--- title: "Distinguishing plane, circular, elliptical light" subject: "Engineering Physics" chapter: 4 key: "semester_1/engineering_physics:distinguish-polarized-nicol-wave-plate" source: https://www.bctengineer.com/q/semester_1/engineering_physics/distinguish-polarized-nicol-wave-plate site: BCT Engineering --- # Distinguishing plane, circular, elliptical light Engineering Physics · Chapter 4: Physical Optics - Asked in: 2074 Chaitra, 5 marks - Video: [Last Lecture 57 | 3rd Semester | Analysis of plane, circularlly & Elliptically polarized light](https://www.youtube.com/watch?v=nVHgYKFZEv0) — Physics for UnderGraduates - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_1/engineering_physics/distinguish-polarized-nicol-wave-plate - Whole chapter: https://www.bctengineer.com/md/semester_1/engineering_physics/chapter-4 ## Question How can you distinguish the plane, circularly and elliptically polarized light by using nicol prism and wave plate? ## Short answer Rotate Nicol alone: extinction twice per turn indicates plane polarization; constant intensity suggests circular or unpolarized light. Insert a quarter-wave plate: circular becomes plane and shows extinction; elliptical with QWP axis along ellipse axis becomes plane and extinguishes. No extinction means unpolarized or partial. ## Answer Plane, circularly and elliptically polarized light can be told apart using a Nicol prism (as analyzer) and a quarter-wave plate (QWP). The same tests also separate them from unpolarized and partially polarized light. ```text beam --> [QWP] --> [Nicol analyzer, rotated] --> eye ``` #### Step 1: Nicol alone Rotate the Nicol through 360° and watch the transmitted intensity. | Observation | Light is | |---|---| | Intensity varies; **zero** twice per turn | Plane polarized | | Intensity varies between max and min, **min not zero** | Elliptical (or partially polarized) | | Intensity **constant** | Circular (or unpolarized) | #### Step 2: Quarter-wave plate in front of the Nicol 1. **Plane polarized**: with the QWP's axis parallel or perpendicular to the vibration, the light stays plane polarized, so extinction by the Nicol remains. At other angles the QWP makes it elliptical and extinction is lost. 2. **Circular**: the QWP introduces a further π/2 phase difference, giving δ = π or 0, and so converts it to **plane** polarized light. On rotating the Nicol, complete extinction occurs twice per turn. Unpolarized light stays unpolarized, so there is no extinction. 3. **Elliptical**: with the QWP's axis along a principal axis of the ellipse, the light becomes plane polarized, and rotating the Nicol now gives complete extinction at some angle. For partially polarized light the minimum never becomes zero. #### Summary - Extinction with the Nicol alone: plane. - Constant with the Nicol, extinction after the QWP: circular. - Minimum not zero with the Nicol, extinction after the QWP (axis set along the ellipse axis): elliptical. - No extinction in any case: unpolarized or partially polarized. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Rotating a Nicol analyzer through 360° shows intensity going to zero twice. The light is: A. Circular B. Plane polarized C. Unpolarized D. Elliptical Answer: B. Complete extinction twice per turn is characteristic of plane polarized light. (Key point: Nicol alone: plane) 2. Intensity stays constant on rotating the Nicol. The light may be: A. Plane polarized B. Elliptical C. Circular or unpolarized D. Only plane polarized Answer: C. Both circular and unpolarized light give constant intensity, so a further test with a QWP is needed. (Key point: Nicol alone: constant) 3. What does a quarter-wave plate do to circularly polarized light? A. Converts it to plane polarized light, so extinction occurs on rotating the Nicol B. Leaves it circular C. Converts it to unpolarized light D. Makes it elliptical Answer: A. The extra π/2 phase difference gives δ = 0 or π, which is plane polarization. (Key point: Circular light after QWP) 4. Which test separates elliptical light from partially polarized light? A. The Nicol minimum is never zero in both cases, so no test is possible B. With the QWP axis along the ellipse axis, elliptical light gives complete extinction but partially polarized light does not C. Elliptical light never gives extinction D. Partially polarized light gives extinction after the QWP Answer: B. The QWP converts elliptical light to plane light (extinction possible), while partially polarized light still has a nonzero minimum. (Key point: Elliptical vs partially polarized) ## Related in Engineering Physics - [Production of plane, circular, elliptical light](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Apolarized-light-production-relations) — Engineering Physics, chapter 4, asked 3× - [Nicol prism as polarizer and analyzer](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Anicol-prism-polarizer-analyzer) — Engineering Physics, chapter 4, asked 3× - [Double refraction, linear and circular light](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adouble-refraction-produce-polarized-light) — Engineering Physics, chapter 4 - [Quarter wave plate thickness, quartz](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aquarter-wave-plate-quartz) — Engineering Physics, chapter 4 - [Quarter wave plate thickness numerical](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aquarter-wave-plate-thickness) — Engineering Physics, chapter 4 ## Related in other subjects - [Plane wave in free space, find β and H](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Az-polarized-wave-find-beta-h) — Electromagnetism, chapter 4 - [Circular well 4 m, light and heavy well](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acircular-well-4m-light-heavy) — Foundation Engineering, chapter 9