The focal length of a lens is greatest for which colour?

$A$ lens made up of different materials forms two images of a point object $O$ as shown in the figure. Then:

An object and a screen are fixed on the uprights of an optical bench. The distance between them is $100 \, cm$. $A$ convex lens is placed in between the object and the screen and the position of the lens is so adjusted that the image of the object is formed on the screen at two conjugate positions of the lens. The distance between these conjugate positions of the lens is $40 \, cm$. What is the focal length of the lens in $cm$?

Look at the ray diagram shown. What will be the focal length of the $1^{st}$ and the $2^{nd}$ lens,if the incident light ray passes without any net deviation?

$A$ lateral object of height $h_o = 0.5\, cm$ is placed on the optical axis of a bi-convex lens of focal length $f = 80\, cm$,at an object distance $u = -60\, cm$. The image formed is:

$A$ white light is falling on a bi-convex lens. Which of the following options represents the correct qualitative behaviour of the focussing of this light?