Bài giảng Vật lý điện tử__emoptics | Môn Vật lý điện tử | Trường Đại học Bách Khoa Hà Nội
Bài giảng Vật lý điện tử__emoptics | Môn Vật lý điện tử | Trường Đại học Bách Khoa Hà Nội. Tài liệu gồm 38 trang giúp bạn tham khảo ôn tập đạt kết quả cao trong kỳ thi sắp tới. Mời bạn đọc đón xem.
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Electron Optics cuu duong than cong . com CuuDuongThanCong.com
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Transmission Electron Microscope
Optical instrument in that it uses a lens to form an image
Scanning Electron Microscope
Not an optical instrument (no image
forming lens) but uses electron optics.
Probe forming-Signal detecting cuu duong than cong . com device. CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics Refraction, or bending of a beam of illumination is caused when the wavelength enters a medium of a different optical cuu duong than cong . com density. CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics
In light optics this is accomplished when a
wavelength of light moves from ai
cuu duong than cong . com r into glass
In EM there is only a vacuum with an optical
density of 1.0 whereas glass is much higher CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics
In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the
same effect of causing the beam of illumination to bend. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics
In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the
same effect of causing the beam of illumination to bend.
Electromagnetic Force or Electrostati cuu duong than cong . com c Force CuuDuongThanCong.com
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Classical optics: The refractive index changes
abruptly at a surface and is constant between the
surfaces. The refraction of light at surfaces separating
media of different refractive indices makes it possible
to construct imaging lenses. Glass surfaces can be shaped.
2) Electron optics: Here, changes in the refractive
index are gradual so rays are continuous curves rather
than broken straight lines. Refraction of electrons
must be accomplished by fields in space around
charged electrodes or solenoids, and thes
cuu duong than cong . com e fields can
assume only certain distributions consistent with field theory. CuuDuongThanCong.com
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Converging (positive) lens: bends rays toward the
axis. It has a positive focal length. Forms a real
inverted image of an object placed to the left of the
first focal point and an erect virtual image of an
object placed between the first focal point and the lens. cuu duong than cong . com CuuDuongThanCong.com
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Diverging (negative) lens: bends the light rays
away from the axis. It has a negative focal length.
An object placed anywhere to the left of a diverging
lens results in an erect virtual image. It is not
possible to construct a negative magnetic lens
although negative electrostatic lenses can be made cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics Electrostatic lens cuu duong than cong . com
Must have very clean and high vacuum
environment to avoid arcing across plates CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Optics Electrostatic lens Converging Lens cuu duong than cong . com Diverging Lens CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electromagnetic Lens
Passing a current through a single c cuu duong than cong . com oil of
wire will produce a strong magnetic field in the center of the coil CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electromagnetic Lens cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electromagnetic Lens Pole Pieces of iron cuu duong than cong . com Concentrate lines of Magnetic force CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electromagnetic Lens cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electromagnetic Lens cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt The two force vectors, one in the direction of the electron trajectory and the other perpendicular to it, causes the electrons to move through
cuu duong than cong . com the magnetic field in a helical manner. CuuDuongThanCong.com
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The strength of the magnetic field is determined by the number of
wraps of the wire and the amount of current passing through the cuu duong than cong . com
wire. A value of zero current (weak lens) would have an infinitely
long focal length while a large amount of current (strong lens)
would have a short focal length. CuuDuongThanCong.com
https://fb.com/tailieudientucntt A TEM image is made up of nonscattered electrons (which strike the screen) and scattered electrons which do not and therefore appear as a dark area cuu duong than cong . com on the screen CuuDuongThanCong.com
https://fb.com/tailieudientucntt Some of the scattered electrons will only be partially scattered and thus will reach the screen in an inappropriate position giving a false signal and thus contributing to a degradation of the image. These forward scattered electrons can be eliminated cuu duong than cong . com by placing an aperture beneath the specimen. CuuDuongThanCong.com
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