Physics

Air moves from the region where the air pressure is high to the region where the pressure is low.

Most commonly observed phenomena with light can be explained by waves. But the photoelectric effect suggested a particle nature for light.

1 kW = 1000 W.

The metal ball can be considered to be made up of several layers of thinner ones. On heating each of these layers will increase in radius. As the inner most layer also increases its radius, the volume inside it i.e, the volume of the hollow portion will also increase.