Why does the top layer of a liquid appear like a thick sheet when you look at it sideways?
Neerad Thanvi, Rajasthan
Such a thick layer can be observed when the liquid is held within a transparent container and the observer is looking at the top most portion of the liquid from the sides of the transparent container. Here the light waves/rays travelling from different directions strike the boundary or the surface layer that is separating the two media namely the liquid and surrounding air at different angles called angles of incidence. Such light waves will be partially refracted or reflected back into the surrounding medium. If the angle of incidence (angle between the incident ray and the normal drawn to the boundary layer at the point of striking or incidence) is such that the light wave or ray propagates almost parallel to the surface layer it will not be refracted but completely reflected back into the medium and it cannot cross the boundary separating the two media. This phenomenon is called total internal reflection and it happens when the boundary layer is separating two media of different optical densities corresponding to two different refractive indices and the light is travelling from an optically denser medium to a rarer medium (of lower optical density). The angle of incidence above which total internal reflection occurs is called the critical angle and it is a constant for a pair of media. For acrylic glass and air it is 41.8°. At critical angle, the angle of refraction will be 90° and light ray grazes along the boundary layer. In the present case, for the observer the light received by him is mostly the total internally reflected light and the liquid molecules are held firmly together at the surface of the liquid by the surface tension experienced by them. Hence it appears as a thick sheet to the person seeing from the sides.
Dr T. Bhavani, Bengaluru
This week’s questions
How does the train stop when somebody pulls the chain?
Maharajan, Kanyakumari
Why do we see an increased number of younger children wearing spectacle? Is it because of increased use of gadgets?
Nisha, Viluppuram, Tamil nadu
