This set of Microbiology Multiple Choice Questions & Answers (MCQs) focuses on “Transmission and Scanning Electron Microscope”.
1. Which of the following is used in electron microscope?
a) electron beams
b) magnetic fields
c) light waves
d) electron beams and magnetic fields
Explanation: Electron Microscope uses electron beams and magnetic fields to produce the image, whereas the light microscope uses light waves and glass lenses. In electron microscopy, a much higher resolution is obtained with extremely short wavelength of the electron beam.
2. Electron Microscope can give a magnification up to ___________
Explanation: The resolving power of the electron microscope is more than 100 times that of the light microscope, and it produces useful magnification up to 400,000X. It is possible to resolve objects as small as 10 Angstrom.
3. Which of the following are true for electron microscopy?
a) specimen should be thin and dry
b) image is obtained on a phosphorescent screen
c) electron beam must pass through evacuated chamber
d) specimen should be thin and dry, image is obtained on a phosphorescent screen and electron beam must pass through evacuated chamber
Explanation: Since electrons can travel only in high vacuum, the entire electron path through the instrument must be evacuated; specimens must be completely dehydrated prior to examination. Only very thin specimens can be observed in the conventional electron microscope since the penetrating power of electrons through matter is weak. The magnified image may be viewed on a phosphorescent or fluorescent screen.
4. Degree of scattering in transmission electron microscope is a function of __________
a) wavelength of electron beam used
b) number of atoms that lie in the electron path
c) number and mass of atoms that lie in the electron path
d) mass of atoms that lie in the electron path
Explanation: In a transmission electron microscope, contrast results from the differential scattering of electrons by the specimen, the degree of scattering being a function of the number and mass of atoms that lie in the electron path.
5. Negative Staining is used for examining _____________
a) virus particles
b) protein molecules
c) bacterial flagella
d) virus particles, protein molecules and bacterial flagella
Explanation: In negative-staining the electron opacity of the surrounding field is increased by using an electron-dense material such as phosphotungstic acid as a stain. Negative staining is particularly valuable for the examination of very small structures such as virus particles, protein molecules and bacterial flagella.
6. Which among the following helps us in getting a three-dimensional picture of the specimen?
a) Transmission Electron Microscope
b) Scanning Electron Microscope
c) Compound Microscope
d) Simple Microscope
Explanation: The scanning electron microscope lacks the resolving power obtainable with the transmission electron microscope but has the advantage of revealing a striking three-dimensional picture. The surface topography of a specimen can be revealed with clarity and depth of field not possible by any other method.
7. The secondary electrons radiated back in scanning microscope is collected by?
c) vacuum chamber
Explanation: In scanning electron microscope (SEM), the surface of the specimen is irradiated with a very narrow beam of electrons. Such irradiations causes low energy (secondary) electrons to be ejected from the specimen which can then be collected on a positively-charged plate or anode thereby generating an electric signal.
8. On what factors do the intensity of secondary electrons depend upon?
a) shape of the irradiated object
b) chemical composition of the irradiated object
c) number of electrons ejected
d) size and chemical composition of the irradiated object, number of electrons ejected and on the number of electrons reabsorbed by surrounding
Explanation: The irradiations in SEM causes secondary electrons to be ejected from the specimen thereby generating a signal that is proportional to the number of electrons striking the anode. The intensity or the number of secondary electrons depends on the shape and the chemical composition of the irradiated object and also on the number of electrons ejected and the number of electrons reabsorbed by surrounding.
9. Where do we obtain the magnified image of the specimen in SEM?
a) cathode ray tube
b) phosphorescent screen
d) scanning generator
Explanation: In TEM, the image is obtained on a phosphorescent screen but in SEM the magnified image of the surface topography of the specimen is obtained on the cathode ray tube. The electronic signals generated scan the specimen in a raster pattern in the manner of a television system to produce an image on a cathode ray tube.
10. Which of the following techniques are used in Transmission Electron Microscopy (TEM) for examining cellular structure?
b) Shadow Casting
c) Ultrathin Sectioning
d) Negative-Staining, Shadow Casting, Ultrathin Sectioning, Freeze-Etching
Explanation: Numerous techniques are available for use with electron microscopy which extends its usefulness in characterizing cellular structure. Some of them are Negative-Staining (which increases the electron opacity of surrounding), Shadow Casting (helps in producing three-dimensional structure of the object), Ultrathin Sectioning and Freeze-Etching.
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