Applying Snell's Law, sinθ₁/sinθ₂ = n₁/n₂
Since sinθ₁ and n₁ are positive but n₂ is negative, the fraction n₂/n₁ × sinθ₁ (say k) is negative. θ₂ = sin⁻¹k where k is negative.
Therefore -π/2 < θ₂ < 0
Note that angles measured in anticlockwise direction is positive and clockwise direction is negative.