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?

For Young's double slit experiment, two statements are given below :

Statement I : If screen is moved away from the plane of slits, angular separation of the fringes remains constant.

Statement II : If the monochromatic source is replaced by another monochromatic source of larger wavelength, the angular separation of fringes decreases.

In the light of the above statements, choose the correct answer from the options given below :

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 :