When we talk about microscope, we all know that it produces a magnified image of an object for us to examine the details which cannot be observed by the naked eye. In fact, we use various types of microscopes to observe different samples, and the image details shown by each microscope are different. The microscopes commonly used can be classified into two types: optical microscope and electron microscope. As the name suggests, the imaging source of an optical microscope is visible light, which passes through the optical lens system to create a magnified image; On the other hand, the imaging source of electron microscopes is electron beam. The computer receives and processes the signals created by electrons to form a microscopic image.
The most common type of microscope found in biology laboratories in local secondary schools is compound light microscope, with magnification power from 40x to 1000x. It is suitable for translucent samples such as cells, tissues or thin organ slides of organisms. Sometimes we have to pretreat the sample before observation, for example, fixing or staining the sample with chemicals. The sample must be mounted on the glass slide and covered with a cover slip too. Since the sample is extremely thin in slice and translucent, compound microscope allows us to observe the internal structures of the sample, such as the organelles in the cell.
Besides, according to the feature of the sample or the details we want to highlight, we can apply some techniques like ‘darkfield’ or ‘phase contrast’ by adding additional parts to alter the light path. To make it simple, ‘brightfield’ is the basic mode of observation with a white background; in ‘darkfield’ microscopy, we use a light stop to block the direct light, making the background becomes black to increase contrast; ‘phase contrast’ microscopy makes use of light wave interferences to show the details of the object.
Dissecting microscopes, or known as stereo microscopes, are another type of optical microscope commonly used for observing the external features of a non-translucent sample, such as the surface of insects or flowers. Its operation requires less technique than that of compound microscopes, and the sample can be put onto the stage for observation directly without any pre-treatment. The magnification power of dissecting microscopes is in a range from around 6x to 50x, which is much lower than that of the compound microscope but still good enough since the sample observed is usually much larger in size.
Comparisons between compound microscope and dissecting microscope
Compound microscopes | Dissecting microscopes | |
Magnification power | Around 40x to 1000x | Around 6x to 50x |
Suitable samples | - Internal structures of thin and translucent samples -Smaller in size | - External features of a sample -Larger in size |
Sample treatment | -Must be mounted on the glass slide and covered with a cover slip | -No pre-treatment needed |
Others | -Additional accessories can be used to present different observation effects | / |
To conclude, we select using different types of microscopes according to the image details we would like to show. Good tools come before good work, so a suitable and sophisticated microscope can become a good partner of the observer; and learning how to get along with your partner is also the first key to enter the microscopic world.
Author: Ms. CHIM Hoi Ying (Biology teacher)
