Theory:

Let us look at an activity to understand the concept 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 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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At infinity
 
A virtual image is formed at the focus when an object is placed at infinity. The size of the image is much smaller than that of the object.
 
Case 2: Between infinity and optical centre of the lens
 
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Beyond c
 
A virtual image is formed between the optical centre and focus of the concave lens when an object is placed at a finite distance from the lens. The image is a fraction of the size of the object.
 
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Image formation
 
However, as the distance between the object and the lens decreases, the distance between the image and the lens decreases. Furthermore, as the distance between the object and the lens decreases, the size of the image formed grows larger. The summary of the above Activity is given in the below table.
 
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