A convex lens of focal length ' $f$ ' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

The critical angle for light going from medium ' $x^{\prime}$ to medium ' $y$ ' is ' $\theta$ '. The speed of light in medium ' $x$ ' is ' $v_x$ '. The speed of light in medium ' $y$ ' is

A telescope has large diameter of the objective. Then its resolving power is

A thin hollow prism of refracting angle $3^{\circ}$, filled with water gives a deviation of $1^{\circ}$. The refractive index of water is