Q. How will you measure thickness of thin silicon-chip using Bi-prism experiment? Derive the formula used.
When a thin silicon chip, thin film of soap bubble etc. is introduced in the path of one of the coherent interfering beam emitting from two virtual coherent sources S and S' then due to insertion of this plate the whole fringe pattern is shifted to a point towards the beam in the path of which the plate is introduced. If we are able to measure this fringe shift we can find the thickness and refractive index of given thin silicon-chip. From fig. let S1 and S2 are two virtual coherent sources made by using Bi-prism. Let a thin film of thickness'', and refractive index 'µ' is interested in path of light from source S1 due to this optical path changes. The change in optical path causes the fringe system to shift.
1. Due to insertion of piate, fringe width does not change.
2. All the fringes are shifted through the same amount.