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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: Object is at infinity
Object is at infinity
Case 2: Object is between infinity and the optical centre of the lens
Object is 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.