The Use Of Lenses in Optical Dvices

 The Differences between Microscope and Telescope
                  The Compound Microscope

           

  -We use it to view very small objects.

  -Microscope consists of two convex lenses:

      a)The objective 

     -the lens which is nearest to object.

     -a powerful convex lens with a short focal length.

     -focal length of the objective is fo.

      b)Eyepiece

     -the lens which is nearest to the eye.

     -eyepiece has a longer focal length than the objective.

     -focal length of the eyepiece is fe.

                          The Telescope

                                        

 

    -Astronomical telescope consists of two convex lenses:

        a)The objective.

        b)The eyepiece.

    -The objective is a convex lens with a wide aperture and long focal length to  enable the telescope collect as much light as possible. 

    -Eyepiece is a convex lens of short focal length.

    -The picture shows three actual rays coming from the top of a distant object.Since the object is far away,the rays are assumed to be parallel when incident on objective lens.

    -The rays at point I will emerge parallel from the eyepiece and appear to come from the top of final image at infinity.

    -The total distance between the objective and the eyepiece is = fo+fe

    -The magnifying power of the telescope :

                     M=fo/fe

    -If the power of eyepiece is increasing,the power of objective is reducing,it can give a high magnification.

   

Linear Magnification

    Linear magnification is:
        the ratio of the size of the image to the size of the object.
    Linear magnification, m = size of image/size of object

Image Formed By Lenses

                                   Ray diagram

      1)-Virtual                    2)-Virtual
          -Upright                     -Upright
          -Magnified                -Magnified
      3)-Real                        4)-Real
         -Inverted                     -Inverted
         -Magnified                   -Same size 
      5)-Real                         6)-Real
         -Inverted                      -Inverted
         -Diminished                  -Diminished


                           Concave Lens                                                                                                                                               

 

           -Always virtual

           -Upright

           -Diminished

Power Of A Lens

The power of a lens is:
            a measure of the ability of the lans to converge or diverge rays of light
               Formula:   1/Focal length(m)  or  P=1/f                      *f is the local length and measure in metres.          
           --The u nit of the power of lens is dioptres or D.
           --The focal length is inversely propotional to power of lens.
           --If focal length is shorter,power of lens is higher.
           --Thick lens has shorter focal length and higher power.
       a)thick lens

      
     b)thin lens