In a myopic eye, the image of a distant object is formed in front of the retina instead of on it, due to excessive curvature of the eye lens or elongation of the eyeball.
A concave lens is a diverging lens. It diverges the incoming light rays before they enter the eye, effectively shifting the image backward onto the retina, thus restoring clear distant vision.
Source: Defects of Vision and Their Correction, Chapter 10
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Examiners expect two clear points: (1) what goes wrong in myopia (image forms in front of retina) and (2) how a concave lens fixes it (diverges rays to push image back onto retina). Mentioning "diverging lens" earns terminology marks. Avoid lengthy diagrams unless asked — just state the cause and correction concisely.