Resolution & LM Types (L4)

Question 1

Resolution?

Answer 1

= the ability of an optical system (eg, microscope) to show detail in the specimen/subject being displayed/viewed.

Question 2

Resolving power (R)?

Answer 2

= the ability of a optical system (eg a microscope) to form distinguishable images of objects separated by small angular distances.

Question 3

Diffraction?

Answer 3

= the bending of light as it passes an obstacle or through an aperture (Kindly get Sir’s definition of this from the Word document).

Question 4

Resolving power attributes? (2)

Answer 4

• Because of diffraction of radiation by the circular aperture of the lens, a point source is not imaged as a point.

• Diffraction disks should be separate to produce a clear (resolved) image.

Question 5

Factors affecting Resolving power? (2)

Answer 5

• Numerical Aperture (N.A.) of the objective lens of the microscope.

• Wavelength (λ) of the radiation used to form the image.

Question 6

Numerical Aperture (N.A.)?

Answer 6

= measure of its ability to gather light & resolve fine specimen detail at a fixed object distance.

Question 7

Equation of N.A.?

Answer 7

N.A. objective = sinα × R.I

Question 8

R.I?

Answer 8

= Refractive Index.

Question 9

α in N.A. equation attributes? (2)

Answer 9

• Maximum α is 90⁰ but is impossible in practice.

• Therefore, α < 90⁰ and sinα < 1.0

Question 10

Practical maximum values for LM objectives:

R.I. for Dry specimens?

Answer 10

1.0

Question 11

Practical maximum values for LM objectives:

N.A. for Dry specimens?

Answer 11

0.95

Question 12

Practical maximum values for LM objectives:

R.I. for Water immersion specimens?

Answer 12

1.33

Question 13

Practical maximum values for LM objectives:

N.A. for Water immersion specimens?

Answer 13

1.25

Question 14

Practical maximum values for LM objectives:

R.I. for Oil immersion specimens?

Answer 14

1.515

Question 15

Practical maximum values for LM objectives:

N.A. for Oil immersion specimens?

Answer 15

1.4–1.5

Question 16

Resolving power (λ of radiation):

What is the λ (nm) of Red light?

Answer 16

700nm.

Question 17

Resolving power (λ of radiation):

What is the λ (nm) of Green light?

Answer 17

550nm.

Question 18

Resolving power (λ of radiation):

What is the λ (nm) of Blue light?

Answer 18

450nm.

Question 19

What is the average imaging wavelength?

Answer 19

550nm.

Question 20

Equation of RP of a microscope?

Answer 20

RP of microscope = 0.61λ / N.A.

Question 21

Relationship between Resolution & N.A.?

Answer 21

High N.A., Better image, Best resolution.

Question 22

Explain the relationship between resolution & N.A.?

Answer 22

A lens with a larger N.A. will be able to resolve smaller features, resulting in a sharper image.

Question 23

Thing to note regarding LM magnification, N.A. & resolution?

Answer 23

Maximum useful magnification for LM is about 1000× the N.A. of the objective in use. If it’s more than this, things look bigger, but you can’t see any more detail.

Question 24

What factors cause the actual RP be less than the theoretical RP?/ Why might the actual RP be less than the theoretical RP? (3)

Answer 24

• Abberations/distortions in the lenses used.

• Specimen thickness.

• Specimen contrast.

Question 25

Aberration?

Answer 25

= deviation of light rays through lenses causing images of objects to be blurred.

Question 26

Types of lens aberrations? (3)

Answer 26

• Spherical aberration.
• Chromatic aberration.
• Field curvature.

Question 27

Spherical aberration?

Answer 27

= when rays from a point in the specimen, that pass through different regions of the lens, focus at different distances resulting in an unsharp image.

Question 28

Spherical aberration attributes?

Answer 28

• Outer parts of a lens don’t bring the same focus as/to? the center part.

Question 29

Chromatic aberration?

Answer 29

= when rays of different wavelengths from a point in the specimen focus at different distances resulting in an unsharp/blurred image.

Question 30

Chromatic aberration attributes?

Answer 30

• Lens cannot bring all the wavelengths….

Question 31

Field curvature?

Answer 31

= when a flat specimen is imaged as curved, so that the centre & edges cannot be focused simultaneously.

Question 32

What does the control of aberrations depend on? (2)

Answer 32

• The use of different types of glass with differing refractive indices & dispersions.

• The different shapes of concave/convex lenses.

Question 33

LM objective designations?

Answer 33

= assume that the objective is assumed dry unless stated otherwise.

Question 34

How do we control aberrations?

Answer 34

By using suitable combinations of positive & negative lenses.

Question 35

Control of aberrations attributes? (2)

Answer 35

• Many aberrations can be controlled in similar manners.

• Flint fixed rays to one point.

Question 36

Cover slip attributes? (4)

Answer 36

• Light rays are refracted at upper surface of cover slip.

• High power objectives are designed to correct for this refraction.

• Wothout cover slip, objective is under corrected, resulting in spherical aberrations & an unsharp image.

• Although exceptions exist, as a rule.of thumb, always use a cover slip with high power objectives.

Question 37

LM objective designation attributes? (2)

Answer 37

• Mechanical tube length.
• Infinity tube length objectives.

Question 38

Mechanical tube length?

Answer 38

= effective optimal distance between the bottom of the nosepiece (where objective screen is) & the top of the eyepiece tube.

Question 39

Why must the Mechanical tube length be correct?

Answer 39

It’s because it affects ¹magnification & ²image quality.

Question 40

Stereobinocular vs Compound microscopes in terms of image? (4)

Answer 40

● Stereobinocular
• erecting prisms.
• image is right way up & right way down.

● Compound
• no erecting prisms.
• image is inverted.