Double-slit interference

EK 14.8.A.1 sets the apparatus: monochromatic light of wavelength $\lambda$ incident on two slits a distance $d$ apart. EK 14.8.A.1.i says that considering interference alone, a double slit creates a pattern of uniformly spaced maxima.

The condition, and what each letter is. Light from the two slits arrives at a point on the screen having travelled different distances. EK 14.8.A.1.iii: the amount of interference between two wavefronts depends on the path length difference $\Delta D$. EK 14.8.A.1.iv gives $\Delta D$ in terms of the geometry.

$d$ is the slit separation, $\theta$ the angle between the direction of propagation of the wavefront and the normal to the opening, and $m$ the order, an integer. Whole wavelengths of path difference put the two arrivals back in step, so $m$ counts bright fringes outward and $m = 0$ is the central maximum.

The small angle form. EK 14.8.A.1.v, for $\theta < 10$ degrees:

where $L$ is the slit to screen distance and $y_{\text{max}}$ the distance from the middle of the central bright fringe to the $m$th maximum. The exam conventions on the sheet confirm the approximation is valid for single- and double-slit diffraction. Note the subscript: this equation locates bright fringes.

Why it mattered historically. EK 14.8.A.2: interference patterns produced by light interacting with a double slit indicate that light has wave properties, and the source of that discovery was Young's double-slit experiment.