Derive the expression for the equivalent focal length of a combination of two thin convex lenses in contact.

Two long straight thin conductors carrying currents I_1 and I_2 respectively along the same direction are placed parallel to each other in air. Derive an expression for the force per unit length acting on any one of the conductors and hence define one ampere current.

Calculate the focal length of the combination of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm. Is the system a converging or diverging lens?

What sign (+ve or-ve) is given to the focal length of a convex mirror?

Find the power of a convex lens of focal length 25 cm.

Find the focal length of a convex mirror whose radius of curvature is 32 cm.

State parallelogram law of vector addition and derive an expression for the resutlant of two vectors vecto P and vecto Q inclined to each other at angle theta .