why does the rainbow appear as an arch even though water droplets are sperical?

Answer 1

Each drop bends the light by the same angle to make one small spot in the rainbow. It is the combined light from millions of drops at different places that makes up the arc of the bow. The arc is simply the collection of places where the angle from the sun to the drop to your eye is equal to the angle at which light in one drop bends. Different colors bend by slightly different angles. Red light bends a little more than blue light, so all the red spots form an arc a little farther from the Sun and the blue spots form an arc a little closer to the sun.

Answer 2

Actually water droplets are spherical. Rainbows appear as an arc, but their actual shape is conical. The apex of the cone is your eye. The center of a rainbow is at the shadow of your head or the opposite direction from the sun.