The human eye has essentially a lens and a retina used as a screen. Which one of the following statements is true about this combination of lens and retina?
1
It is a convex lens with variable focal length and at a fixed distance between the retinal and the lens
2
It is a convex lens with variable focal length and its distance from the retina
3
It is a convex lens of constant focal length at a fixed distance from the retina
4
It is a concave lens of adjustable focal length at a fixed distance from the retina