With the help of ciliary muscles, the human eye can change its curvature and thus alter the focal length of its lens. State the changes that occur in the curvature and focal length of the eye lens while viewing (a) a distance object (b) nearby objects. Explain why a normal eye is not able to see distinctly the objects placed closer than $25 \mathrm{cm}$ without putting any strain on the eye.

Study the diagram below and answer the following question :

(i) Name the defect of vision depicted in the diagram.

(ii) List two causes of the said defect.

(iii) Draw a ray diagram for the correction of the above defect using an appropriate lens.

$\left(a\right)$ Ravi kept a book at a distance of $10 \mathrm{cm}$ from the eye of his friend Hari. Hari is not able to read anything written in the book. Explain why? 

$\left(b\right)$ A lens of focal length $5 \mathrm{cm}$ is being used by a student in the laboratory as a magnifying lens. His least distance of distinct vision is $25 \mathrm{cm}$. What magnification is the student getting? 

$\left(a\right)$ A student is suffering from myopia is not able to see distinctly the objects placed beyond $5 \mathrm{m}$. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain :

$\left(i\right)$ why the student is unable to see distinctly the objects placed beyond $5 \mathrm{m}$ from his eyes?

$\left(ii\right)$ the type of corrective lens used to restore proper vision and how this defect is corrected by the use of this lens

$\left(b\right)$ If, in this case, the numerical value of the focal length of the corrective lens is $5 \mathrm{m}$, find the power of the lens as per the new cartesian sign convention.