Explain the first focal centre (first principal focus) and second focal centre (second principal focus) for a biconvex lens and a biconcave lens with the help of diagrams.

(N/A) $1$. First Principal Focus $(F_1)$: It is a fixed point on the principal axis of a lens such that light rays starting from this point (in a convex lens) or appearing to converge towards this point (in a concave lens) become parallel to the principal axis after refraction through the lens.
$2$. Second Principal Focus $(F_2)$: It is a fixed point on the principal axis of a lens such that light rays coming parallel to the principal axis after refraction through the lens either converge to this point (in a convex lens) or appear to diverge from this point (in a concave lens).
- For a biconvex lens: The first focus is on the left side,and the second focus is on the right side.
- For a biconcave lens: The first focus is on the right side,and the second focus is on the left side.