Interference, Diffraction, and Polarization (4D) Practice Test

A double-slit pattern is used to estimate slit separation $d$ by measuring the distance between adjacent bright fringes $\Delta y$ on a screen at distance $L$. Under small-angle conditions, $\Delta y \approx \lambda L/d$. If $\Delta y$ is measured to be larger than expected while $L$ and $\lambda$ are correct, which change best explains the observation?

A microfluidic cytometry device uses a 532-nm laser to illuminate a double-slit mask placed immediately before a detector. The two slits are separated by $d = 0.20\ \text{mm}$, and the detector screen is $L = 1.5\ \text{m}$ from the mask. The device is calibrated in air ($n \approx 1.00$). Assume small-angle conditions so that $y_m \approx m\lambda L/d$. Which observation is most consistent with the described interference pattern when the slits are illuminated uniformly?

Constants: $\lambda = 532\ \text{nm}$; $1\ \text{nm} = 10^{-9}\ \text{m}$.