What is the iris?

$(a)$ Explain how a normal human eye is able to see distinctly objects placed at a distance as well as those placed at a nearer distance.
$(b)$ What are the far point and near point of a normal human eye?

An old person is unable to see clearly nearby objects as well as distant objects. $(i)$ What defect of vision is he suffering from? $(ii)$ What kind of lens will be required to see clearly the nearby as well as distant objects? Give reasons.

$A$ student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook. Which of the following statements is correct?

$A$ myopic person has been using spectacles of power $-1.00\, D$ for clear vision. During old age,he also needs to use separate reading glasses of power $+2.00\, D$. Explain what may have happened to his eyesight?

Name the outermost layer of the eye. What is its function?