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Directions : (Q. No. 6 to 10)

Read the following text and answer the following questions on the basis of the same:

Electron Microscope

Electron microscopes use electrons to illuminate a sample, hi Transmission Electron Microscopy (TEM), electrons pass through the sample and illuminate film or a digital camera.

Resolution in microscopy is limited to about half of the wavelength of the illumination source used to image the sample. Using visible light the best resolution that can be achieved by microscopes is about ~200 nm. Louis de Broglie showed that every particle or matter propagates like a wave.

The wavelength of propagating electrons at a given accelerating voltage can be determined by

\[l=\frac{h}{\sqrt{2{{m}_{e}}v}}\]

Thus, the wavelength of electrons is calculated to be 3.88 pm when the microscope is operated at 100 ke V, 2.74 pm at 200 ke V and 2.24 pm at 300 ke V.

However, because the velocities of electrons in an electron microscope reach about 70% the speed of light with an accelerating voltage of 200 ke V, there are relativistic effects on these electrons. Due to this effect, the wavelength at 100 ke V, 200 ke V and 300  ke V in electron microscopes is 3,70 pm, 2.51 pm and 1.96 pm, respectively.

Anyhow, the wavelength of electrons is much  smaller than that of photons (2.5 pm at 200 ke V).

Thus if electron wave is used to illuminate the sample, the resolution of an electron microscope theoretically becomes unlimited. Practically, the resolution is limited to ~0.1 nm due to the objective lens system in electron microscopes. Thus, electron microscopy can resolve subcellular structures that could not be visualized using standard fluorescences microscopy.

In electron microscope, electron is used:

A) to charge the sample.

B) to clean the sample.

C) to illuminate the sample.

D) All of the above