It should be easy to see objects which are close to our eyes. But we find it difficult to see objects that are too close to the eye. How can you explain this phenomenon?

Explanation

Since the distance between eye lens and retina is fixed i.e. v is fixed eye has to change its focal length f so as to focus object on retina. The focal length of eye lens is changed by action of ciliary muscles. For focusing near objects, the focal length of eye lens is decreased by putting pressure on eye lens. Radius of eye lens cannot be decreased beyond certain limits and hence objects placed very near to eyes cannot be seen clearly.