From the expression of linear expansion, ∆R=Rα∆T ‌
∆R
R
=α∆T If f is the initial focal length and f′ is the final focal length, then ‌
∆f
f
‌‌=α∆T ∆f‌‌=fα∆T f′−f‌‌=fα∆T f′‌‌=f(1+α∆T) Substituting the given value in above expression, we get f′=150(1+12×10−6×200) f′=150.36cm