# Coming Out The Same Way

A glass prism whose cross-section is an isosceles triangle stands with its (horizontal) base in water; the angles that its two equal sides make with the base are each $$\theta$$.

An incident ray of light, above and parallel to the water surface and perpendicular to the prism's axis, enters the prism and is internally reflected at the glass-water interface and re-emerges into the air. Taking the refractive indices of glass and water to be $$\frac{3}{2}$$ and $$\frac{4}{3}$$, respectively.

Calculate the minimum value of $$\theta$$ (in radians) for total internal reflection to be possible.