The Working Principles of Microscope

Phaos Optic Science Educational Series

8 March, 2021
12:00 PM (GMT)

5 Minutes

Basic Structure of a Microscope

A general optical microscope mainly consists of an objective lens, ocular lens, lens tube, stage, and reflector. An object placed on the specimen stage will be magnified through the objective lens. When the target is focused, a magnified image can be observed through the eyepiece.

Cr/ Microbe Notes

Working Principles

Microscope will emit light onto or through objects (Sample placed on the specimen stage) and magnify the transmitted or reflected light with the objective and ocular lenses.

Microscope Abilities

Firstly, Magnification is the ability of a microscope to product an image of an object at a scale larger (or even smaller) than its actual via the use of objective lens. Difference lens have different magnification such as x 10, x 50 or x 100 to enlarge samples with precision.

Next, Resolution also plays a part in determining the performance of a microscope. It is the ability to identify two light spots separately and is expressed as the shortest distance between two points that can still be distinguished as distinct entities. While a high magnification is needed in order to visualize small objects, the resolution will determine just how clearly they can be seen.

How about the performance of the objective lens?

Users can consider the numerical aperture (NA). It measures the ability to gather light and resolve fine specimen detail while working at a fixed object (or specimen) distance. Image-forming light waves pass through the specimen and enter the objective in an inverted cone. Hence, the smaller the object, the more pronounced the diffraction of incident light rays will be.

Higher values of numerical aperture permit increasingly oblique rays to enter the objective front lens, which produces a more highly resolved image and allows smaller structures to be visualized with higher clarity, thereby, improve its resolution and brightness. [For more information on the topic of numerical aperture and resolution, keep a lookout on our next blog!]

In addition, Aberration, the distortion or blurring of an image caused by imperfections in the lens’ shape, is another factor that influences lens performance. Some of the common aberrations are;

  1. Defocus: The optical system is out of focus, reducing the sharpness of images produced by the system.
  2. Radial distortion: Images with radial distortion usually have symmetric distortion due to the symmetry of a lens.
  3. Astigmatism: Rays that propagate in two perpendicular planes have different foci.

Hence, the more thoroughly the aberration is corrected, the higher the performance of the lens.

In summary, when selecting a microscope, the user must check the NA of the lens, the magnification, and the resolution based on the samples intended for imaging.