Answer
(a) outward
(b) $W=\lambda$
Work Step by Step
(a) We know that $sin\theta=\frac{m\lambda}{W}$. This equation shows that if the width $W$ is decreased then $sin\theta$ increases and as a result, we will have the dark fringes moving outwards.
(b) We know that $sin\theta=\frac{\lambda}{W}$. If $\theta$ is $90^{\circ}$ then the first dark fringe will move outwards to infinity. This can happen when the wavelength of the light is equal to the width of the slit. Thus, the width of the first dark fringe to move outward to infinity is equal to $\lambda$.
