**Category:** Optics  
**Subtype:** Interference (Young's Double Slit Experiment)  
**Problem Description:** This problem involves calculating the minimum distance \(\Delta a\) that one slit must be moved closer to the other in a double-slit interference experiment, while keeping the central bright fringe (O) in the same position. Given the distance between the slits and the screen, the wavelength of the light, and the distance between consecutive bright fringes, the task is to find the smallest shift that maintains the central fringe at the same location.