JEE Main Physics Optics 2027: Mirrors, Lenses, Young's Double Slit & Polarization

Optics is two chapters in one: ray optics, where geometry and sign convention rule, and wave optics, where interference and diffraction decide. Both halves are formula-friendly, and once you fix the sign convention, most numericals become mechanical.

In ray optics, sign convention is the entire game. Choose one and never switch mid-problem.

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Chapter at a Glance

Priority reflects past-paper trends, not an official NTA weightage.

What the NTA Syllabus Covers

• Reflection, spherical mirrors and mirror formula
• Refraction at plane and spherical surfaces, thin lens and lens maker's formula, total internal reflection, magnification, power, lenses in contact, prism
• Microscope and astronomical telescope with their magnifying powers
• Wave optics: Huygens' principle, Young's double slit experiment, coherent sources, single-slit diffraction, polarization, Brewster's law and Polaroids

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Master These Topics

1. Mirrors and Lenses with Cartesian Sign Convention

Measure distances from the pole or optical centre, positive along the direction of incident light.

• Mirror: 1/v + 1/u = 1/f, magnification m = −v/u
• Thin lens: 1/v − 1/u = 1/f, magnification m = v/u
• Lens maker's formula: 1/f = (n − 1)(1/R₁ − 1/R₂)
• Power P = 1/f with f in metres, in dioptres. Lenses in contact: P = P₁ + P₂.

Worked example (mirror): An object is 30 cm from a concave mirror with f = −10 cm. Then 1/v = 1/f − 1/u = −1/10 + 1/30 = −2/30, so v = −15 cm. Magnification m = −v/u = −(−15)/(−30) = −0.5. The image is real, inverted and half the object's size.

Worked example (lens): A convex lens with f = +20 cm and an object at u = −30 cm: 1/v = 1/20 − 1/30 = 1/60, so v = 60 cm and m = v/u = −2, real, inverted and twice as large.

Trap: A concave mirror has negative f and a convex lens has positive f in Cartesian convention. Mixing the two conventions is the most common source of wrong answers.

2. Refraction, TIR and Prism

Snell's law: n₁ sinθ₁ = n₂ sinθ₂. Total internal reflection needs light going from denser to rarer medium beyond the critical angle: sinC = 1/n. For n = 4/3 (water), sinC = 0.75, so C ≈ 49°.

For a thin prism, deviation is δ = (n − 1)A. For a general prism at minimum deviation, n = sin((A + δ_m)/2) / sin(A/2).

3. Young's Double Slit Experiment

Fringe width is β = λD / d, where D is the screen distance and d is slit separation. Bright fringes occur where path difference is nλ, dark where it is (n + ½)λ.

Worked example: λ = 600 nm, D = 1 m, d = 1 mm. Then β = 6 × 10⁻⁷ × 1 / 10⁻³ = 6 × 10⁻⁴ m = 0.6 mm.

For single-slit diffraction of slit width a, the central maximum has width 2λD / a.

4. Polarization

Only transverse waves can be polarized, which is evidence that light is a transverse wave. At the Brewster angle, reflected light is completely plane polarized: tanθ_B = n. For glass with n = 1.5, θ_B ≈ 56°.

5. Optical Instruments

For an astronomical telescope in normal adjustment, M = f₀ / f_e. A compound microscope has magnification roughly (L/f₀)(D/f_e), where D = 25 cm is the least distance of distinct vision.

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Common Traps to Avoid

• Using the mirror formula for a lens or the reverse.
• Forgetting that the object distance u is negative in Cartesian convention.
• Mixing up fringe width with fringe position.
• Applying Brewster's law without checking that the light travels from the rarer to the denser medium.

60-Second Revision Sheet

• Mirror 1/v + 1/u = 1/f; lens 1/v − 1/u = 1/f
• Lens maker 1/f = (n − 1)(1/R₁ − 1/R₂)
• TIR sinC = 1/n; Brewster tanθ_B = n
• YDSE β = λD/d; single-slit central width 2λD/a

Your Study Plan

1. Day 1: reflection, mirrors, sign convention practice.
2. Day 2: lenses, combinations, lens maker's formula and power.
3. Day 3: refraction, TIR, prism and optical instruments.
4. Day 4: wave optics (YDSE, diffraction, polarization), timed set.

Practice Optics Questions Free → (opens in a new tab)

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Continue Your Physics Journey

• Previous chapter: Electromagnetic Waves
• Next chapter: Dual Nature of Matter & Radiation
• All 20 JEE Main Physics chapters
• Complete JEE Main Syllabus 2027 guide

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