optics practicle?

Question

give proper readings to find the focal length of concave lens using a convex lens

Answer 1

Use a convex lens with a known focal length F0 less than the absolute value of the concave lens focal length F2. When you put the two lenses together you have the equivalent of a single convex lens with a focal length F1 > F0. Verify F0 < |F2| by observing convex lens behavior of the combination.
First find F1, either applying parallel rays (e.g., from sun) and measuring distance L2 of focused sun image (in which case F1 = L2), or placing an object at a distance L1 which yields a real, focused image, and measuring image distance L2 (in which case F1 = 1 / (1 / L1 + 1 / L2)).
Once you have F1, compute concave lens focal length F2:
F2 = 1 / (1 / F1 - 1 / F0).

Answer 2

1/O + 1/I = 1/f

I = -O

concave ---- R + R>0
convex ---- R - R<0

Hope it helps!!