Optics Question 955
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:
Options:
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 $
Show Answer
Answer:
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.
