Answer
(a) increase
(b) decrease
Work Step by Step
(a) The bright fringe is given as
$dsin\theta=m\lambda$ $m=0,\pm 1, \pm 2.....$ eq(1)
and the dark fringe is given as
$y=Ltan\theta$...eq(2)
We know that the blue light has a shorter wavelength than red light. Thus, the value of $\lambda$ in eq(1) decreases when blue light is used and the angle of fringes decreases as well. As a result, the spacing between the fringes decreases according to equation (2).
(b) If the separation between the slits is decreased, then the angle of fringes increases according to eq(2). This is because the spacing between the slits is inversely proportional to the angle and thus the spacing between the fringes will increase.
