In a fluorescence microscope, what is the job of the barrier (emission) filter?

To block the exciting light and pass fluorescence

To increase the numerical aperture of the objective

To absorb the light emitted by the fluorochrome

To pass only ultraviolet light to the specimen

Phase-contrast microscopy is especially useful for:

Viewing living, unstained cells

Reading auramine-stained smears

Studying surfaces in three dimensions

General microbiology: microscopes

22 Kerala PSC Lab Technician Gr II practice questions on General microbiology: microscopes, part of Microbiology. The first 22 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
In a fluorescence microscope, what is the job of the barrier (emission) filter?
1. ATo block the exciting light and pass fluorescence
2. BTo increase the numerical aperture of the objective
3. CTo absorb the light emitted by the fluorochrome
4. DTo pass only ultraviolet light to the specimen
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Answer: A · To block the exciting light and pass fluorescence
The exciter filter passes only short-wavelength (ultraviolet or blue) light to the specimen. Fluorochromes such as auramine O absorb this light and emit light of a longer wavelength. The barrier filter, placed between the objective and the eyepiece, stops the exciting light, which would harm the eyes and swamp the image, and lets the emitted light through. Stained objects therefore glow against a dark background.
- B: Numerical aperture is a property of the objective lens, not of a filter.
- C: Absorbing the emitted light would leave nothing to see.
- D: Selecting the exciting light is the job of the exciter filter.
Question 2 · Easy
What focuses the beam in an electron microscope?
1. AGlass objective lenses
2. BQuartz condenser lenses
3. CElectromagnetic lenses
4. DConcave mirrors
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Answer: C · Electromagnetic lenses
Glass cannot bend a beam of electrons, so electron microscopes focus it with electromagnetic coils that act as lenses. The column is kept under vacuum because air molecules scatter electrons. The very short wavelength of electrons gives a resolution hundreds of times better than that of the light microscope. The image is formed on a fluorescent screen or recorded digitally.
- A: Glass lenses bend light, not electrons.
- B: Quartz condensers are used in some ultraviolet light microscopes, not for electrons.
- D: Concave mirrors reflect light in older microscopes and cannot focus electrons.
Question 3 · Hard
Using d = 0.61λ/NA, what is the limit of resolution with green light (λ = 0.55 µm) and an oil objective of numerical aperture 1.30?
1. A0.44 µm
2. B0.42 µm
3. C0.26 µm
4. D3.9 µm
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Answer: C · 0.26 µm
Multiply the wavelength by 0.61: 0.61 × 0.55 µm = 0.336 µm. Divide by the numerical aperture: 0.336 ÷ 1.30 = 0.26 µm, so two points closer than this appear as one. Resolution improves (d becomes smaller) with a shorter wavelength or a higher numerical aperture. That is why oil immersion, which raises the numerical aperture, is used for bacteria.
- A: 0.44 µm multiplies by the numerical aperture instead of dividing by it.
- B: 0.42 µm leaves out the constant 0.61.
- D: 3.9 µm inverts the formula.
Question 4 · Easy
Fluid from a painless genital ulcer is examined for live, motile Treponema pallidum. Which microscope is used?
1. AFluorescence microscope
2. BElectron microscope
3. CDark-field microscope
4. DBright-field microscope
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Answer: C · Dark-field microscope
Treponema pallidum is too thin (about 0.1-0.2 µm) to be seen unstained by ordinary light microscopy. A dark-field condenser lets only light scattered by objects enter the objective, so the spirochaetes shine against a black background and their corkscrew motility can be seen. The fluid must be examined within minutes, before the organisms stop moving. Oral lesions are unsuitable because commensal treponemes look similar.
- A: Fluorescent antibody methods use fixed smears, so motility is lost.
- B: Electron microscopy needs killed, processed specimens and cannot show motility.
- D: T. pallidum is too thin to be seen unstained by bright-field microscopy.
Question 5 · Medium
Phase-contrast microscopy is especially useful for:
1. AViewing living, unstained cells
2. BSeeing viruses in fine detail
3. CReading auramine-stained smears
4. DStudying surfaces in three dimensions
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Answer: A · Viewing living, unstained cells
Living cells are nearly transparent, so they show little contrast in a bright-field microscope. A phase-contrast microscope uses an annular diaphragm and a phase plate to turn small differences in refractive index and thickness into differences in brightness. Internal structures and motility can then be seen without fixing or staining, for example in wet mounts and cell cultures.
- B: Viruses are below the resolution of any light microscope.
- C: Auramine-stained smears are read by fluorescence microscopy.
- D: Three-dimensional surface views need a scanning electron microscope.
Question 6 · Hard
A stained smear looks dim and lacks detail under the 100x oil objective. The condenser is at its lowest position and its iris diaphragm is almost closed. The best correction is to:
1. ASwitch to the 40x objective with oil
2. BAdd a second drop of oil to the slide
3. CChange to a 15x eyepiece and refocus
4. DRaise the condenser and open the iris
Answer and explanation in LearnFree account
Question 7 · Medium
Why is immersion oil placed between the slide and the 100x objective?
1. AIt protects the lens from the stain
2. BIt holds the coverslip firmly in place
3. CIt magnifies the image a second time
4. DIt matches the refractive index of glass
Answer and explanation in LearnFree account
Question 8 · Easy
With a 10x eyepiece and the 100x oil-immersion objective, the total magnification is:
1. A1000x
2. B110x
3. C100x
4. D400x
Answer and explanation in LearnFree account
Question 9 · Hard
After oil-immersion work, a student swings the 40x objective across the oiled slide. Now the 40x image is foggy on every slide, including unmounted smears, while the 10x and 100x images stay sharp. The most likely cause is:
1. ACondenser iris closed too far
2. BSlides mounted with thick coverslips
3. COil on the front lens of the 40x
4. DLamp voltage set too low
Answer and explanation in LearnFree account
Question 10 · Medium
Why must the specimen in an electron microscope be examined in a vacuum?
1. AThe vacuum cools the electromagnetic lenses
2. BAir would absorb the fluorescent light
3. CThe vacuum fixes and stains the specimen
4. DAir molecules would scatter the electrons
Answer and explanation in LearnFree account
Question 11 · Medium
Which change improves the resolving power of a light microscope?
1. AClosing the iris diaphragm
2. BLowering the condenser
3. CUsing a 15x eyepiece instead of 10x
4. DUsing blue light instead of red
Answer and explanation in LearnFree account
Question 12 · Easy
Which part of the compound microscope collects light and focuses it onto the specimen?
1. AObjective
2. BIris diaphragm
3. CCondenser
4. DEyepiece
Answer and explanation in LearnFree account
Question 13 · Medium
Which two special parts convert a light microscope into a phase-contrast microscope?
1. AAnnular diaphragm and phase plate
2. BCentral stop and paraboloid condenser
3. CPolariser and analyser
4. DExciter filter and barrier filter
Answer and explanation in LearnFree account
Question 14 · Easy
In a routine bacteriology microscope, the 40x objective is known as the:
1. AScanning objective
2. BOil-immersion objective
3. CHigh-power dry objective
4. DLow-power objective
Answer and explanation in LearnFree account
Question 15 · Easy
Which microscope gives a three-dimensional image of the surface of a cell or bacterium?
1. AScanning electron microscope
2. BDark-field microscope
3. CPhase-contrast microscope
4. DTransmission electron microscope
Answer and explanation in LearnFree account
Question 16 · Medium
In dark-field microscopy, organisms look bright against a dark background because:
1. AOnly light scattered by them enters the objective
2. BThey take up a stain that reflects light
3. CA phase plate converts thickness into brightness
4. DThey fluoresce under ultraviolet light
Answer and explanation in LearnFree account
Question 17 · Easy
In fluorescence microscopy of sputum for tubercle bacilli, the smear is stained with:
1. AAuramine O
2. BCarbol fuchsin
3. CMethylene blue
4. DCrystal violet
Answer and explanation in LearnFree account
Question 18 · Hard
A dark-field condenser is fitted and oiled to the slide, but with the 100x oil objective the background stays grey instead of black. What is the best correction?
1. AUse a thick slide of 2 mm
2. BClose the objective's iris to lower its NA
3. CIncrease the lamp brightness fully
4. DRemove the oil below the slide
Answer and explanation in LearnFree account
Question 19 · Medium
Virus particles for transmission electron microscopy are commonly negatively stained with:
1. ANigrosin
2. BIndia ink
3. CPhosphotungstic acid
4. DMalachite green
Answer and explanation in LearnFree account
Question 20 · Hard
An auramine smear is positive when read at 10 a.m. A second reader looks at it next afternoon after it lay on a sunny bench, and sees only a few faint bacilli. The most likely cause is:
1. AFading of fluorescence on exposure to light
2. BOvergrowth of contaminating bacteria
3. CChange of auramine into carbol fuchsin
4. DLoss of mycolic acid from the bacilli
Answer and explanation in LearnFree account
Question 21 · Easy
Most viruses cannot be seen with a light microscope because its limit of resolution is about:
1. A20 µm
2. B2 nm
3. C0.2 µm
4. D0.2 nm
Answer and explanation in LearnFree account
Question 22 · Easy
A fluorescence microscope excites the fluorochrome in the specimen with:
1. AA beam of electrons
2. BPolarised white light
3. CUltraviolet or blue light
4. DInfrared light
Answer and explanation in LearnFree account

General microbiology: microscopes

In a fluorescence microscope, what is the job of the barrier (emission) filter?

What focuses the beam in an electron microscope?

Using d = 0.61λ/NA, what is the limit of resolution with green light (λ = 0.55 µm) and an oil objective of numerical aperture 1.30?

Fluid from a painless genital ulcer is examined for live, motile Treponema pallidum. Which microscope is used?

Phase-contrast microscopy is especially useful for:

A stained smear looks dim and lacks detail under the 100x oil objective. The condenser is at its lowest position and its iris diaphragm is almost closed. The best correction is to:

Why is immersion oil placed between the slide and the 100x objective?

With a 10x eyepiece and the 100x oil-immersion objective, the total magnification is:

After oil-immersion work, a student swings the 40x objective across the oiled slide. Now the 40x image is foggy on every slide, including unmounted smears, while the 10x and 100x images stay sharp. The most likely cause is:

Why must the specimen in an electron microscope be examined in a vacuum?

Which change improves the resolving power of a light microscope?

Which part of the compound microscope collects light and focuses it onto the specimen?

Which two special parts convert a light microscope into a phase-contrast microscope?

In a routine bacteriology microscope, the 40x objective is known as the:

Which microscope gives a three-dimensional image of the surface of a cell or bacterium?

In dark-field microscopy, organisms look bright against a dark background because:

In fluorescence microscopy of sputum for tubercle bacilli, the smear is stained with:

A dark-field condenser is fitted and oiled to the slide, but with the 100x oil objective the background stays grey instead of black. What is the best correction?

Virus particles for transmission electron microscopy are commonly negatively stained with:

An auramine smear is positive when read at 10 a.m. A second reader looks at it next afternoon after it lay on a sunny bench, and sees only a few faint bacilli. The most likely cause is:

Most viruses cannot be seen with a light microscope because its limit of resolution is about:

A fluorescence microscope excites the fluorochrome in the specimen with: