Fringe spacing in double-slit interference

Steps:

• Recall the fringe spacing formula: $\Delta y = \frac{\lambda L}{d}$, where $\lambda$ is wavelength, $L$ is screen distance, and $d$ is slit separation.
• Identify that close fringes mean small $\Delta y$.
• To increase $\Delta y$, increase $L$ or $\lambda$, or decrease $d$.
• Evaluate options: only B increases $L$.

Why B is correct:

• Increasing $L$ (distance to screen) directly enlarges $\Delta y$ per the formula, spreading fringes farther apart.

Why the others are wrong:

• A: Brightness affects intensity, not spacing.
• C: Increasing $d$ decreases $\Delta y$, making fringes closer.
• D: Higher frequency shortens $\lambda$, decreasing $\Delta y$.

Final answer: B