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The flashcards below were created by user
DesLee26
on FreezingBlue Flashcards.
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Two types
- a. TEM: form images using electrons that are trasmittted through a specimen
- b. SEM: utilize electrons that have bounced off the surface of the specimen
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I. TEM
a. greater __than LM
b. great resolving power of EM is due to __
- resolution
- wave properties of electrons
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limit of resolution of a microscope is what?
directly proportional to the wavelength of the illuminating light: the longer the wavelength, the worse the resolution
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Wavelength of an electron varies with __, which in turn depends on the __ applied in the microscope.
- with speed at which its traveling
- accelerating voltage
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wavelength equation
- λ = √(150/V)
- where λ is the wavelength in angstroms and V is the accelerating voltage in volts
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a. standard TEMs have voltage from __. Their resolution is __due to serious __, which requires that the numerical aperture of the lens be made very small
i. practical limit of resolution is__; the actual limit is __
- 10,000 to 100,000 V.
- two orders less than its theoretical limit
- spherical aberration of electron-focusing lenses
- 3-5 angstroms
- 10-15
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Structure
a. __ through which the __ passes and a __containing a __ that electronically controls the operation in the column
- tall, hollow, cylindrical column
- electron beam
- console
- panel of dials
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i. top of the column contains __(a tungsten wire filament that is heated to provide a source of electrons)
1. electrons are __ and __ by the __ applied between the __
- cathode
- drawn from the hot filament
- accelerated as a fine beam
- high voltage
- cathode and anode
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1. __is pumped out of the column prior to operation, producing a __
through which the electrons travel
a. if air not removed, electrons would be __
- air
- vacuum
- prematurely scattered by collision with gas molecules
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a. a beam of __ can be focused by __, which are located in the wall of the column
i. strength of magnets controlled by __provided them
b. __ placed between the __ and the specimen and the electron beam is focused on the specimen
- negatively charged electrons
- electromagnetic lenses
- current
- condenser lenses
- electron source
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a. thin, metal grid to support specimen
b. One __ that provides entire range of __ delivered by the instrument
i. image from __ is object for additional lens system; image provided only magnified about __times
ii. by altering __, magnifications can vary from 1000 to 250,000 times
- objective lens
- magnification
- objective lens
- 100
- current
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i. electrons that have passed through the specimen are brought to focus on a __ situated at the bottom of the column
1. electrons striking the screen excite a coating of __--> the image
- phosphorescent screen
- fluorescent crystalsà
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a. Image formation depends on __by parts of the specimen
i. __ by a part of the specimen is proportional to the __ of the atoms that make up the specimen
- differential scattering of electrons
- scattering of electrons
- size of the nuclei
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1. because the insoluble material of cells contains atoms with low atomic numbers, biological material cannot really __
a. therefore, to scatter and get better contrast, tissues are __, where they go into organelles
- scatter electrons
- fixed and stained with heavy metals
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i. the cell parts that bind the most metal atoms allow passage of the __
1. fewer electrons that are focused= __
a. photographs are taken by __ to produce an image
- least number of electrons
- the darker the spot
- putting photographic plate beneath screenà
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