Images Formed By Lenses: Concave And Convex Lens

In our school laboratory we might have seen magnifying glasses. It is used to observe body parts of earth worms and cockroaches. Sometimes for fun, we might have tried studying books through it. We find that everything which we see through a magnifying glass appears bigger than its actual size. This magnifying glass is actually made up of lenses. They are used in spectacles, microscope and telescopes. Ever tried touching the lenses. You will find that some lenses are thicker in the middle than at the edges and are known as convex lens. While some of them are thicker at edges than at the middle, these are known as concave lens.

  1. Convex lens: Now try finding a convex lens and place it in the path of sun-rays. Also place a paper below the lens. Adjust the distance between the paper and the lens until you get a bright spot on the paper. Keep the lens at the same position. You will observe that the paper starts burning at that bright spot. This observation helps us conclude that rays passing through convex lens converge at one point, thus concentrating the sunlight at one particular spot, and hence it is known as converging lens.

Convex lens

If we perform another experiment keeping the candle, convex lens and screen parallel to each other. Change the distance between the lens and the candle every time and notice the image. You will generally find inverted and real images.


  1. Concave lens: If we replace convex lens with concave lens in the above experiment of sunlight and paper. We will find that the light gets scattered on the paper after passing through the concave lens. It shows that the rays diverge when passed through such lens. Hence lenses of such type are known as diverging lens. When convex lens is replaced by concave lens in the second experiment, changing the distance between candle and the concave lens always gives us erect and virtual images.

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Practise This Question

Which mirror or lens can form a virtual, erect and highly diminished image of object?