if a big stone and a small stone is dropped simultaneously from the certain height. they fall together why??

Because it takes more force to accelerate a mass, and gravity exerts more force on a greater mass.  So every object winds up with the same acceleration.  

And THAT, of course, says that all objects in the same gravity will pick up the same speed in the same time, and fall the same distance in the same time.

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.

measuring things 1 litre of water