A diopter
is a unit of refractive power that is equal to the reciprocal of
the focal length (in meters) of a given lens.
Cameras, f stop number = f/D.
Antonie van Leeuwenhoek (1632 - 1723), by
grinding down spherical lenses, produced double-convex lenses
of such short focal length that he could achieve
magnifications of 200 or 300 times.
COMPOUND MICROSCOPE
GALILEO'S (TERRESTRIAL) TELESCOPE
Galileo's original telescope used a convex
objective and a concave secondary lens. The motivation for
this was to produce an erect, virtual image, when the
telescope was used to look at terrestrial objects.
Kepler-type Astronomical
Telescope!
Refracting telescopes used for astronomy
have two converging lenses. [First one made by Kepler in about
1611.] This produces far greater magnification, but the image
is upside down, which does not bother astronomers in any way.
NEWTON'S REFLECTING TELESCOPE
Newton's idea, to use a spherical or
parabolic concave mirror, opened the way to produce
astronomical telescopes of incredible light-gathering power.
Mirrors could be supported from the back, and so could be of
enormous size, whereas lenses of refracting telescopes had to
be supported at their edges, and this mechanical limitation
prevented the lens from having great size or great weight.
MULTIPLE MIRROR TELESCOPES, THE WAVE OF
THE FUTURE.
Resolution of optical instruments is
limited by wave diffraction. In effect, sinθr =
1.22 λ/a, where a is the diameter of the aperture that is
needed to resolve two sources at an angular separation of θr.
