Fill in the blank spaces with appropriate answers:
A double convex lens of focal length 'f' is cut at `90^@` to its principal axis into two identical component lenses. The focal length of each component lens is _________.

Fill in the blank spaces with appropriate answers: A double convex lens of focal length 'f' is cut at 90^@ to its principal axis into two identical component lenses. If D_1 and D_2 are the respective powers then D_2 is _____ than D_1

Fill in the blank spaces with appropriate answers: A double convex air bubble in water behaves as a ______ lens.

If the lens is double concave then its focal length is

The power of a convex lens of focal length 20 cm in dipter unit is