Is this ever a good approximation?

Question

In a two-slit interference pattern on a distant screen, are the bright fringes midway between the dark fringes? Is this ever a good approximation?

Answer 1

Sure, it's a fine approx., especially when the fringe spacing angle is small. Remember, the difference between a bright and a dark fringe is the path length difference from each slit to the point. For a dark fringe, the path difference is lambda/2, for a bright fringe it's lambda.