*For a lens, or a [[sphere|spherical]] or [[parabola|parabolic]] [[mirror]], it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points - one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the ''focal length''. | *For a lens, or a [[sphere|spherical]] or [[parabola|parabolic]] [[mirror]], it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points - one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the ''focal length''. |