In between the plates of parallel plate capacitor of plate separation 'd' a dielectric plate of thickness ' $t$ ' is inserted. The capacitance becomes one-third of the original capacity without dielectric. The dielectric constant of the plate is
A thin rod of length ' 4 L ' and mass ' 4 m ' is bent at the points as shown in the figure. The moment of inertia of the rod about an axis passing through point ' $O$ ' and perpendicular to plane of the paper is

The resistance of a coil for d.c. is $5 \Omega$. In a.c., the resistance will
When a ray of light is refracted from one medium to another, then the wavelength changes from $6000 \mathop {\rm{A}}\limits^ \circ$ to $4000 \mathop {\rm{A}}\limits^ \circ$. The critical angle for the interface will be