MAGNIFICATION versus RESOLUTION of 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 , ~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 µ m. This is 200 nm. Note: 1 µ m = nm 1 µ m = m 1 nm = 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 = 0.94 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 = 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 = 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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