(a) In a double-slit experiment, what ratio of d to a causes diffraction to eliminate the fourth bright side fringe? (b) what other bright fringes are also eliminated?

Explanation

To solve the problem, we need to analyze the conditions under which diffraction causes the elimination of bright fringes in a double-slit experiment.

### Step-by-Step Solution:

#### Part (a): Finding the Ratio of D to A that Eliminates the 4th Bright Fringe

1. **Understanding the Conditions for Minima and Maxima**:
- In a double-slit experiment, the condition for minima (dark fringes) due to diffraction is given by:
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