Find the focal length of a convex lens,if its power is $+2.0 \ D$. (in $m$)

$(a)$ What is meant by refraction of light?
$(b)$ If on applying Cartesian sign convention for spherical lenses the image distance obtained is negative,state the significance of the negative sign.

From which point of a lens are all the distances measured?

Through which point of a convex lens does a ray of light pass without undergoing refraction?

Which of the following always form a virtual image?

What type of image cannot be formed by a convex lens?