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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.
END OF FILE