Concepts of Physics

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

How many black rectangles are there?


How many black rectangles are there?


Our eyes have a limited resolving power due to diffraction. This experiment illustrates this in a simple manner.


A plain paper on which closely spaced rectangles are drawn. To make this, draw a rectangle by any graphic program on computer screen. The width of the rectangle may be around 1 cm and length 7-8 cm. Fill the rectangle with black. Copy it and displace it so that there a very thin gap between them. Repeat to draw 4 such rectangles. Now copy the entire 4 rectangles set and displace the whole thing so that the new 4 rectangles set is separated from the first set by about 1 cm.


  1. Hold the paper in your hands so that the rectangles are vertical. Ask your friend to stand at a distance of about 30 ft.
  2. Ask him/her to tell how many black rectangles he/she can see. The expected answer is 2.
  3. Ask the person to slowly come closer. At some distance the person will be able to recognize that there are 8 rectangles. Do it with several of your friends (including yourself) and write the minimum distances needed to resolve the rectangles.
  4. Question: Does the diffraction takes place at the pupil or at the eye lens?