Microscopy Revision

Which part of the compound microscope helps in gathering and focusing light rays on the specimen to be viewed?

• Eyepiece lens

• Objective lens

• Condenser lens

• Magnifying lens

Answer 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.

What is the minimum distance for the eye to focus any object?

• 11 cm

• 25 cm

• 32 cm

• 42 cm

Answer explanation

he 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.

Resolving power of a microscope is a function of ____________

• Wavelength of light used

• Numerical aperture of lens system

• Refractive index

• Wavelength of light used and numerical aperture of lens system

Answer 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.

Oil immersion objective lens has an NA value of____________

• 0.65

• 0.85

• 1.33

• 1.00

Answer 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.

Which part of the light microscope controls the intensity of light entering the viewing area?

• Coarse adjustment screw

• Find adjustment screw

• Diaphragm

• Condenser lens

Answer 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.

Which of the following is used in electron microscope?

• electron beams

• light waves

• magnetic fields

• electron beams and magnetic fields

Answer 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.

Which of the following are true for electron microscopy?

• specimen should be thin and dry

• image is obtained on a phosphorescent screen

• electron beam must pass through evacuated chamber

• pecimen should be thin and dry, image is obtained on a phosphorescent screen and electron beam must pass through evacuated chamber

Answer 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.

The secondary electrons radiated back in scanning microscope is collected by?

• specimen

• Anode

• vacuum chamber

• Cathode

Answer 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.