Explain how interference fringes are formed in an air wedge.

Explain how interference fringes are formed in an air wedge.

• Monochromatic light from the lamp is reflected by the inclined glass plate so that it is incident normally on the wedge.
• On reaching X, some of the light is reflected at the bottom part of the top glass slide while part is transmitted and reflected at Y on the top part of the bottom glass slide.
• The two waves are thus coherent and interfere forming a series of straight line fringes of bright and dark bands.
• The wave reflected at x suffers no phase change while that from Y reflected from at boundary with a denser medium suffers a phase change of π due to reflection which is equivalent to an additional path of λ/2
• Therefore a dark band is formed at the point of contact of the slide.
• Where the path difference is an integral multiple of wavelength, a bright band is formed  when 2t = (m ± λ/2) , where m = 0, 1, 2, 3, … due to reinforcement.
• Where the path difference is an odd multiple of half the λ, a dark band is formed (i.e. 2t = mλ); where m = 0, 1, 2, 3, …

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