An object is mounted on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens placed between these walls. The lens is kept at distance x in front of the second wall. The required focal length of the lens will be :

Light travels a distance $$\mathrm{x}$$ in time $$t_{1}$$ in air and $$10 \mathrm{x}$$ in time $$t_{2}$$ in another denser medium. What is the critical angle for this medium?

In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin) ?

Two thin lenses are of same focal lengths $$(f)$$, but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be :