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either transmitted in their original direction or are deflected by some change in its direction (refraction). Thus light passing through water retains the original photon-energy. Hence it is transparent. 10) Surface tension:Water surface, in contact with air exhibits tension, called surface tension. Under its influence, the surface of water tends to contract, until it ocupies the minimum area, consistent with its fixed boundaries. It takes the shape of a sphere, becuase sphere represents the smallest area for a given volume of liquid. Due to the influence of surface tension, drops of water always tend to take the shape of a sphere. Surface tension exists at the interface of water and air, as well as also at the interface of water and its container wall. The value of surface tension of water adjoining ambient air is 7.28 dyn/cm at 20° C. It decreases with the increase of temperature. Water bubbles have double walled layers of surfaces with liquid water filled into it. 11) Potential Energy We know that water flows from a higher altitude to a lower level. This property is responsible for the flow of rivers from water-sources at an altitude to the oceans or seas. These rivers may be flowing on the earth surface or may be flowing under the ground. The potential energy of water at an altitude is converted into kinetic energy of its flow. This conversion is either wasted in flow energy or can be used in moving turbines to generate power. 12) Ocean currents: Another important property of water is that it expands, and becomes lighter on heating. This property is responsible for creating Ocean Rivers, called ocean-currents. In a sense they are similar to surface rivers. There are two types of currents - hot water and cold water currents. They are generated by differential heating of ocean water at different locations by the sun. As the surface near equator becomes warmer due to the sun, it starts moving northward or southward, causing convection currents in the water mass, just like the water being heated in a pot on a burner. This induces oceanic currents. If near a continent, it is moving northward in norhtern hemisphere, a southward counter current will follow on the opposite side of the land
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