1. One of the two slits in a double-slit interference arrangement is closed with a thin strip of glass of refractive index 1.4 and the other slit is closed with a thin strip of transparent plastic with refractive index 1.7. On doing this, the centre of the interference pattern on the screen is seen to be displaced to the position of the fifth bright fringe (order 5) in the original pattern. If the wavelength of the monochromatic light used is 480 nm and if the strips have a thickness of t each, what is the value of t ? I can't really understand with the question whehter I should use the Young Modulus double slit formula or the air wedge formula because Young Mudulus double slit does not involved refractive index ,n and in the air wedge,there is question on calculating the thickness. So,I don't know what to do now because I get confuse with the question and have try so many times on calculation but still cannot get the answer.Help,I need somebody the help me figure out and explain to me what the question want.Thanks for anybody that spend sometime on this question.