numerical aperture for a lens =
= n * SIN(angle alpha)
where:
• n = refractive index for the medium (air, oil, water, etc.) that the light is passing through. For air n = 1.
• Angle alpha is the angle between the vertical axis through the center of the lens and the ray of light reaching the lateral edge of the lens (= the half angle of the "acceptance" for the lens)
Numerical aperture for an objective:
(1) for "dry" lens: 0.05 to 0.95
(2) for oil lens: up to 1.4
(3) written as a decimal number (a) just below the magnification number on the lens and (b) just above the tube length (usually 160 mm) and cover thickness for which the lens is corrected.
f number for lens =
= 1 / (2 * (numerical aperture))
minimum resolved distance in µm =
= (wavelength of light in µm) / (2 * (numerical aperture for lens))
where:
• 100 nanometers = 0.1 µm = 1,000 Angstrom units
• visible spectrum of light ranges from 400 to 700 nm: violet 400-420 nm, blue 424-491 nm, yellow 575-585 nm, orange 585-647 nm, red 647-700 nm
