Everything you need to know about Microscopy. Some of the most frequently asked questions are as follows:- .
Q.1. What is a Micrometer (µm)?
Ans. A micrometer, also called a micron (µ) is equal to 10-6 m, i.e., 1/1000th mm.
Q.2. What is a Nanometer (nm)?
Ans. It was formerly called millimicron (mµ) and is equal to 10-9 m or 10-1 or 10A.
Q.3. What is Angstrom (A)?
Ans. An angstrom is equal to 10-10 m.
Q.4. How many lenses were there in the Microscope prepared by Leeuwenhoek in seventeenth century?
Ans. Only one lens (Fig. 3.1).
Q. 5. Who Invented the first Compound Microscope?
Ans. Zaccharias Janssen is known to have made the first compound microscope in 1600. However, they were of poor quality.
Q.6. Who made improvements on the crude compound microscopes made by Zaccharias Janssen?
Ans. Joseph Jackson Lister, father of Joseph Lister made the improvements.
Q.7. When did Robert Hooke observe cork cells under a compound microscope?
Ans. In 1665.
Q.8. Name the important parts of a compound light microscope and also give their functions.
Ans. Illuminator or source of light, condenser (focuses light through the specimen to be seen), eye piece or ocular lens (which magnifies the image formed by objective the lens), body tube (transits the image from the objective lens to the ocular lens), objective lenses (primary lenses that magnify the object), stage (the platform that holds the slide), diaphragm (which controls the amount of light entering the condenser), coarse focusing knob (for major adjustments) fine focusing knob (for precise adjustments) (Fig. 3.2).
Q.9. Which lens of the objective is used for maximum magnification?
Ans. Oil immersion lens (for which a drop of immersion oil, generally cedar wood oil is placed on the slide).
Q.10. What is resolution of a microscope?
Ans. It is the resolving power of the combination of lenses to distinguish fine details.
Q.11. What is the main characteristic of a dark-field microscope?
Ans. Instead of a normal condenser a dark field microscope contains an opaque disc. As there is no direct background light the specimen appears light against a black background or the dark- field. It is used to examine unstained microorganisms suspended in liquid, e.g., to see spirochetes like Treponema pallidum the causal organism of syphilis.
Q.12. Give the principle and main characteristics of phase contrast microscope.
Ans. The principle of phase contrast microscope is based on slight variations in refractive index. While the rays pass from the light source through the specimen, their velocity may be altered because of the differences in the thickness and physical properties of various portions of the specimen.
A phase contrast microscope is provided with special condensers which contain annular or ring shaped diaphragms. The diaphragm permits a ring of light to pass through the condenser focusing light on the specimen and a ring shaped diffraction plate in the objective lens. So, the diffracted and un-diffracted rays are then brought into phase with each other to produce the image that meets the eye.
Q.13. What is differential interference contrast (DIC) microscopy?
Ans. The DIC microscopy like phase contrast microscopy uses differences in refractive indexes. However, a DIC microscope uses two beams of light instead of one. Besides it the prisms split each light beam adding contrasting colours to the specimen. In this way resolution of a DIC microscope is higher than that of a phase contrast microscope. The image is brightly coloured and is nearly three-dimensional in appearance.
Q.14. What is a fluorescent microscope?
Ans. A fluorescent microscope uses fluorochromes (fluorescent dye) to stain the specimen, e.g., the fluorochrome auramine O glows yellow on exposure to UV light which is strongly absorbed by Mycobacterium tuberculosis the bacterium that causes tuberculosis.
The bacterium can be detected as bright yellow organisms against a dark background. Bacillus anthracis causal organism of anthrax, however, appears apple green while stained with another fluorochrome fluorescein isothiocyanate (Fig. 3.3).
Q.15. Write the principle of immuno fluorescence.
Ans. A fluorochrome is combined with antibodies against a specific type of bacterium. On adding the preparation to bacterial cells on a glass slide the antibodies attach to the bacterial cells and the cells fluoresce on illuminating with ultra violet (UV) light.
Q.16. Give the main characteristics of an electron microscope.
Ans. Such a microscope uses electrons in place of light and instead of using glass lenses uses electromagnetic lenses to focus on a specimen. The beam of electrons travels through an evacuated tube.
There are two types of electron microscopes:
(1) Transmission electron microscope (TEM) and
(2) Scanning electron microscope (SEM).
Q.17. What is a scanning electron microscope? What is it used for?
Ans. Scanning Electron Microscopes provide three dimensional views of the specimens. An electron gun produces a finely focused beam of electrons known as primary electron beam. These electrons pass through electromagnetic lenses and are directed over the surface of the object to be seen.
The primary electron beam knocks electrons out of the surface of the specimen resulting in the production of secondary electrons that are transmitted to an electron collector, amplified and used to give an image on the viewing screen or photographic plate. The scanning electron microscope is used to study surface features as that of leaf-peelings, pollen grains, fungal spores and bacteria.
Q.18. Why are electron beams used instead of light in TEM and SEM?
Ans. Because electrons have shorter wave length, and specimens smaller than 0.2 pm which cannot be resolved under light microscopes can be resolved and studied with TEM and SEM.
Q.19. Who invented atomic force microscope?
Ans. The precursor to AFM (atomic force microscope), the scanning tunneling microscope (STM) was developed by Gerd Binning and Heinrich Rohrer, in the early 1980s at IBM Research – Zurich a development which earned them Nobel Prize in Physics in 1986. However, Binning, Quate and Gerber invented the first atomic force microscope in 1986. The commercially available atomic force microscope was introduced in 1989.