Theory:

Sun is a good natural source of light and heat. Living things on Earth survive with the energy provided by the Sun. Let us look at a simple activity that clearly shows the converging power of a convex lens.
Important!
Perform the following activity in the sunlight. You can look at the image of the sun only when it is reflected on the screen or a wall. But, never focus the sunlight anywhere on your body or look at the sun directly as it will damage your eyes.
  1. A convex lens (for example, a magnifying glass) is held facing the sun.
  2. A sheet of paper is taken, and the sunlight is focussed on it using the lens.
  3. The distance of the lens is adjusted until a sharp, bright spot is formed on the paper.
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Observation
At first, a real image is formed on the paper. But, after some time, the paper begins to burn due to the converging light rays of the Sun.
 
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Now, for the same experiment, a concave lens is used instead of a convex lens.
 
Observation
The bright spot in a diminished manner is formed on the sheet due to diverging nature of the lens.
 
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Like mirrors, lenses also have different nature and size for the image.