Analyse the observation table which shows variation of image distance [v] with object distance [u] in the case of a convex lens. Answer the questions that follow without doing any calculations.{:("Sl.No","Object distance","Image distance"),(,"u [cm]","v [cm]"),(1,-90,+18),(2,-60,+20),(3,-30,+30),(4,-20,+60),(5,-18,+90),(6,-10,+100):}(a) What is the focal length of the convex lens ? Given reasons to support your answer.(b) Write the Sl.No of the observation which is not correct. How did you arrive at this conciusion ?(c ) Use an appropriate scale to draw the ray diagram for the observation at Sl. No 4 and find the approximate value of magnification.

Analyse the observation table which shows variation of image distance

1.	Analyse the observation table which shows variation of image distance [v] with object distance [u] in the case of a convex lens. Answer the questions that follow without doing any calculations.{:("Sl.No","Object distance","Image distance"),(,"u [cm]","v [cm]"),(1,-90,+18),(2,-60,+20),(3,-30,+30),(4,-20,+60),(5,-18,+90),(6,-10,+100):}(a) What is the focal length of the convex lens ? Given reasons to support your answer.(b) Write the Sl.No of the observation which is not correct. How did you arrive at this conciusion ?(c ) Use an appropriate scale to draw the ray diagram for the observation at Sl. No 4 and find the approximate value of magnification.
Answer» SOLUTION :(a) When `u=-30 cm , v=+30 cm`. This indicates that the OBJECT is at C1 [which is `2F_(1)`] R = 2f, 30 = 2f, f = 15 cm Focal length of the convex lens is + 15 cm. (b) If Object distance `u=-10 cm`, it means the object is placed between the optic centre and focus, a virtual erect image formed in this case on the same side as the object. Therefore v should be NEGATIVE according to Cartesian sign convention ie., `v=-100 cm` and not `v=+100` cm as mentioned in the table. Therefore SI. No 6 is incorrect. (c ) SI. No 4 `u=-20 cm , v=+60 cm F_(1)=-15 cm 2F_(1)=-30 cm` The object is between `F_(1)` and `2F_(1)`, an enlarged real and inverted image is formed BEYOND 2F, Magnification `=m = (v)/(u)=((+60))/((-20))=-3`