Overview

Diffraction rounds out the wave optics trio, and it tends to blend into Interference in students’ minds — which is exactly why examiners love asking you to distinguish between the two. Expect a steady numerical every year, almost always built around a diffraction grating: given lines per mm and an incident wavelength, find the diffraction angle, the wavelength, or the maximum number of visible orders. The “central diffraction peak width” problems from a single slit are the other recurring numerical type, and they follow the same handful of setups with different numbers plugged in each year.

On the theory side, the chapter splits into single-slit diffraction (deriving the condition for the first minimum, secondary maxima and minima) and grating diffraction (explaining how a transmission grating is constructed and how its pattern forms). A common trap in the short-answer questions is the Fresnel vs. Fraunhofer distinction — Fraunhofer diffraction assumes a plane wavefront and parallel rays, which is the version this chapter almost exclusively deals with, while Fresnel diffraction involves diverging or converging wavefronts and rarely appears directly in the numericals. Also watch for the “why can we hear around a corner but not see around one” conceptual question — it’s really just testing your understanding of how wavelength relative to obstacle size affects diffraction.

Below is the full set of MCQs, theory questions, and numerical problems from past NEB board exams, organized by course and question type.