$A$ student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw a ray diagram for the correction of this defect.

The student is suffering from myopia,also known as near-sightedness. In this defect,a person can see nearby objects clearly but cannot see distant objects distinctly because the image of a distant object is formed in front of the retina.
To correct this defect,a doctor will advise her to use a concave lens of appropriate power. The concave lens diverges the incoming light rays from a distant object,effectively shifting the image back onto the retina.