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LENSES

 
Objective Aperture (ii)
    G: 12 of 14
 

By inserting the aperture or tilting the beam, different types of images can be formed.

The most common conditions are:

  • No aperture - the diffraction pattern is centered on the optical axis.
  • Aperture is centred on the optical axis.
  • Aperture displaced, selecting a diffracted beam.
  • Beam is tilted so that the diffracted beam is on the optical axis.

 

Use the controls to try the different combinations, noting the effect on the appearance of the diffraction pattern.

The objective aperture is to be used to allow the undeflected beam to pass but to stop electrons diffracted from the 200 planes of copper. What is the maximum size the aperture could be?
(Lattice parameter of copper =0.36 nm, wavelength of electrons = 3.7 pm, focal length of objective lens = 1 mm.)

 
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