In a biprism experiment a steady interference pattern is observed on the screen using a light of wavelength 5000 Å . Without disturbing the set up of the experiment, the source of light is replaced by a source of wavelength 6400 Å . The fringe width will

Step-by-step solution

Fringe width in a Fresnel biprism is given by = D d , where is wavelength, D is source-screen distance, and d is virtual source separation. Since D and d remain unchanged, the fringe width is directly proportional to wavelength: . With initial wavelength ₁ = 5000 A increasing to ₂ = 6400 A , the fringe width ratio becomes ₂ ₁ = 6400 5000 = 1.28 . The percentage change in fringe width is (1.28 - 1) 100 % = 28 % . Since the ratio exceeds unity, the fringe width increases by 28 % . The correct answer is D .