Theory:

Let us look at an activity to understand the concept of image formation by a concave lens.
  • Take a concave lens and fix it up on a lens stand.
  • On one side of the lens, place a lit candle.
  • Examine the image through the lens from the other side. If at all possible, get the image on a screen. If not, look at the image through the lens directly.
  • Make a note of the image's nature, relative size, and approximate location.
  • Move the candle away from the lens. Note the change in the size of the image. When the candle is placed too far away from the lens, what happens to the image size?
Case 1: At infinity
 
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Object at infinity
 
Case 2: Between infinity and optical centre of the lens
 
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Object between infinity and optical centre
 
A concave lens will always give a virtual, erect and diminished image, irrespective of the position of the object. The summary of the above activity is given in the below table.
 
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