Because a concave lens cannot form a real image of a real object, it is difficult to measure its focal length precisely. One method uses a second,...

Hello!

We need a formula which establishes a relation between a lens' focal length a distance from an object to the lens and a distance from an image to a lens.

For a convex (collecting) lens and a real image the formula is

for a concave (diverging) lens and a virtual image the formula is

Please look at the picture attached. The focal length of the convex lens is

...

Hello!

We need a formula which establishes a relation between a lens' focal length a distance from an object to the lens and a distance from an image to a lens.

For a convex (collecting) lens and a real image the formula is

for a concave (diverging) lens and a virtual image the formula is

Please look at the picture attached. The focal length of the convex lens is

For the concave lens the object is the real image produced by the convex lens. So the distance between the lens and the object is 37.5-15=22.5(cm). The distance from the concave lens to its real image is given (35 cm). Therefore its focal length is

Yes, it is negative because the lens is diverging.

The answer: the focal length for the convex lens is 19.8 cm and for the concave lens is 63 cm (or -63 cm).