Explore Key Concepts and Solutions for Class 12 Physics Wave Optics
FAQs on Master Wave Optics with Free Class 12 NCERT Book & Solutions (2025-26)
1. What are the most important topics in Wave Optics for the CBSE Class 12 Board Exam 2025-26?
For the CBSE Class 12 Physics exam, the most important topics in Wave Optics that are frequently asked include:
Huygens' Principle: Its statement and using it to prove the laws of reflection and refraction.
Interference: Derivation for fringe width in Young's Double-Slit Experiment (YDSE), conditions for constructive and destructive interference, and conditions for sustained interference.
Diffraction: Diffraction due to a single slit, the width of the central maximum, and the conditions for minima and maxima.
Polarisation: Brewster's law, and the concept of plane-polarised light.
Students should focus on the derivations from these topics, as they often appear as 3-mark or 5-mark questions.
2. How is the marks weightage for Wave Optics in the Class 12 Physics board exam?
As per the CBSE 2025-26 syllabus, the unit 'Optics,' which includes both Ray Optics and Wave Optics, carries a very high weightage. Typically, questions from Wave Optics, including derivations, numericals, and conceptual problems, contribute significantly to this total. You can expect questions ranging from 1-mark MCQs to 5-mark long-answer questions from this chapter.
3. State Huygens' principle. How can it be used to derive the law of reflection?
Huygens' principle is a key concept often asked for 3 or 5 marks. It states that:
Every point on a given wavefront acts as a fresh source of new disturbances, called secondary wavelets, which travel in all directions with the speed of light in the medium.
The forward envelope, or the surface tangent to these secondary wavelets at any instant, gives the position of the new wavefront at that instant.
To prove the law of reflection (i = r), we consider a plane wavefront AB incident on a reflecting surface XY. By drawing secondary wavelets from points A and B and applying geometric principles using congruent triangles, we can demonstrate that the angle of incidence is equal to the angle of reflection. This is a crucial derivation for board exams.
4. Why are two independent light sources, like two separate bulbs, considered incoherent?
This is a frequently asked conceptual question. Two independent light sources are incoherent because they cannot maintain a constant phase difference between the light waves they emit. Light from a source is produced by the random de-excitation of billions of atoms. Even in two identical bulbs, these atomic emissions are independent and random, causing the phase difference between the waves from the two sources to fluctuate rapidly and randomly. For sustained interference, a constant phase difference is a necessary condition, which cannot be met by independent sources.
5. What is a key difference between the interference pattern and the diffraction pattern in terms of fringe width?
A key difference, often asked in board exams to test conceptual clarity, lies in the fringe width:
In an interference pattern (like in YDSE), all the bright and dark fringes are of equal width.
In a diffraction pattern (due to a single slit), the fringes are of unequal width. The central bright fringe is the widest and brightest, while the secondary bright fringes become progressively narrower and less intense as their distance from the centre increases.
6. What is the formula for fringe width in Young's Double-Slit Experiment (YDSE), and what type of numericals are expected?
The expression for the fringe width (β) in YDSE is a very important derivation, and the formula is central to solving numericals. The formula is: β = λD/d, where:
λ is the wavelength of the light used.
D is the distance between the slits and the screen.
d is the distance between the two slits.
For the 2025-26 exam, you can expect numericals asking you to calculate the fringe width, the wavelength, or the change in fringe width when D, d, or λ is altered.
7. What happens to the interference fringes in a YDSE if the entire apparatus is immersed in a transparent liquid like water?
This is a higher-order thinking skills (HOTS) question. When the YDSE setup is immersed in water (or any medium with refractive index 'n'), the wavelength of light decreases. The new wavelength (λ') becomes λ/n, where λ is the wavelength in a vacuum or air. Since the fringe width is directly proportional to the wavelength (β = λD/d), the fringe width will decrease. The interference fringes will become narrower and appear closer together.
8. State Brewster's Law of Polarisation. At what angle is reflected light completely plane-polarised?
Brewster's Law states that for a particular angle of incidence, the reflected light is completely plane-polarised. This angle, known as the polarising angle (iₚ) or Brewster's angle, is such that the tangent of the polarising angle is equal to the refractive index (n) of the medium.
Mathematically: n = tan(iₚ).
At this angle, the reflected and the refracted rays are perpendicular to each other. This is a very important 2-mark question for the Class 12 board exam.
9. To score full marks in Wave Optics, which question types should I prioritise: derivations, numericals, or conceptual questions?
For a strong performance in Wave Optics in the CBSE 2025-26 exam, a balanced approach is crucial, but with a specific focus:
Derivations (High Priority): Questions like proving laws of reflection/refraction using Huygens' principle, deriving fringe width in YDSE, and showing the width of the central maximum in diffraction are frequently asked as 3 or 5-mark questions. Master them first.
Conceptual Questions (High Priority): Focus on 'why' questions, such as conditions for interference, the difference between interference and diffraction, and effects of changing parameters in YDSE. These are common in 1, 2, and 3-mark sections.
Numericals (Medium Priority): Numericals are generally formula-based, primarily from YDSE (β = λD/d) and diffraction. They are important but usually straightforward if you know the formulas and concepts.
Prioritising derivations and concepts will build a strong foundation to solve any type of question from this chapter.
