Irodov Part 5 Optics — Which Problems to Solve for JEE
A section-by-section map of Part Five of I.E. Irodov’s Problems in General Physics, checked against the JEE Main and JEE Advanced syllabuses.
Part Five is the worst value in Irodov for a JEE candidate. It holds 292 problems and not one of its seven sections is wholly on the syllabus. Two sections are off it entirely, and the other five need picking through problem by problem. The realistic yield is around 116 problems, under two in five.
The reason is that Irodov’s optics is a physics course rather than an exam topic. Where the syllabus wants ray optics, Young’s double slit and a single slit, Irodov wants Fresnel zones, diffraction gratings, crystal optics and the dispersion theory of a medium.
If your time is short, do 5.1 and leave the rest. If you have finished Parts One and Three and still have months, come back for the interference and photoelectric material.
Key insight. Geometrical optics in this part is fully examinable and the wave optics mostly is not. That inversion catches people out, because wave optics is the half that feels advanced — and it is exactly the half where Irodov goes furthest past what can be asked.
The sections at a glance
| Section | Problems | Count | Verdict | Roughly in scope |
|---|---|---|---|---|
| 5.1 Photometry and Geometrical Optics | 5.1–5.63 | 63 | Mostly solve | ~44 |
| 5.2 Interference of Light | 5.64–5.96 | 33 | Pick carefully | ~12 |
| 5.3 Diffraction of Light | 5.97–5.156 | 60 | Little in scope | ~12 |
| 5.4 Polarization of Light | 5.157–5.199 | 43 | Pick carefully | ~17 |
| 5.5 Dispersion and Absorption of Light | 5.200–5.223 | 24 | Skip entirely | 0 |
| 5.6 Optics of Moving Sources | 5.224–5.245 | 22 | Skip entirely | 0 |
| 5.7 Thermal Radiation. Quantum Nature of Light | 5.246–5.292 | 47 | Mostly solve | ~31 |
Section 5.1 — Photometry and Geometrical Optics
Sixty-three problems, of which about forty-four are usable — the best pool in Part Five and the section to do if you do only one. Geometrical optics is on the syllabus in full: reflection and refraction at plane and spherical surfaces, total internal reflection, deviation and dispersion by a prism, thin lenses, mirror-and-lens combinations and magnification.
Solve all of that. Skip the photometry at the start — luminous flux, luminous intensity, illuminance and the units that go with them are on neither syllabus, and they are a distinct block you can recognise immediately by the units.
Before you start it: Ray Optics and Optical Instruments.
Section 5.2 — Interference of Light
Thirty-three problems, of which only about twelve are in scope, and the reason is worth stating precisely. The JEE Advanced syllabus carries interference limited to Young’s double slit experiment, in those words. That single clause removes most of the section.
Solve the double-slit problems: fringe width, path difference, a slab introduced in one arm, a source displaced.
Skip thin films, Newton’s rings, the Fresnel biprism, Lloyd’s mirror and the Michelson interferometer as Irodov poses them. Thin-film interference is taught for JEE by most coaching and does reach Main papers in a simplified form, so it is worth knowing — but Irodov’s thin-film problems are pitched far past that, and they are not the place to learn it.
Before you start it: Wave Optics.
Section 5.3 — Diffraction of Light
Sixty problems and roughly twelve worth attempting — the worst ratio of any section in the book that is not skipped outright. The syllabus carries diffraction due to a single slit and stops there.
Solve the single-slit problems. Skip Fresnel zones, zone plates, the spiral construction, diffraction gratings and resolving power — which together are the great bulk of the section. Resolving power is not on either syllabus.
Given the yield, many candidates should simply skip 5.3 and get their single-slit practice from past papers instead.
Section 5.4 — Polarization of Light
Forty-three problems, about seventeen of them usable. Plane polarised light, Brewster’s law and polaroids are on the syllabus, and Malus’s law comes with them in every standard treatment.
Solve the problems on Malus’s law, Brewster’s angle, polarisation by reflection and crossed polaroids. Skip birefringence, quarter- and half-wave plates, optical activity and the crystal optics that occupies the back of the section.
Section 5.5 — Dispersion and Absorption of Light
Skip the entire section. Twenty-four problems on the dispersion theory of a medium and absorption coefficients. Prism dispersion is on the syllabus; this is not that topic, despite the name.
Section 5.6 — Optics of Moving Sources
Skip the entire section. Twenty-two problems on the relativistic Doppler effect and aberration of light, resting on the relativity that is absent from both syllabuses.
Section 5.7 — Thermal Radiation. Quantum Nature of Light
Forty-seven problems, of which about thirty-one are usable — the second-best pool in this part, and a section students skip because its title sounds advanced. It is not. Blackbody radiation, absorptive and emissive power, Kirchhoff’s law, Wien’s displacement law and Stefan’s law are all on the syllabus under thermal physics, and the photoelectric effect is on it under modern physics.
Solve the blackbody problems, the Wien and Stefan problems, and the photoelectric ones. Skip the Compton effect and the radiation-pressure problems, which are on neither syllabus.
Before you start it: Thermal Properties of Matter and Dual Nature of Radiation and Matter.
Common mistakes with Part Five
Doing wave optics and skipping geometrical optics. It is the wrong way round. 5.1 is the most usable section here and the one people treat as beneath them.
Grinding through 5.3 because diffraction is on the syllabus. Single-slit diffraction is. Fresnel zones are not, and they are most of those sixty problems.
Skipping 5.7 because of its title. Thermal radiation reads like university physics and is in fact examinable school physics, with about thirty-one usable problems — more than interference and diffraction put together.
Using Irodov to learn thin films. The topic is worth knowing; this is not the book to learn it from, because its thin-film problems assume a level of interference theory the exam never asks for.