Interference fringes are observed on a screen by illuminating two thin slits 1 mm apart with a light source ( $\lambda=632.8 \mathrm{~nm}$ ). The distance between the screen and the slits is 100 cm . If a bright fringe is observed on a screen at a distance of 1.27 mm from the central bright fringe, then the path difference between the waves, which are reaching this point from the slits is close is