A lens of power +5 D is put in contact with a lens of power -3 D . The combination will behave like a

A plane wavefront of width ' $x$ ' is incident on an air-water interface and the corresponding refracted wavefront has a width ' y ' as shown in figure. The refractive index of air with respect to water in terms of distances ' $w$ ' and ' $z$ ' is $(\mathrm{AD}=\mathrm{w}, \mathrm{CB}=\mathrm{z})$

The intermediate image formed by an objective lens of a compound microscope is

A thin concavo-convex lens with convex face receiving incident rays has radii of curvatures 12 cm and 24 cm respectively. If refractive index of material of lens is 1.5 , then the focal length of the lens is