Question

A thread is slightly loose to a wire frame as in the figure and the frame is dipped into a soap solution and taken out. The frame is completely covered with the film. When portion A is punctured with a pin, the thread…….

(a) Becomes concave towards A.
(b) Becomes convex towards A.
(c) Either (a) or (b) depending on the size of A with respect to B.
(d) Remain in the initial position.

Answer 1

A soap film covers the whole frame and it is divided into two sections. When puncturing section A, with a pin the thread can be either in concave or in convex shapes or remains in the initial position. The principle of the surface tension has to be concluded here to get the idea of the shape of the thread.

Complete step-by-step solution:
The given figure describes the change in the surface tension at the boundary of the two parts A and B. The wireframe is poured into a soap solution and when it is taken out of the solution it becomes covered by a film layer of soap. The soap film consists of two sections A and B separated by the thread. The thread is tied to the frame. It is shown in the figure shown below.

At the boundary of the two portions A and B, the surface tension of the film is balanced. But when portion A got punctured with a pin, the surface tension will be directed towards section B. This results in the movement of the thread towards B.
Hence, the correct answer is the thread becomes concave towards A.

Note: A clear curved piece of material used to bend the rays of light to form an image is called a lens.
The soap film constitutes a lens.
A concave lens has at least one spherical surface curved inward.
If two surfaces that make up the lens are curved outward which a lens is said to be convex.