Question

Derive Newton's formula for thin lens.

Answer 1

Newton law states that

$f_1\times f_2=x_1\times x_2$

if $R_2=-R_1=R$ & $f_2=-f_1=f$ then,

$x_1\times x_2=-f^2$

Law: The product of distance of object and image from respective foci is equal to the product of focal length