Question

A student finds the writing on the blackboard as blurred and unclear when sitting on the last desk in the classroom. He, however, sees it clearly when sitting on the front desk at an approximate distance of 2 m from the blackboard. Draw ray diagrams to illustrate the formation of the image of the blackboard writing by his eye-lens when he is seated at the (i) last desk (ii) front desk. Name the kind of lens that would help him to see clearly even when he is seated at the last desk. Draw a ray diagram to illustrate how this lens helps him to see clearly.

Answer 1

The student is suffering from the Near-sightedness or Myopia. That's why he is able to see blackboard writing when he is nearby the blackboard, but he is unable to see the blackboard writing clearly when he is sitting far at the last desk.

(i) When he is sitting at the last desk, the image of the blackboard writing is forming in front of the retina due to myopia effect. That's why he is seeing a blurred and unclear image. The following ray diagram explains the defect of the student.



(ii) When the student is sitting on the front desk, he is able to see the writing on the blackboard because his eye is forming the image of the nearby object on the retina. That's why he is able to see the blackboard clearly. The following diagram shows the image formation by his eye when he is sitting on the front desk:



The student must use a concave lens to correct the defect of his eye. The concave lens will diverge the rays coming from the blackboard and will form an image on the retina when he will be seeing the blackboard from a far distance. i.e. Last desk of this class. The following figure shows, how a concave lens can help him to see clearly.