Fundamentals Of Nanoscience Set 8

Answer: 4Θ

Explaination:
the diffracted beams in the powder method forms a cone, if the bragg’s angle is Θ, then the angle between diffracted and undiffracted beams is 2Θ and the angle of the cone is 4Θ. each set of planes gives its own cone of radiation.

Answer: cones intersects the strip of the films

Explaination:
in the powder method, the cones are detected by a thin strip of film wrapped around the sample, each cone intersects the film as two short arcs which are symmetrical about the two holes in the film(these allow entry and exit of an incident and undiffracted beams).

Answer: exposure time is long

Explaination:
the disadvantage of using debye-scherrer film method is that exposure times are long and that closely spaced arcs are not well resolved. this is because, although the incident beam enters the camera through a pinhole slit and collimator tube, the beam is somewhat divergent and the spread increases in the diffracted beams.

Answer: bent single crystal of quartz

Explaination:
convergent beam is obtained by placing a bent single crystal of quartz or graphite between the x-ray source and the sample. the orientation of this bent crystal is adjusted so that it diffracts the incident beam and converts it from the divergent beam into a convergent one.

Answer: electron beams and magnetic fields

Explaination:
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.

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

Explaination:
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

Answer: number and mass of atoms that lie in the electron path

Explaination:
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.

Answer: virus particles, protein molecules and bacterial flagella

Explaination:
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.

Answer: scanning electron microscope

Explaination:
the scanning electron microscope lacks the resolving power obtainable with the transmission electron

Answer: anode

Explaination:
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.

Answer: size and chemical composition of the irradiated object, number of electrons ejected and on the number of electrons reabsorbed by surrounding

Explaination:
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.

Answer: true

Explaination:
the resolution that can be attained with a standard transmission electron microscope is about two orders of magnitude less than the theoretical value. this is due to spherical aberration of electron-focusing lenses.

Answer: condenser lens

Explaination:
the condenser lens focuses the electron beam on to the specimen, in case of transmission electron microscope. the specimen is supported on the grid holder and placed inside the column.

Answer: differential scattering

Explaination:
in case of the electron microscope, the image formation is based on the differential scattering of the electrons by parts of the specimen. the scattering of electrons is proportional to the size nuclei of the atoms that make up the sample.

Answer: scatter electrons

Explaination:
the insoluble materials of cells contain atoms of low atomic number such as carbon, hydrogen, oxygen and nitrogen. the biological materials therefore have very little intrinsic capability of scattering the electrons.

Answer: fixative

Explaination:
glutaraldehyde and osmium tetroxide are common fixatives used in the transmission electron microscopy for the

Answer: heavy metal

Explaination:
osmium is a heavy metal that reacts with fatty acids leading to the preservation of membranes. osmium tetroxide is used as a fixative in transmission electron microscopy.

Answer: heavy metal soutions

Explaination:
the tissue is stained by floating on drops of uranyl acetate and lead citrate (heavy metal solutions). these solutions when bound to the specimen, provide the density required to scatter the electron beam.

Answer: isolated particles

Explaination:
shadow casting is a technique of viewing isolated particles. the particles are made to cast shadows after their placement in sealed chambers. the chamber contains a filament of carbon and a heavy metal.

Answer: one, non-uniform

Explaination:
surface is more than one atomic layer deep and is a region of non-uniform atomic potentials. the outermost layer of atoms is called a surface.

Answer: true

Explaination:
surface analysis can provide information that classic methods like microscopic cannot. it is better than reflectivity, adsorption isotherms, etc.

Answer: reflected beam

Explaination:
when a beam is focussed on a surface, one beam enters the material and a second beam is reflected. the reflected beam is analysed.

Answer: auger spectroscopy

Explaination:
auger spectroscopy is a type of electron spectroscopy. electron spectroscopy for chemical analysis is also a type of electron spectroscopy.

Answer: false

Explaination:
surface analysis can provide chemical information. electron and ion spectroscopic techniques are types of surface analysis.

Answer: electron spectroscopy for chemical analysis

Explaination:
electron spectroscopy for chemical analysis is also known as x-ray photoelectron spectroscopy. it is an effective technique for detecting the elements.