Question

what is the formula of lenses concave

Answer 1

Select the distance between the lens(combination of convex lens andconcave lens) and the object (wire gauze) using the slider. ... The focal length of convex lens (f1) is shown inside the simulator window. You can calculate the focal length of theconcave lens using the formula f2 = (f1F/(f1-F).

Answer 2

Formula for lenses is :-
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
where f is focal length ( f is positive in convex lens and negative for concave lens)
v is distance to the image from mirror
and u is the distance to the object from the mirror ( u is always negative)