Overview

Quantization of Energy is the broadest chapter in modern physics for Grade XII, weaving together Bohr’s atomic model, de Broglie’s matter-wave hypothesis, X-ray production and Bragg’s law, and a touch of Heisenberg’s uncertainty principle. Despite covering so much ground, the exam pattern is remarkably consistent: expect a theory question on Bohr’s postulates (often paired with deriving the radius or energy of the nth orbit), and a numerical calculating de Broglie wavelength for an electron, proton, or neutron accelerated through a given potential difference — the single most repeated numerical type in this entire chapter.

The X-ray sub-topic follows two threads: production (via the Coolidge tube, and why it’s called the “inverse photoelectric effect” since electron kinetic energy converts into photon energy rather than the reverse) and diffraction (Bragg’s law, nλ = 2d sinθ, used to find crystal spacing or X-ray wavelength). A recurring conceptual question compares de Broglie wavelengths between particles of different mass — proton vs. electron, or proton vs. alpha particle — at equal kinetic energy or equal accelerating voltage, which tests whether you know λ = h/p depends on mass through momentum, not directly.

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

⚠️ Common Mistakes Students Make

• Confusing “same kinetic energy” with “same accelerating voltage” in de Broglie comparisons. These give different formulas: for equal KE, λ ∝ 1/√m; for equal accelerating voltage, λ ∝ 1/√(mq). A proton and electron problem answered with the wrong assumption gives the wrong “which has longer wavelength” conclusion.
• Mixing up which energy is asked for in Bohr orbit problems. Total energy, potential energy, and kinetic energy in the nth orbit are all different values with a fixed ratio (PE = 2×Total, KE = −Total) — plugging total energy into a question asking for potential energy is a frequent slip.
• Forgetting that X-ray production requires deceleration, not acceleration, of electrons. The minimum wavelength (cutoff wavelength) corresponds to all the electron’s kinetic energy converting to one photon — students sometimes try to apply photoelectric-effect logic (work function, threshold frequency) here, which doesn’t apply to X-ray production.