Sodium light (589.3 nm) from a narrow slit illuminates a Fresnel biprism made of glass of...

Sodium light (589.3 nm) from a narrow slit illuminates a Fresnel biprism made of glass of index 1.50. The biprism is twice as far from a screen on which fringes are observed as it is from the slit. The fringes are observed to be separated by 0.03 cm. What is the biprism angle α?

Figure 1