In a Std. X class, out of 40 students, 10 students use spectacles, 2 students have positive power and 8 students have negative power and 8 students have negative power of lenses in their spectacles .
Give two possible reasons for the above defect .

Explanation

Two possible reasons for myopia
Possible reasons of myopia : (1) The curvature of the cornea and the eye lens increases. The muscles near the lens cannot relax so that the converging power of the lens remains large. (2) The distance between the eye lens and the retina increases as the eyeball elongates.
Myopia is corrected using a suitable concave lens. Light rays are diverged by the concave lens before they strike the eye lens. A concave lens of proper focallength is chosen to produce the required divergence. Hence, after the converging action of the eye lens, the image is formed on the retina.