CBSE Class 12 Physics 2026-27: high-weightage chapters and how to revise them

Unit marks for 2026-27

Source: CBSE Physics (042) syllabus 2026-27. Content marked in the NCERT textbooks as excluded for 2026-27 is not assessed in the board exam.

How the 70 theory marks are split by question type

The 2026-27 sample paper has 33 questions: Section A, 12 MCQs and 4 assertion-reason (1 mark each); B, 5 of 2 marks; C, 7 of 3 marks; D, 2 case studies of 4 marks; E, 3 long answers of 5 marks.

Worked example 1 (electrostatics): capacitors in series

Capacitors of 2 µF and 3 µF are connected in series across a 10 V supply. Find the equivalent capacitance and the energy stored.

1. For two capacitors in series, C = (C1 × C2) ÷ (C1 + C2) = (2 × 3) ÷ (2 + 3) = 1.2 µF.
2. Energy stored U = (1/2) C V^2 = 0.5 × 1.2 × 10^-6 × 10^2 = 6 × 10^-5 J.
3. Sense check: the series combination is smaller than the smaller capacitor, as it must be.

Worked example 2 (wave optics): fringe width

In Young's double-slit experiment, light of wavelength 600 nm is used, the slits are 0.3 mm apart and the screen is 1.2 m away. Find the fringe width.

1. Fringe width β = λD/d (the syllabus asks for the final expression only, no derivation).
2. Convert to metres: λ = 600 × 10^-9 m, d = 0.3 × 10^-3 m, D = 1.2 m.
3. β = (600 × 10^-9 × 1.2) ÷ (0.3 × 10^-3) = 2.4 × 10^-3 m = 2.4 mm.

Worked example 3 (modern physics): photoelectric effect

Photons of energy 4.0 eV fall on a metal with work function 2.5 eV. Find the maximum kinetic energy of the photoelectrons and the stopping potential.

1. Einstein's equation: K_max = hν - φ = 4.0 - 2.5 = 1.5 eV.
2. The stopping potential V0 satisfies eV0 = K_max, so V0 = 1.5 V.
3. If the intensity doubles at the same frequency, K_max stays 1.5 eV; only the number of photoelectrons increases. This is a favourite assertion-reason question.

Worked example 4 (atoms): Bohr model

Find the energy of the photon emitted when an electron in hydrogen falls from n = 3 to n = 2.

1. Energy levels: E_n = -13.6/n^2 eV, so E_3 = -1.51 eV and E_2 = -3.4 eV.
2. Photon energy = E_3 - E_2 = 13.6 × (1/4 - 1/9) = 13.6 × 0.1389 ≈ 1.89 eV.
3. This is the red line of the Balmer series; the syllabus treats line spectra qualitatively, so expect the energy calculation rather than a full series derivation.

Common mistakes

• Not converting nm, mm and µF to SI units before substituting.
• Sign convention errors in mirror and lens questions in ray optics.
• Applying Kirchhoff's loop rule with inconsistent current directions.
• Using Fleming's rule or the right-hand rule the wrong way round when finding force or induced current direction.
• Writing derivations the syllabus does not require while missing the final expression it does ask for.
• Drawing diode characteristics or ray diagrams without labelled axes, arrows or scale.

A revision order and how a tutor helps

Start with the electricity and magnetism chapters (1 to 7), which share ideas and carry 33 marks between them. Then ray and wave optics, which supply many long answers and diagrams. Leave modern physics (chapters 11 to 13) and semiconductors (14) for later: they are short, formula-based and quick to revise, so they make good final-month work. The practical, worth 30 marks, is assessed separately, so keep the record and investigatory project up to date through the year.

In one-to-one lessons a tutor works through numericals on the shared whiteboard, checking units and signs at each step, and has your child explain derivations aloud so gaps in understanding show up. For case-based questions the tutor practises reading the given data first and answering from it. Physics also feeds entrance exams for engineering and medicine, so lessons can go beyond the board paper where a student needs it.

Self-check

• Two 4 ohm resistors in parallel are connected to a 2 V cell of negligible internal resistance. What current flows from the cell? (Answer: 1 A)
• What is the de Broglie wavelength relation? (Answer: λ = h/p)
• A convex lens has power +2.5 D. What is its focal length? (Answer: 40 cm)
• Why does the fringe width increase if the screen is moved further away? (Answer: β is proportional to D)
• Which type of semiconductor is made by doping silicon with a pentavalent impurity? (Answer: n-type)