Wave Optics Part 2

At the interface between two materials having refractive indices n1 and n2, the critical angle for reflection of an em wave is θ1c. The n2 material is replaced by another material having refractive index n3 such that the critical angle at the interface between n1 and n3 materials is θ2c. If n3>n2>n1; ‌

n2

n3

=‌

2

5

and sin‌θ2c−sin‌θ1c=‌

1

2

, then θ1c is