Answer to Question #188288 in Optics for Robin

Question

a thin convex lens is examined in an optical bench. a screen is placed at a distance of 3.6m from an object. when the lens is placed in a place between the object and the screen, a sharp image twice the size of the object is obtained on the screen. how large is the focal length of the examined lens

Answer 1

$\text{Thin lens equation:}$

$\frac{1}{o}+\frac{1}{i}=\frac{1}{f};$

$\text{where:}$

$o -\text{object distance}$

$i -\text{image distance}$

$f -\text{ focal length}$

$\text{According to the condition of the task:}$

$-o+i =3.6;-\frac{i}{o}=2;$

$i = -2o;$

$-o-2o=3.6$

$o=-1.2;i=2.4$

$-\frac{1}{1.2}+\frac{1}{2.4}=\frac{1}{f}$

$f = -1.2$

$\text{Answer: }f=-1.2$