$(a)$ Explain the following terms used in relation to defects of vision and corrections provided for them:
$(i)$ Myopia $(ii)$ Astigmatism $(iii)$ Bifocal lenses $(iv)$ Far-sightedness (Hypermetropia).
$(b)$ Describe with a ray diagram how a person with myopia can be helped by spectacles.

(N/A) $(i)$ Myopia (Near-sightedness): $A$ defect where a person can see nearby objects clearly but cannot see distant objects distinctly. It is corrected using a concave lens.
$(ii)$ Astigmatism: $A$ defect where the cornea or lens has an irregular curvature,causing blurred vision at all distances. It is corrected using cylindrical lenses.
$(iii)$ Bifocal lenses: Lenses consisting of both concave and convex parts,used to correct Presbyopia,where a person has both Myopia and Hypermetropia.
$(iv)$ Far-sightedness (Hypermetropia): $A$ defect where a person can see distant objects clearly but cannot see nearby objects distinctly. It is corrected using a convex lens.
$(b)$ In Myopia,the image of a distant object forms in front of the retina. $A$ concave lens of suitable focal length is placed in front of the eye to diverge the incoming light rays,shifting the image back onto the retina.