses are made up of high-quality optic glass, which is very different from the ordinary ones we use.
A light microscope has two lenses; the Objective Lens and an Eyepiece. The product of the power of both gives the magnification of that microscope, usually ranges from 10x to 2000x.
The microscope generates a highly magnified, visibly cleared, and illuminated image with the help of different types of lenses used in microscopes and a light source.
1. Ocular Lens
An ocular lens or eyepiece is placed at the end of the body tube. The observer views a magnified image of the specimen through the eyepiece. The eyepiece further magnifies the real image created by the objective lens and forms a virtual image.
A light microscope usually uses two types of eyepieces; the negative eyepiece having an internal diaphragm and the positive eyepiece having diaphragm below the eyepiece.
The negative eyepiece has two lenses, and both are plano-convex lenses, one close to the observer’s eye, called the upper lens or the eye lens, and the other that is beneath the diaphragm is called the lower lens or the field lens.
The positive eyepiece also has two lenses; an eye lens and a field lens. They are plano-convex lenses. The curved surface of the field lens is faced towards the eye lens.
The eyepiece has a unique phenomenon of the field stop. Field stop blocks excess light rays, hence increasing the filed of view.
The following are the types of the eyepiece based on the structure of the field stop.
Ramsden is another name of the positive eyepiece. It consists of two plano-convex lenses, with the curved surface facing inwards. The field stop of the Ramsden is located outside the lens tube.
It is the simplest form of the negative eyepiece. Huygens’s lens consists of two plano-convex lenses that face inwards. The field stop is located within the lens tube. They are suitable for low magnification and are used frequently in school laboratories.
Kellner lens is a modified version of the Ramsden lens. It features three lenses. The Kellner lens system uses an achromatic doublet in place of a plano-convex lens in the Ramsden lens. They offer a much larger field of view in comparison to the Huygens lens and Ramsden lens.
A wide-field lens is used to view live specimens.
2. Objective Lens
The objective lenses are placed near the specimen. It creates a magnified virtual image of the object. They are attached to a rotating circular disc on the nose piece.
Objective lenses of a light microscope have two features; magnification and numerical aperture. The magnification ranges from 4x to 100x, and the numerical aperture ranges from 0.10 to 1.25.
A light microscope usually has the following four objective lenses
- Scanning Objective lens that has a magnification power of 4x
- A small objective lens that has a magnification power of 10x
- A large objective lens having up to 100x magnification
- Oil-immersion lens having magnification higher than 100x
To improve color aberration created by the objective lens, different lenses are available which are as follows
The apochromatic lens has a large numerical aperture and a better resolution. It makes the refractive index of three wavelengths of the light the same.
Semi Apochromatic Lens
Semi apochromatic lens is also known as a fluorite lens. It is used for fluorescence observation. It makes the refractive indexes of three wavelengths of the light the same.
The achromatic lens makes the refractive index of the two wavelengths of the light the same.
There are two immersion lenses available; water immersion lens and oil immersion lens. The basic phenomenon is using a liquid between the objective lens and specimen.
3. Condenser Lens
A condenser lens is placed under the stage before the diaphragm. It focuses the rays of light on to the specimen through the diaphragm. It is essential for clarity of the image.
For color aberration, the following are the types of condenser lenses available.
Abbe Condensor has two lense; a plano-convex lens and a bi-convex lens. It is used for routine observations in schools and hospitals.
The achromatic condenser has four lenses. It corrects spherical and chromatic aberrations. It is widely used in laboratories.
The Aplanatic Condensor uses five lens elements. It focuses light in a single plane. Aplanatic Condensors are used for producing white and black images.
A light microscope uses a series of lenses to produce a highly magnified image. The quality of lenses describes the worth of the microscope. These lenses are available in the best possible types to remove the color aberration and improve the field stop.
Objective lens and eyepiece are the two primary lenses for magnification, whereas the third type is known as the condenser lens that focuses the light on to the specimen. All these collectively work in the microscope to bring your desired level of zooming the object.