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Glass lens | Thick lens


A round glass piece which is thick at the centre and thin at the periphery is picture of the convex lens we have in our mind. Similarly a round glass piece which is thin at the centre and thick at the periphery is the picture of a concave lens we have in our minds.

In this demonstration we try to redefine this narrow picture of lenses and give a broader perspective of this concept of lens. The use of milky water and smoke makes the path of light visible by scattering the light that makes the demonstration very beautiful.


You need a plastic box with a lid, incense stick, milky water, a 100 ml glass, two laser torches.

  1. Light the incense stick and fill the plastic box with the smoke of the incense stick.
  2. Tie the two laser torches with a rubber band and make them on by pressing their switches. Check whether you can see two parallel beams of light in the smoke filled box.
  3. Fill the glass with milky water and put it in the middle of the smoke box. See that the parallel beams of light get converged when they pass through the glass. The beams of light meet at a point and then continue their journey forward.
  4. Now take out the milky water from the glass and pour it in the plastic box. Add some more milky water to fill the plastic box to half of its capacity.
  5. Fill the empty glass with smoke and put a lid on it.
  6. Hold the empty glass filled with smoke in the milky water in the centre of the box.
  7. Again shine the two parallel beams of light on the glass. See that the parallel beams of light get diverged when they pass through the glass.


Water is optically denser than air. The parallel beams of light go from a rarer medium to a denser medium when they pass from air in the plastic box to the water filled glass. The glass with its round surfaces and filled with water acts as a convex lens. So the beams of light get converged to a point which is focus of the convex glass lens.

When the water is filled inside the box and the glass is kept empty, the parallel light beams go from the denser medium of water in the box to the rarer medium of air inside the glass. This glass with its round surface and filled with air and smoke acts as a concave lens. So the beams of light get diverged when they pass through the glass. These diverged beams seem to come from behind the glass. This point is the virtual focus of the concave glass lens.

So we see that the same glass can act as both a convex as well as concave lens. The type of lens is determined by the medium inside the glass and the medium surrounding it.


You need a glass of water, stiff sheet of paper to make slits, and a screen.

glass lens

Cut two slits on a stiff piece of paper. Make it stand by fixing it over a window on one side of a cardboard box. Remove the opposite side of the box and let sunlight or torchlight fall on the slits to create `rays'. Place a cylindrical tumbler filled with water in the path of the rays. You will find that the direction of the rays change after refraction. We use the refraction at a curved surface to make lenses.


  1. Refraction at Plane and Spherical Surfaces
  2. Thin lenses
  3. Convex lens
  4. Experiments

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