Explain the following with the help of ray diagram in relation to the main types of spherical lense:
( a) principal focus
(b) focal length

1.Convex lens-When rays come parallel to the principle axis they meet at a point ,that point is the principle focus...the image is thus formed at the focus.
2.Concave lens-When rays come parallel to the principle axis, they appear to come through the focus...a virtual image is thus formed at the focus.
focal length is the distance between the optical. centre and the principle focus.