Compound light microscope: A compound light microscope often contains four objective lenses: a) Scanning lens (4X) b) Low-power lens (10X) c) High-power lens (40 X) d) Oil-immersion lens (100 X)
Magnification: With an ocular lens that magnifies an object about 10 times, the total magnifications possible will be: 40 X with the scanning lens, 100 X with the low-power lens, 400 X with the high-power lens, 1000 X with the oil-immersion lens
Parfocal: Most microscopes are parfocal. This term means that the microscope remains in focus when one switches from one objective to the next objective.
Resolution of the microscope: The ability to see clearly two items as separate objects under the microscope is called the resolution of the microscope. The resolution is determined in part by the wavelength of the light used for observing. Visible light has a wavelength of about 550 nm, while ultraviolet light has a wavelength of about 400 nm or less. The resolution of a microscope increases as the wavelength decreases, so ultraviolet light allows one to detect objects not seen with visible light.
Resolving power of a lens: The resolving power of a lens refers to the size of the smallest object that can be seen with that lens. The resolving power is based on the wavelength of the light used and the numerical aperture of the lens.
Numerical aperture: The numerical aperture (NA) refers to the widest cone of light that can enter the lens; the NA is engraved on the side of the objective lens.