3. Astigmatism : The curvature of cornea               Telescope
     becomes irregular and image is not clear.
     Cylindrical lens is used for correcting this         It is used to see the magnified images of
     defect.                                               the very distance objects.
4. Presbyopia          :     The     power     of         Generally, there are two types of
     accommodation of the eye lens decrease in             telescopes
     the old age. Therefore neither near nor              Astronomical telescope : the objective
     distant objects are clearly seen. Presbyopia          lens is a convex lens of large focal length,
     can be removed by using bifocal lenses.               but eye-piece is a convex lens of short
5. Cataract : An opaque, white membrane is                 focal length.
     developed on cornea due to which a                   Galilean telescope : the objective lens is a
     person loses power of vision partially or             convex lens of large focal length, but the
     completely. This defect can be removed                eye-piece is a concave lens of short focal
     by removing this membrane through                     length.
     surgery.                                             The first recognized practical telescopes
6. Glaucoma : The eye produces a clear                     were invented in the Netherlands at the
     fluid (aqueous humor) that fills the space            start of the 17th century, using glass
     between the cornea and the iris. This fluid           lenses. They found use in terrestrial
     filters out through a complex drainage                applications and astronomy.
     system.                                              There are a wide range of telescopes that
                                                           work in different ways based on what you
Simple Microscope                                          need to use them for. Different types of
                                                           telescopes include optical telescopes,
    It (magnifying glass) is simply a single
                                                           refracting telescopes, reflecting telescopes,
     biconvex lens of a short focal length.
                                                           radio telescopes, x-ray telescopes, gamma-
    It is used to see the magnified images of
                                                           ray telescopes and high energy particle
     very small objects.
    Magnifying power = 1 + (D/f)
                                                       Facts about Telescope
Compound Microscope
                                                          The famous astronomer Galileo Galilei
It consists of two convex lenses.
                                                           used a refracting telescope.
It is used to view smaller specimens such as              Isaac Newton invented the reflecting
cell structures which cannot be seen at lower              telescope which was much more powerful
levels of magnification.                                   than the refracting telescope that Galileo
                    v0  D                               The Hubble telescope sends 120 gigabytes
 MagnifyingPower      1  
                    u0    fe                             of data to Earth each week and is powered
                                                           by energy from the Sun using solar panels.
                                                          The James Webb Space Telescope is the
 v0 = distance of image from the Objective                 successor to the Hubble telescope and is
                                                           due to be launched in 2013.
u0 = distance of object from the objective