In the displacement method, a convex lens is placed in between an object and a screen. If the magnification in the two positions be m1 and m2 and the displacement of the lens between the two positions is X, then the focal length of the lens is :
Answer & explanation
Correct answer: option 2
$m_1=\frac{x_1}{x_2}$ & $m_2=\frac{x_2}{x_1}$
$⇒m_1-m_2=\frac{x_1x_2}{x_1+x_2}$
$⇒f=\frac{x}{m_1-m_2}$