AP EAMCET Engineering 20 Apr 2019 Shift 1 Paper

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