Two identical small balls carry equal charges. They hang from the same point by two light insulating threads of equal length and come to rest with the threads at an angle $2\theta$ to each other, as shown in the figure. The whole arrangement is then dipped into an oil of density $800\ \text{kg/m}^3$ and dielectric constant $2$. Strangely, the angle between the threads stays exactly the same. The density of the material of the balls is

Fig. 1: Two small charged balls hanging from one point on equal threads, pushed apart so that the threads make an angle 2 theta, inside a beaker of oil.