CBSE Class 12 Physics: Ray Optics and Wave Optics — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Derive the Lens Maker's Formula.
  Ans: Consider a thin lens of refractive index n₂ in medium n₁, with radii of curvature R₁ and R₂. For refraction at first surface: n₁/u + n₂/v₁ = (n₂-n₁)/R₁. For refraction at second surface: n₂/v₁ + n₁/v = (n₁-n₂)/R₂. Adding: n₁/v - n₁/u = (n₂-n₁)(1/R₁ - 1/R₂). For object at infinity (u=∞), v=f: 1/f = (n₂/n₁ - 1)(1/R₁ - 1/R₂). For lens in air: 1/f = (μ-1)(1/R₁ - 1/R₂).
• Q: Explain Young's Double Slit Experiment. Derive the expression for fringe width.
  Ans: Setup: Monochromatic light passes through two narrow slits S₁, S₂ separated by d. Interference pattern on screen at distance D. At point P, path difference = S₂P - S₁P = yd/D (for small angles). Bright fringes: yd/D = nλ → y_n = nλD/d. Dark fringes: yd/D = (2n-1)λ/2. Fringe width β = y_n+1 - y_n = λD/d. β is same for bright and dark fringes. Wider slits (smaller d) → wider fringes.
• Q: Define total internal reflection. Explain the principle of optical fibre.
  Ans: Total internal reflection occurs when light travels from a denser to rarer medium and the angle of incidence exceeds the critical angle. Conditions: (1) Light must travel from denser to rarer medium. (2) Angle of incidence > critical angle (sin C = n₂/n₁). Optical fibre: Uses total internal reflection to transmit light over long distances. Core (high n) surrounded by cladding (lower n). Light enters and undergoes repeated total internal reflections, staying within the core. Used in communication, endoscopy.
• Q: Differentiate between interference and diffraction.
  Ans: Interference: Superposition of waves from TWO coherent sources. Bright and dark fringes are equally spaced. All bright fringes have equal intensity. Minimum two sources needed. Diffraction: Bending of waves around edges of a single obstacle/slit. Central maximum is broadest and brightest. Intensity decreases for successive maxima. Single slit is sufficient. Both are based on superposition principle.

Long Answer / Application Questions (4-6 Marks)

• Q: An object is placed 15 cm from a convex lens of focal length 10 cm. Find image position, nature, and magnification.
  Ans: Using lens formula: 1/v - 1/u = 1/f. u = -15 cm, f = +10 cm. 1/v = 1/10 + 1/(-15) = 1/10 - 1/15 = (3-2)/30 = 1/30. v = +30 cm (real image, on other side). Magnification m = v/u = 30/(-15) = -2 (inverted, magnified twice).

Exam Tips for This Chapter

• Revise all definitions and laws from Ray Optics and Wave Optics — commonly asked as 1-2 mark questions
• Practice diagrams related to Ray Optics and Wave Optics — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Ray Optics and Wave Optics carries significant marks in the board exam