An objective in
optics is the
lens or mirror in a
microscope,
telescope,
camera or other
optical instrument that receives the first
light rays from the object being observed. The objective is also called the object lens, object glass, and objective glass.Microscope objectives are typically designed to be
parfocal, which means that when one changes from one lens to another on a microscope, the sample stays in
focus. Microscope objectives are characterized by two parameters, namely,
magnification and
numerical aperture. The former typically ranges from 5X to 100X while the latter ranges from 0.14 to 0.7, corresponding to
focal lengths of about 40 to 2 mm, respectively. For high magnification applications, oil-immersion objective has to be used. The objective is specially designed and has to be immersed in refractive index matching oil (an
index-matching material).
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