Why the angle in fresnels biprism is low?

The angle in Fresnel's biprism is kept low to ensure that the light waves passing through the prism remain nearly parallel, which is crucial for creating clear interference patterns. A small angle minimizes the deviation of light rays, allowing for a larger overlap of the wavefronts and enhancing the visibility of the interference fringes. Additionally, a low angle helps to maintain the coherence of the light waves, which is essential for observing distinct interference effects.