Total Internal Reflection

It is found that all electromagnetic signals sent from $P$ towards $Q$ reach point $R$. The speed of electromagnetic signals in glass cannot be

The value of critical angle is least for which of the following colours of light?

A ray falls on a prism $ABC, \left(AB=BC\right)$ and travels as shown in the figure. The minimum refractive index of the material of the prism should be

If the critical angle for total internal reflection from a medium to vacuum is $30°,$ the velocity of light in the medium is

A transparent solid cylindrical rod has a refractive index of $\frac{2}{\sqrt{3}}.$ It is surrounded by air. A light ray is incident at the mid-point of one end of the rod as shown in the figure.

The incident angle $\theta$ for which the light ray grazes along the wall of the rod is