Question

derive x=vnott+1/2at^2 using the calculas methods where symbols have usual meaning​

Answer 1

The sign of power depends on the direction in which a light ray is deviated by the lens. The power could be positive or negative. If a lens deviates a ray towards its centre (converges), the power is positive and if it deviates the ray away from its centre (diverges), the power is negative.