What do you mean by efficiency of a machine?​

Efficiency in general means How well an object does with respect to its designated purpose for example a battery with Voltage of 10 volts won't exert 10 volts but slightly less due to internal resistance let's say 8V so efficiency will be 8/10 x100 = 80%

Same thing with a machine/engine If an engine is designed to produce 150 horsepower it won't give you the full energy due to part of the energy being dissipated in form of heat due to friction so let's say 145 hp is produced which is 96% efficiency

convex lenses are thicker at the middle. rays of light that pass through the lens are brought closer together (they converge). a convex lens is a converging lens. when parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.

when heated, the valance electrons increased their speed and transferred the heat from electron to electron.   i just took the test.