question_answer

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) \[1.0\times {{10}^{8}}m/s\]

B) \[2.4\times {{10}^{8}}m/s\]

C) \[2\times {{10}^{7}}m/s\]

D) \[4\times {{10}^{7}}m/s\]

Correct Answer: A

Solution :

[a] This is a case of total internal reflection.

\[\therefore \]    \[\theta >{{\theta }_{c}}(={{\sin }^{-1}}\frac{1}{\mu })\]

\[\frac{1}{\mu }<\sin \theta \]\[\frac{1}{\mu }<\sin 45{}^\circ \]

\[\mu >1/\sin 45{}^\circ \]\[\mu >\sqrt{2}\]

As        \[v=\frac{c}{\mu }\]

\[v<\frac{c}{\sqrt{2}}=\frac{3\times {{10}^{8}}}{\sqrt{2}}\]

\[v<2.1\times {{10}^{8}}\]

\[\therefore \]    only (2) is not possible.