Q: What is the principal advantage of an electron microscope over an optical microscope?
A: Electron microscopes have a far greater resolution compared to optical microscopes. Consequently, a much higher magnification is possible. Optical microscopes can magnify up to about 1000x, electron microscopes up to about 1 000 000x.

Q: How to increase resolution of image?
A: The resolution of am image can not simply be increased, once a picture has been taken through the microscope. Information which is not present in the first place can not simply be created. When taking pictures with the microscope, one should make sure that all the parameters are optimized to reach the maximum theoretical resolution. This includes a steady camera-microscope connection, the correct condenser diaphragm setting, the optimum mounting medium, etc.

Q: Parts of the microscope and their functions?
A: This question can not simply be answered in a line or two. I would recommend to watch the video, or read the post: Parts of a Compound Microscope

Q: What are some microbes that you can see under a microscope?
A: Ultimately you can see all types of microbes, provided you have the right type of microscope and use the appropriate technique. Viruses can be seen with electron microscopes, but not with light microscopes. Bacteria can best be seen with light microscopes that use phase contrast optics. Single celled eukaryotes (ciliates, algae etc.) as well as multicellular microorganisms can be seen with bight-field compound microscopes and also with stereo microscopes.

Q: How many different types of microscopes are there?
A: It depends on what system of classification you use and how many subdivisions you include. One common way to classify microscopes is into optical and non-optical microscopes. I already wrote a post on different types of microscopes:

Q: Which type of microscope would be best to use if you wanted a 3-dimensional view of a bacteria cell?
A: Here you have to be careful, the question can be misinterpreted. For true 3D, stereoscopic views two different images are needed.

There are two types of microscopes that provide 3-D (stereoscopic) views:

• Scanning electron microscopes: These devices scan the surface of the object. One single image is produced, which appears 3D (including “shadows” and surface texture). An example image can be found in this article:
• Confocal laser microscopes: These are highly specialized optical microscopes, in which a computer computes a final. In this case it is possible to compute two different pictures, one for the left and one for the right eye. The image is then truly stereoscopic

Q: Compare the kind of image obtained with scanning electron microscope with that obtained using transmission electron microscopy.
A: In short, scanning electron microscope (SEMs) produce images that have a 3D appearance, Transmission electron microscopes (TEMs) produce 2D images.

Q: Why is it desirable that microscope objectives be parfocal?
A: Parfocal objectives are not only desirable, but (in my humble view) a necessity for efficient microscopic work. Parfocal objectives manufactured in such a way that a change in objective will not result in a significant loss of focus. If the image is in focus using a 4x objective, then the image is also in focus when a 10x objective is used. Significant refocussing is not necessary with parfocal objectives.

Q: Part of the microscope that contains the ocular lens
A: One word answer: the eyepiece. Sometimes the terms “eyepiece” and “ocular lens” are used interchangeably, but the eye piece contains more than one lens element.

Q: Different types of microbes
A: The term “microbe” is a colloquial term which refers to organisms (living things) that are too small to be seen with the unaided eye. The term is somewhat unclear, because microscopic insects (and other multicellular organisms) generally are not included. Viruses are not alive and therefore do not quality as microorganisms. Without going into too much detail, microorganisms include prokaryotes (Bacteria, Archaea), microscopic fungi, single-celled algae and protozoa (ciliates and amoeba belong to this category, among others).

Q: Who invented the microscope?
A: Which microscope? There are many kinds. In 1931, Ernst Ruska and Max Knoll constructed the first prototype of an electron microscope. Optical microscopes as we know them today evolved over a longer time period. Many people contributed to the developments. Two notable figures are Antonie van Leeuwenhoek (1632 – 1723) and Robert Hook (1635 – 1703). Leeuwenhoek made single-lens microscopes with which he discovered bacteria. Hook constructed compound microscopes (composed of objective and ocular lenses) and coined the term “cell”.

The numbers written on an objective designate different optical characteristics and standards.

What do the numbers and abbreviations on an objective mean? Especially when buying used microscopes from research laboratories or hospitals a basic knowledge of the text written on the optics can become handy. You don’t want to buy things that you don’t need.

• A or ACHRO (depending on brand): This signifies that the objective is an achromat. This means that chromatic abberration was corrected for 2 colors (in contrast to the expensive APOchromatic lenses). Achromatic lenses are those most commonly found in education, they are the cheapest.
• PLAN: These objectives produce an image which is in focus from edge to edge. They are used for photographic work and are more expensive.
• PLANAPO: This refers to a planapochromatic objective. It produces a flat image (in focus from edge to edge) and it is has a chromatic abberration correction for 4 colors. Expensive and not needed for educational work.
• PLANFL: A Planfluorite objective. A bit less expensive than the planapochromats but also not as fully corrected.
• 160: This represents the standard tube length of 160mm. Objectives with this standard are interchangeable between manufacturers.
• 0.17: This represents the thickness of the cover slip to be used in mm. Coverslips with a deviating thickness will result is an image of lower resolution.
• 4, 10, 20, 40, 100: This represents the magnification of the objective. The total magnification is calculated by multiplying the magnification of the objective with the magnification of the ocular (eye piece), which is usually 10x. The magnification is also indicated by the ring colors:
  • red: 4x or 5x
  • yellow: 10x
  • green: 20x
  • blue: 40x, 50x or 60x
  • white: 100x
• OIL: This designates an oil immersion objectives. Do not immerse non-oil objectives into immersion oil!
• WI: Water Immersion. Here water is used instead of oil.
• 0.65 (etc): This is the numerical aperture. This value indicates the angle to which an objective is able to receive light. This value also determines the resolution of the system. For maximum resolution, the iris diaphragm should be set to a value equal or larger than the numerical aperture of the objective in use.
• NCG or NC: These abbreviations stand for “No cover glass”. These objectives are designed to be used without a cover glass. They are useful in the medical area where blood smears etc. are observed.
• LWD or ULWD: These abbreviations stand for “long working distance” or “ultra-long working distance”. These objectives are able to work with a large specimen-objective distance and are used for specific applications.
• P, POL or SF: These objectives are designed to be used for polarization microscopy. The objectives are strain-free (SF) and will therefore not modify the polarization of the light. They are not necessary for simple polarization microscopy conducted in classrooms.
• PL or NH: These are designation of objectives used for phase contrast microscopy. A PL (positive low) objective produces an image of a specimen which is darker than the background, a NH (negative high) objective produces an image which is brighter than the background.
• NIC or DIC: Nomarski Interference Contrast or Differential Interference Contrast objectives produce an image of a specimen which appears to be slightly 3 dimensional. If you use a filter to achieve oblique illumination, then the result will look similar.

Spirogyra alga. We are at the limit of resolution for this objective. Further magnification of the image will not reveal more details. The only possibility to increase resolution is to switch to an objective with a higher resolving power, to use a shorter wavelength of light or to generally improve the optics. But there is a physical limit.

A part of the above image was further magnified 2x. No additional details become visible. This is referred to as 'empty magnification.'

Let’s start this topic with a little example. You are in a shop and have a choice between two microscopes. One is capable of magnifying 100x, the other one is capable of magnifying 400x. Which one is better? Given only this information, most people would opt for the 400x device. A larger number, in the mind of the uninitiated consumer, means a better quality and more value. Technical and scientific instruments and even consumer electronics, such as digital cameras, have become increasingly complex and the consumers often demand a quick and easy measure to compare the different instruments. In the case of microscopes it is often the magnification, in the case of digital cameras it is the number of mega pixels, and computers are often compared using CPU speed and memory. Simple numbers, simple comparison. So a 400x microscope, in the mind of a lay person, will show you 4 times as much as a 100x device. It is not unusual to see „department store microscopes“ advertised with a maximum magnification of 1250x. This drive for large numbers is even visible in other optical devices, such as telescopes. I once saw an advertisement of a children’s telescope advertised with magnifications of 650x. The maximum useful magnification of astronomical telescopes is around 300x.

One thing that must be made clear to students is, that a high magnification is probably the easiest thing to achieve! Just take a picture of the image and then enlarge it to fill your living room wall. The only problem is, that you are not going to see more detail. The image is larger for sure, but also blurry and soft. A larger magnification does not always mean that the resulting image has a higher information content and more detail.

The maximum useful magnification for compound light microscopes is around 1000x. Everything above this value will result in „empty magnification“, that is magnification without further detail. The reason for this limit lies not in the manufacturing limitations of the optics, but rather in the physical nature of light. It is not possible to resolve details that are smaller than the wave length of the light used. In simple words, from a certain magnification upwards, the light is too „coarse“ to resolve more details.

Let’s go back to the 100x and 400x microscope. Which one is better? The answer is simple: it depends on the resolution that they are able to produce. A high-resolution 100x microscope will show more detail than a 400x microscope with a poor resolution. If the resolution of the 400x microscope is also high, however, then one would see more with the 400x instrument. In summary, a combination of both magnification and resolution determines how much one is able to see. A high useful magnification is only possible when the resolution is also high.