In a single slit diffraction experiment, slit of width ' $a$ ' is illuminated by light of wavelength ' $\lambda$ ' and the width of the central maxima in diffraction pattern is measured as ' $y$ '. When half of the slit is covered and illuminated by light of wavelength (1.5) $\lambda$, the width of the central maximum in diffraction pattern becomes

To protect the instrument from magnetic field, it is completely surrounded by