Definition of optical microscope
An optical microscope ( or light microscope) which utilize visible light rays and a lens for the formation of magnified image of a small objects, optical microscopes has been designed around 17th century so tis is one of the oldest microscopes.
Components of optical microscopes
A microscope comprises the following structural parts:
Eyepiece | Ocular lens :
The eyepiece forms the viewing part of the microscope from where the image can be observed. The eyepiece is basically a cylindrical tube having one or more lenses attached to the top of the tube. The eyepiece does not allow the ocular lens to fall or experience damage. This enhances the clarity of the lens.
Objective turret | Revolver or revolving nose piece:
These are used for holding the objective lenses and allowing the user to rotate as per requirements.
The objective lens is attached to the lower end of the microscope tube and is directed towards the sample. A microscope can have one or more objectives for collecting light reflected off the sample object. Microscope objective lenses are parfocal i.e. the even after switching the lenses, the sample object is kept in focus. Objective lenses are used depending on the required magnification and numerical aperture. With the increase in magnification, the numerical aperture also increases. High-performing microscopes design special objective-eyepiece pairs for better performance.
Focus knobs can adjust the level of the stage up and down. This feature is required especially for adjusting the focus of a sample with varied thickness. In modern-day microscopes, the stage is mobile and the tube is stationary whereas older microscopic designs had a mobile tube and a stage was stationary.
Coarse adjustment knob is capable of adjusting the focus for a sample to a great extent.
Fine adjustment is capable of adjusting the focus for specimen by small amounts or minutely.
The stage gives a platform for keeping the sample directly below the objective lens for observation. The stage can illuminate or pass the light beam to the sample through a transparent circle on which the slide is kept. The stage has arms that can be adjusted precisely to secure the slide in its place for different microscope objective lenses. The stage is usually mobile and can be adjusted upwards and downwards.
Microscope can have several types of light sources. Simple microscopes use sunlight directly to illuminate the sample with the help of a mirror. Advanced microscopic designs have artificial lighting sources attached in the microscope stage for illuminating the specimen. The light intensity and luminaire brightness can be manually varied depending on the user’s needs. The source of illumination can be a halogen lamp, an LED, or a laser. The most expensive microscopes use Köhler illumination as an illuminating source.
Diaphragm | Condenser:
A condenser is a lens or set of lenses that are used to focus the light rays from the illuminating source to the sample. The condenser has diaphragms or filters that can be used to further adjust the intensity of illumination. Certain illumination methods need the sample to be perfectly aligned with the optical path; for example phase contrast, differential interference contrast, and darkfield.
What are the types of optical microscopes?
Optical microscopes can be broadly divided into three types:
In a simple microscope, the phenomenon of angular magnification is used for obtaining an erect magnified image. Such microscopes may use a single lens or a set of lenses. Examples of simple microscopes are loupes, magnifying glass, telescopes eyepieces, and microscopes eyepieces.
In a compound microscope, one lens is used for collecting light from the specimen and focusing a real image of it inside the microscope and another lens is used for further magnifying that real image to form an inverted vertical image. These microscopes can provide much higher magnifications and are used for several purposes.
In a digital microscope the image of the sample is taken via a digital camera and observed by using a computer. The microscope design may be partly or totally controlled by a computer. Certain microscopes have CCD (charge-coupled device) in place of an eyepiece. The digital microscope allows a more detailed analysis of the sample specimen.
How does an optical microscope work?
In standard compound-microscopes, the sample first staged then it’s illuminated as per the light requirement’s and light from specimen will passed thru the objective-lens and form an erect real image with the help micros coping tube and The point at which the real image is formed is generally known as focus of the eyepiece lens. The ray from this point will passing thru the eyepiece lens to finally produce an inverting magnified image of the sample object. For few microscopes, a CCD in the microscopic tube replacing the eyepiece lens, in that magnified image will form in a computer .
What is an oil immersion objective?
When the magnification of the objective increases, its resolution decreases and we are not able to distinguish between two points properly. So, to increase the image resolution, the numerical aperture should be increased. As the numerical aperture is directly proportional to the refractive index of the medium, if we increase the refractive index of the medium in which the objective lens is kept, the numerical aperture will increase. So oil immersion objectives are used. One can achieve a numerical aperture as high as 1.6 by using an oil immersion objective.
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