Saniya and Shikha are friends studying together in grade `IV`. While Saniya can read a book keeping it at normal distance of about `25 cm`, Shikha has to keep the book at about `50 cm` from her eyes. Shikha feels it is a natural defect of her eyes, which cannot be corrected. Saniya is smarter. She tells her that medical science is advanced and there must be treatment for this problem. The doctor, on checking, prescribes spectacles of suitable power to Shikha and her vision with glasses becomes normal.
What the above passage and answer the following quentions :
(i) What defect was there in the vision of Shikha ?
(ii) What could be the causes of this defect ?
(iii) Calculate nature and power of the lens prescribed by the doctor. Assume that Shikha can read the book from a distance of `25 cm`, which she could do from a distance of `50 cm` without glasses.
(iv) What are the values of life displayed by Saniya ?]

Explanation

(i) Shikha is suffering from Hypermetropia or long sightedness.
(ii) Two possible causes of this defect are :
decrease in size of eye ball , increase in focal length of eye lens.
(iii) Here, `u = -25 cm, v = -50 cm`
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(-50)+(1)/(25)=(1)/(50), f = 50 cm`
`P = (100)/(f) = (100)/(50) = + 2 D`
The lens must be convex.
(iv) Saniya has displayed concern for the health of Shikha whi is her friend. Her awareness has helped Shikha to get her problem treated. But for Saniya, Shikha has resigned herself to live with the problem. Saniya provided that a friend in need is a friend indeed.