Q. 274.5( 4 Votes )

A student finds the writing on the blackboard as blurred and unclear when sitting on the last desk of the classroom. He, however, sees clearly when sitting on the front desk at an approximate distance 2 m from the blackboard.
(i) Draw the ray diagram to illustrate the formation of image of the blackboard writing by his eye lens when he sits at the (a) last desk, (b) front desk

(ii) Name the defect of vision the student is suffering from. Also, list two causes of this defect.

(iii)Name the kind of lens would enable him to see clearly when he is seated at the last desk. Draw the ray diagram to illustrate how this helps him to see clearly.

Answer :

(i) The following is the ray diagram to illustrate the formation of image of the blackboard writing by his eye lens when he sits at the


(a) At last desk: due to myopia image is formed in front of retina.



(b) Front desk



(ii) The student suffers from Myopia. Myopia is the defect of vision due to which a person cannot see the distant objects clearly (although he can see the nearby objects clearly.


The far point of an eye suffering from myopia is less than infinity. It is caused:


Due to high converging power of the eye lens(because of its short focal length) or


Due to ye ball being too long.


(iii) The concave lens would help him to see clearly when he is seated at the last desk.



The whole purpose of concave lens is to decrease the converging power of the eye lens and focus the image on the retina.

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