(a) The focal length of the eye lens cannot be decreased below a certain minimum limit. As a result, the sharp image of the object is not formed on the retina but behind the retina of the lens.
(b) The necessary conditions for the formation of a rainbow are:
(i) The presence of water droplets in the atmosphere, and
(ii) The sun must be at the back of the observer, i.e., the observer must stand with his back towards the sun.