A Fresnel biprism experiment uses light of wavelength 589 nm. The virtual sources are separated by d = 2.0 mm and the screen is D = 1.5 m away. (a) Calculate the fringe width. (b) The fringe pattern shifts by 8 fringes when a thin glass plate is introduced in one beam; find the extra optical path introduced. Solution: (a) Fringe width β = λD/d = (589 × 10⁻⁹ m)(1.5 m) / (2.0 × 10⁻³ m) = 4.42 × 10⁻⁴ m ≈ 0.44 mm. So neighbouring bright bands are less than half a millimetre apart — this is why a micrometer eyepiece is used to measure them. (b) A shift of N fringes means the extra path difference = Nλ = 8 × 589 nm = 4712 nm ≈ 4.71 × 10⁻⁶ m. Hence the glass plate adds about 4.7 μm of extra optical path, which can be converted to the plate's thickness if the refractive index is known.