Consider the following statements,
1. The focal length of the convex lens will always be positive in any medium.
2. The focal length of the convex lens will always be positive in the air.
3. The focal length of the convex lens may be negative or positive depending upon the refractive index of the medium.
4. The focal length of the concave lens will always be negative in any air.
Which of the following statements are correct?
1
1 only
2
1 and 3 only
3
2, 3 and 4 only
4
1, 2 and 3 only