Question

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

• A. $1.0\times 10^{8}m{s}^{-1}$
• B. $2.15\times 10^{8}m{s}^{-1}$
• C. $2\times 10^{7}m{s}^{-1}$
• D. $4\times 10^{7}m{s}^{-1}$

Answer 1

Answer: (B)

There TIR at B,
$i>i_C$
$\therefore 45^{\circ }>{\sin }^{-1}\left(\frac{1}{\mu }\right)$
$\therefore 45^{\circ }>{\sin }^{-1}\left(\frac{v_g}{c}\right)$
$\therefore sin45^{\circ }>\frac{v_g}{C}$
$\therefore v_g<c\times sin45^{\circ }$
$\therefore v_g<3\times 10^8\times \frac{1}{\sqrt{2}}$
$\therefore v_g<2.14\times 10^{8}m{s}^{-1}$