Question

solve question no. 1 please

Answer 1

see diagram.

The angle of incidence is same at both interfaces, as the surfaces are parallel.

Critical angle for top interface/top reflection: Sin⁻¹ √2/2 = 45°
Critical angle for the bottom interface (2nd reflection)
        Sin⁻¹ √3/2 = 60°

Hence, the angle of incidence must be more than or equal to 60° for total internal reflection to occur at both interfaces.