What will be the minimum angle of incidence such that the total internal reflection occurs on both the surfaces?

The side of a cube is measured by vernier callipers (10 divisions of a vernier scale coincide with 9 divisions of main scale, where 1 division of main scale is 1 mm). The main scale reads 10 mm and first division of vernier scale coincides with the main scale; Mass of the cube is 2.736 g. Find the density of the cube in appropriate significant figures.

An unknown resistance X is to be determined using resistances R$$_1$$, R$$_2$$ or R$$_3$$. Their corresponding null points are A, B and C. Find which of the above will give the most accurate reading and why ?

A transverse harmonic disturbance is produced in a string. The maximum transverse velocity is 3 m/s and the maximum transverse acceleration is 90 m/s$$^2$$ . If the wave velocity is 20 m/s, then find the waveform.