Sodium light (589.3 nm) from a narrow slit illuminates a Fresnel bi-prism made of glass of refraction index 1.5. The biprism is twice as far from a screen on which the fringes are observed as it is from the slit. The fringes are separated by 0.03 cm. What is the biprism angle? (The slit is perpendicular both to the optical axis and to the long side of the biprism.)