What is the condition for achromatism of two lenses of same material separted by certain distance?

What is the essential condition for addition of two vectors?

What is the focal length of a combination of two concave lenses both of focal length 20 cm ?Is the system,a converging or a diverging lens?Ignore thickness of the lenses.

A currentof 5.0 A is flowing in eah of the two parallel conductors of infinite length separated by certain distance apart in vacuum in the same directions. Each conductor expereinces a force of 2xx10^-4N m^-1 due to the other. Find the separation between the two conductors.

Two heating coils, one of fine wire and the other of thick wire of the same material and of the same length are connected in series and in parallel. Which of the following statement is correct ?

Compare the resistance of two wires of same material. Their lengths are in the ratio 2:3 and their diameters are in the ratio 1:2

What is the distance between two consecutive nodes?