Electromagnetic Lenses

Question 1

Define resolution

Answer 1

Distance that two object can be placed apart and still be seen as separate entities

Question 2

Why does the SEM employ electromagnetic lenses instead of glass lenses?

Answer 2

Glass or plastic lens will stop electrons so they have no value in an electron microscope
Since electrons are charged particles they can be bent in a magnetic fieldinstead

Question 3

What is the function of the first condenser lens (C1)?

Answer 3

Begins demagnification (decreases spot size) of focused spot of electrons formed by the electron gun

Question 4

What is spherical aberration? How can it be corrected?

Answer 4

Spherical aberration occurs when electrons along the beam axis are deflected less than electrons passing through beam periphery, results in more than one focal point
May be reduced by using an aperture to eliminate some of the peripheral rays

Question 5

What is working distance?

What happens to depth of field and resolution as working distance is increased?

Answer 5

Working distance:
Distance between the bottom of the objective lens and the specimen
As working distance is increased, depth of field is increased, resolution is decreased