Last Updated: April 2026
Optics is one of the most scoring units in NEET Physics, contributing 3-5 questions per year from two chapters: Ray Optics (Class 12 Chapter 9) and Wave Optics (Class 12 Chapter 10). With clear concepts, a good formula sheet, and systematic practice, students can consistently score full marks in this unit. This guide covers every key concept with derivations, formulas, and practice problems.
NEET Optics — Weight and Coverage
| Chapter | NEET Questions/Year | Class | NCERT Chapter |
|---|---|---|---|
| Ray Optics and Optical Instruments | 2-3 | Class 12 | Chapter 9 |
| Wave Optics | 1-2 | Class 12 | Chapter 10 |
| Total Optics | 3-5 | — | — |
Part 1: Ray Optics
Reflection from Curved Mirrors
Key formulas for mirrors (sign convention: distances measured from pole):
- Mirror formula: 1/v + 1/u = 1/f
- Magnification: m = -v/u = h’/h
- Focal length: f = R/2 (R = radius of curvature)
- For concave mirror: f is negative; for convex: f is positive (New Cartesian sign convention)
Refraction — Snell’s Law
Snell’s Law: n₁ sin θ₁ = n₂ sin θ₂
Where n₁, n₂ are refractive indices of the two media and θ₁, θ₂ are angles with the normal.
Total Internal Reflection (TIR)
TIR occurs when light travels from a denser medium to a rarer medium and the angle of incidence exceeds the critical angle.
Critical angle formula: sin θc = n₂/n₁ (where n₁ > n₂)
Applications: Optical fibres, mirage, diamond sparkle, endoscopes
Refraction through Prisms
Prism formula: μ = sin[(A+D)/2] / sin(A/2)
Where A = angle of prism, D = angle of minimum deviation, μ = refractive index
Thin Lens Formula
- Lens formula: 1/v – 1/u = 1/f
- Lens maker’s equation: 1/f = (μ-1)(1/R₁ – 1/R₂)
- Power of lens: P = 1/f (in dioptres, f in metres)
- Combined power: P = P₁ + P₂ + P₃…
- Magnification: m = v/u
Optical Instruments
| Instrument | Magnification | Key Feature |
|---|---|---|
| Simple Microscope | m = 1 + D/f (near point), m = D/f (far point) | D = 25 cm (least distance of distinct vision) |
| Compound Microscope | m = L/fo × D/fe | L = tube length, fo = objective focal length |
| Astronomical Telescope | m = -fo/fe (at infinity) | Inverted image |
| Terrestrial Telescope | m = fo/fe | Erect image (erecting lens added) |
Part 2: Wave Optics
Huygens’ Principle
Every point on a wavefront acts as a secondary source of spherical wavelets. The new wavefront is the common tangent to all these secondary wavelets.
Young’s Double Slit Experiment (YDSE)
Key formulas:
- Fringe width: β = λD/d (λ = wavelength, D = screen distance, d = slit separation)
- Position of bright fringe: y_n = nλD/d
- Position of dark fringe: y_n = (2n-1)λD/(2d)
- Condition for constructive interference: Path difference Δ = nλ
- Condition for destructive interference: Δ = (2n-1)λ/2
Diffraction
Bending of light around obstacles. In single-slit diffraction:
- Central maxima width = 2λD/a (a = slit width, D = screen distance)
- First minimum at angle θ where a sin θ = λ
Polarisation
Light is a transverse wave — polarisation confirms this. Malus’ Law: I = I₀ cos²θ (θ = angle between polariser and analyser)
Important NEET Problems and Solutions
Problem 1: An object is placed 15 cm from a concave mirror of focal length 10 cm. Find the image position and magnification.
Solution: Using 1/v + 1/u = 1/f; u = -15 cm, f = -10 cm
1/v = 1/f – 1/u = 1/(-10) – 1/(-15) = -1/10 + 1/15 = (-3+2)/30 = -1/30
v = -30 cm (real, inverted image on same side as object)
m = -v/u = -(-30)/(-15) = -2 (magnification = 2, inverted)
Problem 2: In YDSE, λ = 500 nm, D = 1 m, d = 0.5 mm. Find the fringe width.
Solution: β = λD/d = (500×10⁻⁹ × 1)/(0.5×10⁻³) = 500×10⁻⁹/5×10⁻⁴ = 10⁻³ m = 1 mm
Problem 3: Find the critical angle for glass (μ = 1.5) in air.
Solution: sin θc = 1/μ = 1/1.5 = 2/3; θc = sin⁻¹(2/3) ≈ 42°
Practice MCQs — NEET Physics Optics 2027
Q1. A convex lens has a focal length of 20 cm. Its power is:
(A) 0.05 D (B) 5 D (C) 20 D (D) 0.2 D
Answer: (B) P = 1/f = 1/0.20 = 5 D
Q2. When monochromatic light is used in YDSE, fringe width increases when:
(A) Wavelength decreases (B) Distance between slits increases (C) Wavelength increases (D) Screen moves closer
Answer: (C) β = λD/d — fringe width is directly proportional to wavelength
Q3. An optical fibre works on the principle of:
(A) Refraction (B) Diffraction (C) Total Internal Reflection (D) Polarisation
Answer: (C)
Q4. Malus’ Law states I = I₀ cos²θ. If the intensity reduces to I₀/4 after passing through an analyser, the angle θ between polariser and analyser is:
(A) 30° (B) 45° (C) 60° (D) 90°
Answer: (C) cos²θ = 1/4 → cos θ = 1/2 → θ = 60°
Q5. The image formed by a plane mirror is:
(A) Real and inverted (B) Virtual and erect (C) Real and erect (D) Virtual and inverted
Answer: (B)
Frequently Asked Questions
How many questions from Optics appear in NEET every year?
NEET Physics Optics (Ray Optics + Wave Optics combined) typically contributes 3-5 questions per year. Ray Optics (Chapter 9) usually has 2-3 questions focusing on mirrors, lenses, refraction, TIR, and optical instruments. Wave Optics (Chapter 10) contributes 1-2 questions on YDSE, interference, diffraction, and polarisation.
Which is more important for NEET — Ray Optics or Wave Optics?
Ray Optics carries slightly more NEET weightage than Wave Optics. However, both are important. In Ray Optics, focus heavily on lens-mirror formula numericals, TIR, and the prism formula. In Wave Optics, YDSE fringe width calculations, conditions for constructive/destructive interference, and Malus’ Law are most frequently tested in NEET.
What sign convention should I use for NEET Optics?
NCERT and NEET use the New Cartesian Sign Convention: all distances are measured from the pole (for mirrors) or optical centre (for lenses); distances in the direction of incident light are positive; distances against the direction of incident light are negative; heights above the principal axis are positive; heights below are negative. Stick consistently to this convention to avoid sign errors.
Also explore: NEET Physics Laws of Motion | Free NEET Mock Test | NEET 2027 Full Syllabus