A thin plano-convex lens of focal length f is split into two halves. One of the halves is shifted along the optical axis as shown in figure. The separation between object and image planes is 1.8 m. The magnification of the image, formed by one of the ball lens is 2. Find the focal length of the lens and separation between the two halves. Draw the ray diagram for image formation.

Answer

D

Explanation

Given `u_2+v_2=1.8m`…(i)
The magnification of lens (1) is 2
`:'``2=(v_2+d)/(u_2-d)` ….(ii)
From (i) and (ii)
`u_2=0.6+d`,v_2=1.2-d`
Applying lens formula
`(1)/(v_2+d)+(1)/(u_2-d)=(1)/(f)`….(iii) for lens (1)
`(1)/(v_2)+(1)/(u_2)=(1)/(f)`....(iv) for lens (2)
From (iii) and (iv)
`(1)/(v_2+d)+(1)/(u_2-d)=(1)/(v_2)+(1)/(u_2)`
`implies` `(1)/(1.2-d+d)+(1)/(0.6+d-d)=(1)/(1.2-d)+(1)/(0.6+d)`
On solving, we get `implies``d=0.6m`
Subsstituting this value in (iv)
`(1)/(1.2-0.6)+(1)/(0.6+0.6)=(1)/(f)` `:'` `f=0.4m`