What will be change in wave length, if a light of wave length $$600 \mathrm{~nm}$$ travels from air enters a medium of refractive index 1.5 and continues its journey through that medium?

A point object is placed on the optic axis of a convex lens of focal length $$f$$ at a distance of $$2f$$ to the left of it. The diameter of the lens is $$d$$. An eye is placed at a distance of $$3f$$ to the right of the lens and a distance $$h$$ below the optic axis. The maximum value of $$h$$ to see the image is

A plane glass mirror of thickness 3 cm of material of $$\mu=\frac{3}{2}$$ is silvered on the black surface. When a point object is placed 9 cm from the front surface of the mirror, then the position of the brightest image from the front surface is

Two converging lenses of focal length 20 cm and 40 cm are placed in contact. The effective power of the combination is