(a) (i) The power of a lens 'X' is $-2.5\ D$. Name the lens and determine its focal length in cm. For which eye defect of vision will an optician prescribe this type of lens as a corrective lens? (ii) "The value of magnification 'm' for a lens is $-2$." Using new Cartesian Sign Convention and considering that an object is placed at a distance of $20\ cm$ from the optical centre of this lens, state : (I) the nature of the image formed; (II) size of the image compared to the size of the object; (III) position of the image, and (IV) sign of the height of the image. (iii) The numerical values of the focal lengths of two lenses A and B are $10\ cm$ and $20\ cm$ respectively. Which one of the two will show higher degree of convergence/divergence? Give reason to justify your answer. OR (b) (i) Draw a ray diagram to show the refraction of a ray of light through a rectangular glass slab when it falls obliquely from air into glass. (ii) State Snell's law of refraction of light. (iii) Differentiate between the virtual images formed by a convex lens and a concave lens on the basis of : (I) object distance, and (II) magnification.