A monochromatic light beam
*b + n5ws0l+xqwemgu8 of wavelength λ is diffracted at a single slit of wid
+8 + *slbqw5em0xw nguth $b$. After passing the slit, a diffraction pattern is observed on a screen a large distance away. The width of the central maximum is $w$.
The screen is moved closer to the slit. Which of the following changes, done separately, could result in no change in $w$?