Optics (NEET Physics)
Why This Chapter Matters
Optics carries 6-10 marks in NEET Physics. Ray optics (mirrors, lenses), wave optics (diffraction, interference), and optical instruments are all tested with numerical problems.
Core Concepts
1. Reflection (Mirrors)
Mirror formula: 1/v + 1/u = 1/f | f = R/2
Magnification: m = -v/u = hi/ho
Sign convention: All distances from pole. Incident light direction = positive.
Concave mirror: f negative | Convex mirror: f positive
Concave mirror uses: Shaving mirrors, searchlights, solar concentrators, ENT doctors.
Convex mirror uses: Rear-view mirrors, security mirrors (wider field of view, always virtual/erect/diminished).
2. Refraction (Snell's Law)
n₁ sinθ₁ = n₂ sinθ₂
n = c/v = speed of light in vacuum / speed in medium
Critical angle (θc): When refracted angle = 90°. sinθc = n₂/n₁ (for n₁ > n₂)
Total Internal Reflection: When angle of incidence > θc. Used in optical fibres.
Refraction through prism:
Refractive index n = sin((A+D_min)/2) / sin(A/2)
where A = prism angle, D_min = minimum deviation
Dispersion: Different colours refract at different angles. Violet bends most, Red least.
Rainbow: refraction + TIR + dispersion in water droplets.
3. Thin Lens Formula
1/v - 1/u = 1/f (same sign convention as mirror)
Magnification: m = v/u
Power: P = 1/f (f in metres) | Unit: Dioptre (D)
Lens maker's equation:
1/f = (n-1)[1/R₁ - 1/R₂]
where n = refractive index of lens, R₁ R₂ = radii of curvature.
Combined lenses: 1/f = 1/f₁ + 1/f₂ | P = P₁ + P₂
4. Optical Instruments
Simple microscope: m = 1 + D/f (D = 25 cm, least distance of distinct vision)
Compound microscope: m = mo × me = (L/fo)(1 + D/fe)
where L = image distance from objective, fo fe = focal lengths
Telescope (refracting): m = fo/fe (angular magnification)
Length = fo + fe
5. Wave Optics
Wavefront: Locus of points at same phase.
Huygens' principle: each point on wavefront is secondary source of new waves.
Young's Double Slit Experiment (YDSE):
Fringe width: β = λD/d
where λ = wavelength, D = screen distance, d = slit separation
Bright fringe: path difference = nλ (n = 0, 1, 2...)
Dark fringe: path difference = (2n-1)λ/2
Diffraction:
Single slit: first minimum at sinθ = λ/a (a = slit width)
Diffraction grating: dsinθ = nλ (d = grating spacing)
PYQs (NEET)
NEET 2023: A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 40 cm. Power of combination?
P = 1/0.2 + 1/(-0.4) = 5 - 2.5 = +2.5 D (converging combination)
NEET 2022: In YDSE, if wavelength is doubled and slit separation halved, fringe width becomes:
β = λD/d → new β = (2λ)D/(d/2) = 4λD/d = 4β. Fringe width becomes 4 times.
NEET 2021: Critical angle for total internal reflection in glass-air interface (n_glass = 1.5)?
sinθc = 1/n = 1/1.5 = 2/3 → θc = sin⁻¹(2/3) ≈ 41.8°