Activity Based Science Learning >Underwater optics

Objective: To study the effect of water surrounding a lens
Equipment: Two laser torches, a long trough (say 20 cm), a convex lens of focal length about 10 cm or less
Introduction: The focal length of a lens depends on the curvatures of the two lens surfaces and on the refractive indices of the lens and the surrounding medium. Thus if the lens is kept inside water, its focal length will increase.

Procedure: Take a convex lens and measure its focal length (roughly) by imaging distant objects on a wall or a paper. Put two laser torches parallel to each other on a base and fix them. Connect to power. Take care you do not stare into the laser. You get parallel beams of red light. Put water in a long trough and put a few drops of soap solution in it. Position the laser base so that light goes in water along the length of the trough. The two beams will be clearly visible from the top. If it is not so, you have most likely put more soap solution than required. Adjust it again. The beams are quite narrow and you can treat each of these as a ray of light.
Put a convex lens inside the water so that the axis is parallel to the beams of light. The two beams will bend towards each other and will intersect. Measure the distance of the lens from the place where the two beams intersect. This is the focal length of the lens in water.
Check that the focal length in water is much larger than that in air.
Discussion: The basic theme is to make the rays visible in water. Once that tool is there you can design many experiments and perform them.
Supervise students and repeatedly remind them that laser light should not fall directly into eyes.
When to do: Students may be introduced to this activity generally after they have done refraction at spherical surfaces (Class 11-12). However class 10 students may also do it and learn from experiments that putting a lens in water increases its focal length. They learn the theory later.
Instrument cost and availability: Laser torche Rs 30, convex lens available in scientific stores and may also be found in gift shops with the name magnifying glass. The school laboratory will definitely have it.







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