COMEDK Exam 2019 Solved Paper

The combined focal length of plano-convex lens is
‌

1

f

=‌

1

f1

+‌

1

f2

where, f1=∞ for the plane surface and f2=f
‌∴‌‌‌

1

f

=‌

1

∞

+‌

1

f

‌⇒‌‌f=f
‌ Now, when concave lens of same focal length is joined to first lens, then combined focal length
‌

1

F

‌=‌

1

F1

+‌

1

F2

⇒‌‌‌

1

F

‌=‌

1

f

−‌

1

f

‌‌(∵F1=f,F2=−f)
⇒‌‌‌

1

F

‌=0⇒F=∞
Thus, the image can be focused on infinity or focus shifts to infinity.