Microbiology Questions and Answers – Microorganisms Microscopic Examination – Light Microscope

This set of Microbiology Multiple Choice Questions & Answers (MCQs) focuses on “Microorganisms Microscopic Examination – Light Microscope”.

1. Which part of the compound microscope helps in gathering and focusing light rays on the specimen to be viewed?
a) Eyepiece lens
b) Objective lens
c) Condenser lens
d) Magnifying lens

Explanation: Compound microscope contains three separate lens systems. The condenser lens is placed between the light source and the specimen and it gathers and focuses the light rays in the plane of the microscopic field to view the specimen.

2. What is the minimum distance for the eye to focus any object?
a) 11 cm
b) 25 cm
c) 32 cm
d) 4 2 cm

Explanation: The eye cannot focus on objects brought closer to it less than 25 cm; this is, accordingly the distance of maximal effective magnification. An object must also subtend an angle at the eye of 1 degree or greater.

3. Resolving power of a microscope is a function of ____________
a) Wavelength of light used
b) Numerical aperture of lens system
c) Refractive index
d) Wavelength of light used and numerical aperture of lens system

Explanation: The ability of a microscope to distinguish two adjacent points as distinct and separate is known as resolving power. Resolving power is a function of wavelength of light used and the numerical aperture (NA) of the lens system. NA refers to the refractive index of the medium multiplied with the sine value of the half-aperture angle.

4. The greatest resolution in light microscopy can be obtained with ___________
a) Longest wavelength of visible light used
b) An objective with minimum numerical aperture
c) Shortest wavelength of visible light used
d) Shortest wavelength of visible light used and an objective with the maximum numerical aperture

Explanation: The relationship between numerical aperture (NA) and resolution is:-
Resolution (d) = wavelength / 2(NA)
Thus maximum resolution is obtained with the shortest wavelength of visible light and an objective with the maximum NA.

5. Oil immersion objective lens has an NA value of____________
a) 0.65
b) 0.85
c) 1.33
d) 1.00

Explanation: NA = refractive index * sine (half-aperture angle).
The maximum NA for a dry objective is less than 1.0 as the refractive index of air is 1. The values of NA for oil immersion lens is slightly greater than 1.0 in the range of (1.2 to 1.4) as the refractive index of oil is 1.56.

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6. In fluorescence microscopy, which of the following performs the function of removing all light except the blue light?
a) Exciter filter
b) Barrier filter
c) Dichroic mirror
d) Mercury arc lamp

Explanation: In fluorescence microscopy, the function of the exciter filter is to remove all but the blue light; the barrier filter blocks out blue light and allows any other light emitted by the fluorescing specimen to pass through and reach the eye.

7. Total Magnification is obtained by __________
a) Magnifying power of the objective lens
b) Magnifying power of eyepiece
c) Magnifying power of condenser lens
d) Magnifying power of both the objective lens and eyepiece

Explanation: The total magnification is determined by multiplying the magnifying power of the objective by that of the eyepiece. Generally, an eyepiece having a magnification of 10X is used although eyepieces of higher or lower magnifications are available.

8. In light microscopy, which of the following is used as fixatives prior to staining technique?
a) Osmic acid
b) Glutaraldehyde
c) Heat
d) Osmic acid, glutaraldehyde, heat

Explanation: Most staining techniques kill cells and so preliminary to staining, the cells are sometimes fixed. Commonly used chemical fixatives include osmic acid and mainly glutaraldehyde. But for light microscopy heat is the most commonly used fixative.

9. In Phase contrast microscopy, the rate at which light enters through objects is __________
a) Constant
b) Inversely proportional to their refractive indices
c) Directly proportional to their refractive indices
d) Exponentially related to their refractive indices

Explanation: Phase contrast microscopy is based on the fact that the rate at which light travels through objects is inversely related to their refractive indices. Since the frequency of light waves is independent of the medium through which they travel, the phase of a light ray passing through an object of the higher refractive index than the surrounding medium will be relatively retarded.

10. Which part of the light microscope controls the intensity of light entering the viewing area?
c) Diaphragm
d) Condenser lens

Explanation: On the condenser is mounted a shutter like an apparatus called the diaphragm which opens and closes to permit more or less light into the viewing area. Condenser lens just helps in condensing the light rays. Coarse and fine adjustment screws are used for focusing under different power lens.

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