Now that we understand how lenses work lets try to understand how these lenses can be put together to build a microscope. Basic microscopes can be divided into two broad categories:
a. Finite Tube Length Microscope
b. Infinite Tube Length Microscope
Finite Tube Length Microscope
Finite tube length microscopes represent the earliest designs of light microscopy. It primarily uses two lenses:
a. Objective Lens: So called because it is placed close to the object or the specimen
b. Eye Piece Lens: So called because it is placed close to the eye of the observer
As can be seen from Figure 9 if an object is placed between the focal length (f) and twice the focal length (2f) then an inverted an magnified image is formed between the 2f and infinity on the other side of the lens. the magnification increases as the object get away from 2f and closer to f. This effect is exploited in this configuration.
It would then look sufficient to have only one lens to make a microscope. This is true if we are to capture this image on a screen, a photographic film or the sensor chip of a camera. However, we need to note that if an observer needs to see the magnified image of the sample through the microscope having only one lens is not sufficient. This is because the eye has a lens of its own. The eye requires parallel rays to enter it so that an image could be formed on the retina.
This problem is solved using the eyepiece which collects the light from the image created by the objective and presents a parallel beam of light to the eye of the observer so as to form an image on the retina.
The construction of a finite tube length microscope is shown in Figure 10. Here fo is the objective focal length and fe is the eye piece focal length.
For this arrangement to work the specimen is placed between f0 and 2fo. This creates a magnified and inverted image beyond 2fo on the other side of the objective lens. The image created by the objective now acts as the object for eyepiece lens. The eyepiece lens needs to be placed so that the image formed by the objective is eyepiece focal length distance (fe) away from it. As can be seen from Figure 5 if an object is placed at focal distance light rays emerge parallel. When the observer places her/his eye in this path the parallel rays are converged by the lens of the eye to create an image on the retina.
It can be appreciated that the distance between objective and eyepiece is fixed and cannot be altered at will. Traditionally these two lenses were mounted on opposite sides of a tube and the tube length was designed to correspond to this fixed distance. Hence the name finite tube length microscope.
Infinite Tube Length Microscope
Modern microscopes use an infinite tube length configuration. This is because of a number of practical problems in a finite tube lengths configuration. Microscope designers wanted more flexibility in the length of the microscope tube and wanted the sample to be placed at a focal distance away from the objective. This gave rise to infinite tube length configuration.
The construction of an infinite tube length microscope is shown in Figure 11. Here a third lens called the intermediate lens or tube lens is introduced between the objective and the tube lens. In Figure 11 'fo' is the focal length of the tube lens, 'fi' the focal length of the intermediate lens and 'fe' the focal length of the eyepiece lens. The following arrangement is used:
a. The object is placed at a focal distance (fo) away from the objective. This makes the light from a given point on the object come out collimated on the other side of the objective.
b. Since the light out of the objective is collimated the intermediate lens or tube lens can be placed ideally at any distance from the objective. This in principle creates an infinite space between the objective and the intermediate lens. Microscope designers could then chose a tube length of their choice independent of the focal lengths of the lenses used in the microscope. The intermediate lens creates a magnified and inverted image on the other side at the intermediate lens focal length (fi) distance from it. This image is called the intermediate image.
c. The eyepiece is placed similarly to that in a finite tube length configuration. The image created by the intermediate lens acts as the object for the eyepiece. The eyepiece lens is placed so that the image formed by the intermediate lens is eyepiece focal length (fe) distance away from it. This makes the light from a given point on the intermediate image come out collimated on the other side of the eyepiece. When the observer places her/his eye in this path the parallel rays are converged by the lens of the eye to create the final image on the retina.