Assertion (A): In a single-slit diffraction experiment with monochromatic light, the central bright band on the screen becomes narrower when the slit is made narrower, with the source and the screen left where they were. Reason (R): The first dark band of the pattern lies at an angle $\theta$ from the axis given by $\sin\theta = \dfrac{\lambda}{a}$, where $\lambda$ is the wavelength of the light and $a$ is the width of the slit.