Lecture 1

Question 1

What is the equation for the numerical aperture?

Answer 1

sin theta

Question 2

What is the equation for resolution?

Answer 2

=1.2 lambda / sin theta

Question 3

What does the electron gun consist of?

Answer 3

A filament, a wehnelt cup and an anode plate

Question 4

Why do we wantt he tip of the filament to be as small as possible?

Answer 4

Electric field = 1/bending rate of tip

Question 5

What are the different types of abberration?

Answer 5

Chromatic

Spherical

Question 6

How does spherical abberation occur?

Answer 6

A good lens should have a spherical surface which flips the spherical wave causing overlapping thus creating a focal point. In reality waves are bent as the lens is not entirely spherical. Beams far off the optic axis have an earlier convergence point

Question 7

What is the plane of least confusion?

Answer 7

The plane in which the disc of focus is the smallest. In EM you always defocus to the plane of least confusion

Question 8

How does chromatic aberration occur?

Answer 8

Due to the differing diffraction rates of different wavelengths. this can be somewhat fixed by using monochromatic light however they still have different work functions.

Question 9

What is astigmatism?

Answer 9

Different curvatures in the lens leads to different focal lines creating a isc of least confusion

Question 10

What can you use to tell how much energy is lost?

Answer 10

Using energy dispersive -ray analysis or parallel electron energy loss spectrometer

Question 11

What is the function of the Wehnelt cup?

Answer 11

It indirectly heats the filament and narrows the beam