The torque τ on a current-carrying coil in a magnetic field B is:
\[ \tau = M \times B \]
Explain the construction of a spherical wavefront by using Huygens' principle.
The slope of the tangent to the curve \( x = \sin\theta \) and \( y = \cos 2\theta \) at \( \theta = \frac{\pi}{6} \) is ___________.