Answer 1

 (a) _air n_x = (Speed of light in air) / (Speed of light in medium X)
= (3 × 10⁸) / (2 × 10⁸)
= 1.5
(b) _air n _{Y =} (Speed of light in air) / (Speed of light in medium ​Y)
= (3 × 10⁸) / (2.5 × 10⁸)
= 1.2
(c) _x n_Y = (Speed of light in medium X) / (Speed of light in medium ​Y)
= (2 × 10⁸) / (2.5 × 10⁸)
= 0.8