14.3 Diffraction and Interference

Diffraction is spreading when a wave passes through an opening or around an obstacle. The effect is weak when wavelength is much smaller than the aperture and strong when wavelength is comparable to aperture size. That scale comparison, not wave intensity, is the primary control variable for geometric spreading.

Diffraction is often introduced qualitatively, but exam problems quickly connect it to measurable fringe widths and minima positions. Therefore keep both conceptual and quantitative views active: wavefront reconstruction explains why spreading happens, while path-difference formulas predict where bright and dark regions appear.

Constructive interference means in-phase arrival and bright intensity. Destructive interference means approximately antiphase arrival and dark intensity. The same logic works for sound, water, and light; only typical wavelength and geometry scales change.

Here D is slit-screen distance and d is slit separation. Increasing D or wavelength increases fringe spacing, while increasing d compresses fringes. The relation is a small-angle approximation, so use the exact trigonometric form when angles are not small.

Many questions provide fringe spacing directly. If adjacent bright fringes are separated by Delta y, then Delta y = lambda D / d. This rearranged form is often the fastest path to unknown wavelength or slit spacing in practical setups.