An optical fiber consists of core of \( \mu _ { 1 } \) surrounded by a cladding of \( \mu _ { 2 } < \mu _ { 1 } \). A beam of light enters from air at an angle \( \alpha \) with axis of fiber. The highest \( \alpha \) for which ray can be traveled through fiber is
1
\( \cos ^ { - 1 } \sqrt { \mu _ { 2 } ^ { 2 } - \mu _ { 1 } ^ { 2 } } \)
2
\( \sin ^ { - 1 } \sqrt { \mu _ { 1 } ^ { 2 } - \mu _ { 2 } ^ { 2 } } \)
3
\( \tan ^ { - 1 } \sqrt { \mu _ { 1 } ^ { 2 } - \mu _ { 2 } ^ { 2 } } \)
4
\( \sec ^ { - 1 } \sqrt { \mu _ { 1 } ^ { 2 } - \mu _ { 2 } ^ { 2 } } \)