Objective: To study total internal
reflection Equipment: A dettol bottle, some common salt, a laser
torch Introduction: When light goes from a denser medium to a
rarer medium, and the angle of incidence is larger than a
critical value, called critical angle, whole of the light
will get reflected at the surface. If the angle of incidence
is smaller than the critical angle, part of the light is
reflected and part of it is refracted. In this inexpensive
demo we show the light paths as the medium changes and hence
all phenomena on refraction can be visually seen. It is very
simple and has been widely appreciated wherever we have
shown it.
Procedure: Take a dettol bottle and fill water in it up
to say three fourths of its height. Put some common salt in it.
Tighten the cap. Put on a laser torch and send light from
outside into the water obliquely. The front end of the torch
should be in contact with the thinner side of the bottle at a
height covered by water, and the incline of the torch should be
adjusted by tilting the torch with your hand. Adjust the
orientation so that the light goes parallel to the flat faces of
the bottle, but at an angle to the surface of the water.
You should be able to see the path of the laser beam in the
water. If it is not clearly visible, invert the bottle for a
second. This will bring the salt sitting at the bottom in the
whole water and make the path visible. Slowly change the
orientation of the laser torch and you very clearly see the
total internal reflection. If you make the torch closer to
vertical, thus decreasing the angle of incidence, the reflected
beam will loose its intensity. A spot will form on the opposite
wall of the bottle above the water surface. This tells that a
part of the beam is getting transmitted.
Now you put smoke of an incense stick into the bottle. It will
collect above the water surface. Now when you send laser beam
and if it gets transmitted to air side the transmitted beam will
also be clearly seen, though it will be hazy. So for angles less
than the critical angle you see both reflected and transmitted
beams, and for angles larger than the critical angle, only
reflected beam is observed. Discussion: Dettol bottle suites this demo because the
walls are flat. This demo can be extended to informal lab, by
encouraging students to adjust the torch for grazing angle
refraction and measuring the inclination angle from vertical
which is also the normal to the surface. This angle is the
critical angle. When to do: Class 10 when you are teaching total internal
reflection Instrument cost and availability: Laser torch
