The radii of curvature of the faces of a double convex lens are $10\, cm$ and $15\, cm$. Its focal length is $12\, cm$. What is the refractive index of the glass?

Two spherical black bodies have radii $r_1$ and $r_2$. Their surface temperatures are $T_1$ and $T_2$. If they radiate the same power,then $\frac{r_2}{r_1}$ is:

There is a horizontal film of soap solution. On it,a thread is placed in the form of a loop. The film is pierced inside the loop,and the thread becomes a circular loop of radius $R$. If the surface tension of the soap solution is $T$,then what will be the tension in the thread?

Which of the following metal ions plays an important role in muscle contraction?

The current in an $AC$ circuit is given by $i = i_1 \sin \omega t + i_2 \cos \omega t$. Its $r.m.s.$ value is:

Two identical charged spherical drops, each of capacitance $C$, merge to form a single drop. The resultant capacitance is: