Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Role of bigger objective in a telescope


Bigger objective helps in getting better resolving power of telescope. Magnification and resolving power are different characteristics of a telescope. In this experiment we try to give this feeling.


Two convex lenses of nearly same focal length \(f_o\) but of different apertures, a concave lens of smaller focal length \(f_e\).


  1. Choose a sheet of paper where matter is written in different fonts. Posters with heading, subheading and text will be ok. Fix it on a wall at working height.
  2. At about 30 ft or more, put up the convex lens with bigger aperture in a stand. The axis of the lens should be passing through the paper.
  3. Place the concave lens at a distance \(f_o-f_e\) from the convex lens. In the present case \(f_o\approx20\,\mathrm{cm}\), \(f_e=5\,\mathrm{cm}\) so that separation will be \(\approx 15\,\mahtrm{cm}\). Make sure they are coaxial.
  4. Look through the concave lens. You will see the circular boundary of the convex lens and within that enlarged image of objects beyond the lens system. Adjust the separation between the lenses and lateral positions so that the printed paper comes in view.
  5. Try to read the headings and sub headings. Focus on the matter (heading/sub heading) in which you are just able to differentiate the letters and they are not mixed up. Just concentrate on one such word.
  6. Now replace the convex lens with the other convex lens with smaller aperture. Do the same exercise. Focus on the same word. Are the letters clearly distinguishable?
  7. It is the personal eye conditions that play important role. But in general you will find the smaller aperture of convex lens (object) has somewhat blurred the image.