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.
