Angular separation of the fringes remains constant (= λ / d).The actual separation of the fringe increases in proportion to the distance of the screen from the plane of the two slits.

The separation of the fringes (and also angular separation) decrease.

The separation of the fringe (and also angular separation) decreases.

By slightly increasing the width of the slits ,we are only increasing the intensity of incident

beam.Again no change in λ , D, d. so β unchanged but sharpness of the fringe increase.

Sol. We know β =

if D is increased ⇒ β increases.

so the width of interference fringes increases.

λ decreases ⇒ β decrease again.

d is increased ⇒ β decreases.

If the source slit is moved nearer. no effects on λ , D or d, so β remains same.

By slightly increasing the width of the slits ,we are only increasing the intensity of incident beam.Again no change in λ , D, d. so β unchanged but sharpness of the fringe increase.