A Young's double-slit screen extends from −4.2β to +4.2β, where β is the fringe width. How many dark fringes are visible?

Question

G1 Physics · Accepted Answer

Dark fringes occur at half-integer multiples of β. The visible positions are ±0.5β, ±1.5β, ±2.5β, ±3.5β. The next pair at ±4.5β lies outside. Therefore there are 8 dark fringes. Hence the answer is 8.