Question

Why is a normal eye not able to see clearly the objects placed closed than $25cm$

Answer 1

Hint: Normal eye not able to see clearly the objects placed closer than $25cm$ because the eye lens will have some standard focal length and we cannot reduced the focal length to a certain limit and also for clear vision light rays must be projected onto the light sensitive retina of our eye .which lies at fixed distance from the lens and normal eye is sensitive for different colour.

Complete step by step answer:
The eye is the most important of all optical instruments, the eye in which we will study or see how images formation will take place and with the clear detail and colour of object it can detect however our eye needs some modification to reach or to have a normal vision but should be called ideal rather than normal .image formation by our eyes and common vision modification are easily done by using optics .A real image must be projected onto the light sensitive part of our eye that is called retina which will be at fixed distance from the lens ,the lens of eye adjust its power to produce an image on the retina for object which are placed at different distance .The center of image falls on the fovea which as the greatest density of light receptors and the highest sharpness in the field of vision ,the variable opening of the eye and also with chemical adaptation allows the eye to detect light intensities from the lowest limit of observable to${{10}^{10}}$ time greater without damaging the eye. This is the highest range of detection our eyes can perform a several number of functions such as sense direction ,movement ,sophisticated colour and distance, optic nerve conveys signal received by the eye to brain.
 Normal eye is not able to see clearly the objects placed closer than $25cm$ because all the power of compromise or settlement of the eye is reached when the object is placed at $25cm$ from the eye.

Note: The corona and lens of an eye forms act together to form a real image on the retina which lies at fixed distance from the lens which as it concentration of receptors in the fovea and a blind spot over the optic nerve, the power of lens of an eye is adjustable to provide an image on the retina for an object placed at different distance.