What will be the focal length of a lens whose power is given as $+2.0 \ D$?

Define linear magnification produced by a spherical mirror. Also,give its relation.

What type of mirror is used to obtain a real image?

What is the point,where the rays parallel to the principal axis meet after reflection,called?

$(a)$ Write the relation between $u, v, f$ for lenses and for mirrors,where $u, v, f$ are object distance,image distance,and focal length respectively.
$(b)$ The magnification produced by a concave mirror is $m = +4$. Write the information about the image given by this statement.
$(c)$ Draw a ray diagram for the following and show the formation of the images in the case of a concave mirror when the object is placed:
$(i)$ Between the pole and the focus point.
$(ii)$ At the centre of curvature.

What changes; frequency or wavelength, when light passes from a rarer to a denser medium?