A steel wire is supported vertically frm a rigid support . When loaded with weight in air, it extends by l n when it is immersed completely in water extension is reduced to l .relative density of maternal of the weight???

Many elevators and garage-door systems use a beam of light and a photoelectric device known as a photocell as a safety feature. As long as the beam of light strikes the photocell, the photoelectric effect generates enough ejected electrons to produce a detectable electric current. When the light beam is blocked (by a person) the electric current is interrupted and the doors are signaled to open.
Photocells are also the basic unit in the solar energy panels that convert some of the energy in sunlight into electrical energy. These panels are able to operate billboards and safety lights in remote areas far from power lines. Large solar panels (240 ft.) will power the International Space Station.
Television Camera Tubes
Light-Activated Counters
Automatic Doors
Intrusion Alarms
Turn on Street Lights at Dawn
Solar calculators.