PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 2 marks · Very short answer

CBSE Class 12 Physics sample paper 2026-27: question 21(B)

From the official CBSE sample question paper for 2026-27, section B, question 21(B), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(B) · 2 marks · Very short answerAtoms

State Bohr’s quantization condition for defining stationary orbit. How does de Broglie hypothesis explain the stationary orbit?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Bohr's condition is about angular momentum.

Hint 2

For a standing wave on a circle, how many wavelengths must fit around it?

Step-by-step solution

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Bohr's quantisation condition: an electron revolves without radiating only in those orbits for which its angular momentum is an integral multiple of $h /2 π$ : $m v r = \frac{nh}{2 π}$ , $n = 1, 2, 3, \dots$
1 mark
de Broglie explanation: the electron has wavelength $λ = h / m v$ . A stable orbit is one where this wave forms a standing wave, i.e. the circumference holds a whole number of wavelengths: $2 π r = nλ = \frac{nh}{m v} \Rightarrow m v r = \frac{nh}{2 π}$ , which is exactly Bohr's condition.
1 mark

Answer: mvr = nh/2π; stationary orbits are those whose circumference fits a whole number of de Broglie wavelengths (2πr = nλ).

Where the marks are

States $m v r = nh /2 π$ (1 mark)
Uses $2 π r = nλ$ with $λ = h / m v$ to obtain the condition (1 mark)

Revise the chapter

Chapter: Atoms (notes, formulas, where marks are lost and every past question)
Topic: Bohr Model of the Hydrogen Atom

Practise this chapterBoard-style questions from Atoms, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

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CBSE Class 12 Physics sample paper 2026-27: question 21(B)

Hints

Step-by-step solution

Revise the chapter

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