A student sets up a double–slit experiment with slit separation d and screen distance D. Answer the following qualitatively, giving reasoned explanations in each case.
(a) Predict how the fringe width will change if the wavelength of light is doubled while d and D remain fixed.
(b) Explain the effect on fringe width if the screen is brought to half the original distance from the slits, keeping λ and d unchanged.
(c) A thin transparent sheet of refractive index μ and thickness t is introduced in the path of one of the interfering beams. State whether the fringe width changes and justify your answer.
(d) In the situation of part (c), describe what happens to the position of the central bright fringe on the screen and explain why.