$6.(15$ points$)$ A monochromatic broad source of red light with a wavelength of $660$ nm shines normally onto two microscopic slides $7\mathrm{.}5$ cm long that are separated by a wire spacer $0\mathrm{.}045$ mm in diameter at one end and touch at the other end. How many bright fringes appear in the reflected light over the $7\mathrm{.}5$ cm distance?