Question

By a surface of a liquid we mean
(a) a geometrical plane like x = 0
(b) all molecules exposed to the atmosphere
(c) a layer of thickness of the order of 10−8m
(d) a layer of thickness of the order of 10−4m

Answer 1

Answer:

(d) a layer of thickness of the order of 10-4m.

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Answer 2

We mean a layer of thickness of the order of $10^{-8} m.$

Explanation:

• In a liquid, the surface tension is effective from the open surface to a $10^{-15}$molecular depth.  
• Therefore, in the surface tension’s context, the liquid’s surface is meant by this highly thicker layer. This comes to $10^{-8} m$ order.
• A free surface can be defined as the fluid's surface that is subject to the shear stress of zero parallel. It is just like to interface between two fluids that are homogenous.
• For instance, the air and liquid water in the atmosphere of the earth.