An isosceles triangular glass prism stands with its base in water as shown. The angles that its two equal sides make with the base are θ each. An incident ray of light parallel to the water surface internally reflects at the glass-water interface and subsequently reemerges into the air. Take the refractive indices of glass and water to be 3/2 and 4/3 respectively. The least value of θ for this to happen must be