On reducing the focal length of a lens, its power :

1. decreases
2. increases
3. does not change
4. first increases then decreases.

The lens of power + 1.0 D is:

1. convex of focal length 1.0 cm
2. convex of focal length 1.0 m
3. concave of focal length 1.0 cm
4. concave of focal length 1.0 m.

The power of a concave lens of focal length 20 cm is :

1. 5D
2. 10D
3. -10D
4. -5D

The focal length of a lens is (i) positive, (ii) negative. In each case, state the kind of lens.