With respect to air,the critical angle in a medium for light of red colour $[{\lambda _1}]$ is $\theta$. Other factors remaining the same,the critical angle for light of yellow colour $[{\lambda _2}]$ will be:

$A$ ray of light is incident on one face of a $90^{\circ}$ prism and undergoes total internal reflection at the glass-air interface. If the angle of incidence is $45^{\circ}$,then the refractive index $n$ is:

The phenomenon involved in the reflection of radio waves by the ionosphere is similar to:

$A$ ray of light travelling inside a rectangular glass block of refractive index $\sqrt{2}$ is incident on the glass-air surface at an angle of incidence of $45^{\circ}$. The refractive index of air is $1$. Under these conditions,the ray:

$A$ transparent solid cylindrical rod has a refractive index of $\frac{4}{\sqrt{3}}$. It is surrounded by a medium of refractive index $2$. $A$ 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

It is found that all electromagnetic signals sent from $A$ towards $B$ reach point $C$. The speed of electromagnetic signals in glass can not be