<P> In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image . Objectives can be a single lens or mirror, or combinations of several optical elements . They are used in microscopes, telescopes, cameras, slide projectors, CD players and many other optical instruments . Objectives are also called object lenses, object glasses, or objective glasses . </P> <P> The objective lens of a microscope is the one at the bottom near the sample . At its simplest, it is a very high - powered magnifying glass, with very short focal length . This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube . The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample . </P> <P> Microscope objectives are characterized by two parameters: magnification and numerical aperture . The magnification typically ranges from 4 × to 100 × . It is combined with the magnification of the eyepiece to determine the overall magnification of the microscope; a 4 × objective with a 10 × eyepiece produces an image that is 40 times the size of the object . Numerical aperture for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively . </P>

What do the objectives do on a microscope