MAGNIFICATIONversus

RESOLUTIONof a

Microscope

GEOL 3213,

Micropaleontology

Low-Power Dissecting Light Microscopy

• Low to moderate resolution is usually adequate for forams, ostracodes, and other microfossils around a tenth to a few mm in size.

• Some additional information with moderate enlargement

Transmitted Light, Polarizing Microscopy

• Whole specimen mounts

• Low-power objective used with NA = 0.10

• Magnification does not seem to improve visibility, so resolution is low

Transmitted Light, Polarizing Microscopy

• Higher power objective used with a larger NA

• More details visible with magnification, so the resolution is high.

Magnifying Low-Resolution Microscopy• If resolution is low =NA, for example, is low• “Empty magnification” = no additional information• Just a larger fuzzy image of a fuzzy image

SEM• SEM = Scanning electron microscopy• Resolution is about 10 times the best

light microscope

Magnification

• Ratio of apparent enlarged image size to object viewed

apparent size

= magnification

actual size

• In terms of lenses:

(objective lens power) X (ocular lens power) = magnification

For a light microscope, maximum useful magnification, assuming a NA for a very good lens to be 1.4-1.5,

~1000 X 1.4 = 1400 times• Usually highest magnification is ~1000X.• “Empty magnification” above this because a fuzzy

image enlarged is just a larger fuzzy image

Stopped Here on Friday

Resolution (r)• Minimum distance apart that 2 points can be distinguished• Smaller the value, the better is the resolution• Maximum r for a light microscope is about 0.2 µm. This is 200 nm. Note: 1 µm = 1 000 nm

1 µm = 10-6 m 1 nm = 10-9 m

• Abbe’s relationship:0.61

r =n sin

• n sin is also called the numerical aperture (NA) = wavelength or average wavelength of light used• n is the refractive index of medium between objective lens and

specimen. n is 1.0 for air (& about 1.4 for immersion oil) is the aperture angle of lens. Good lenses have larger angles to

accept more light. For a good lens it is about 70 degrees.

Resolution (r)• For a very good lens:

NA = n sin = (1) sin 70° = 1 x 0.9397 = 0.94• With white light, average = ~550 nm, so for a good lens:

r = (0.61 x 550) / (0.94) = 360 nm = 0.36 um

• With a good lens & blue light ( = ~450 nm):

r = (0.61 x 450) / (0.94) = 292 nm = 0.292 um = ~0.3 um

• Repeat above with oil-immersion lens (n = 1.4):

r = (0.61 x 450) / (1.4 x 0.94) = 209 nm = 0.209 um = ~0.2 um

• If UV ( = ~350 nm) used, then r = ~ 0.1 um, but must photo.

Homework #1

• Solve the numerical problems on the handout sheet.

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