question_answer

The central fringe obtained with a Lloyd's mirror set up for interference is found to be dark, whereas it is bright with a biprism. Why?                                                       

Answer:

                The central fringe obtained with a Lloyd's mirror is dark because the reflection of light from the denser mirror surface introduces a path difference of\[\lambda /2.\] In case of biprism, the waves from both the coherent sources reach the central point in the same phase so the central fringe is bright.