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

Answer 1

Answer: (B)

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