Define aperture of a spherical mirror

The diameter of the reflecting surface of spherical mirror is called its aperture. Mirrors whose aperture is much smaller than its radius of curvature, we use R=2f.

grav forces affect all objects magnetic forces affect magnetic objects and electric currents

igneous rocks also form where plates converge. the subducting plate melts as it sinks into the crust of the earth, and the melt rises into the overriding plate forming volcanoes. metamorphic rocks are formed mainly in the lithosphere, wherever there is high pressure and high temperature. -woods