NEET Physics — Optics Complete Chapter Guide

Chapter Overview & Weightage

Optics covers ray optics (reflection, refraction, lenses, optical instruments) and wave optics (interference, diffraction, polarization). NEET asks 4-5 questions, with a heavy tilt towards ray optics.

🎯 Exam Insider

Optics carries 8-10% weightage in NEET Physics — 4-5 questions. Mirror/lens formula applications and refraction (Snell's law, TIR) are tested every year. Wave optics questions are typically 1 per paper.

Key Concepts You Must Know

Tier 1 (Core)

Mirror formula: $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ with sign convention
Lens formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
Magnification: mirrors $m = -v/u$ , lenses $m = v/u$
Snell's law: $n_1 \sin i = n_2 \sin r$
Total Internal Reflection: critical angle $\sin C = n_2/n_1$ (when $n_1 > n_2$ )
Power of lens: $P = 1/f$ (in metres), unit: dioptre

Tier 2 (Frequently tested)

Lens maker's equation: $\frac{1}{f} = (n-1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$
Combination of thin lenses: $\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}$
Young's double slit: fringe width $\beta = \lambda D/d$ , bright fringe condition $d\sin\theta = n\lambda$
Prism: $\delta_{min}$ when $r_1 = r_2 = A/2$ , and $n = \frac{\sin\frac{A+\delta_m}{2}}{\sin\frac{A}{2}}$
Human eye defects: myopia (concave lens), hypermetropia (convex lens)

Important Formulas

Mirror and Lens Formulas

Mirror: $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$

Lens: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

Sign convention (Cartesian): distances measured from the pole/optical centre. Along the incident light = positive. Against = negative. Object is usually on the left → $u$ is negative.

Magnification: Mirror: $m = -\frac{v}{u}$ . Lens: $m = \frac{v}{u}$

Refraction and TIR

Snell's law: $n_1 \sin i = n_2 \sin r$

Critical angle: $\sin C = \frac{n_2}{n_1}$ (for $n_1 > n_2$ , light going from denser to rarer)

TIR condition: angle of incidence > critical angle AND light travelling from denser to rarer medium

Young's Double Slit Experiment

Fringe width: $\beta = \frac{\lambda D}{d}$

Where $\lambda$ = wavelength, $D$ = distance to screen, $d$ = slit separation

Bright fringes: $d \sin\theta = n\lambda$ ( $n = 0, 1, 2, ...$ ) Dark fringes: $d \sin\theta = (2n-1)\frac{\lambda}{2}$

💡 Expert Tip

Sign convention is the #1 source of errors in optics problems. For NEET, always use Cartesian convention and write $u$ as negative (object on the left). If you get a negative $v$ for a mirror, the image is real (in front of mirror). If positive, the image is virtual (behind mirror).

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: An object is placed 30 cm in front of a concave mirror of focal length 20 cm. Find the position and nature of the image.

Solution:

$u = -30$ cm, $f = -20$ cm (concave mirror)

$\frac{1}{v} + \frac{1}{-30} = \frac{1}{-20}$

$\frac{1}{v} = -\frac{1}{20} + \frac{1}{30} = \frac{-3 + 2}{60} = -\frac{1}{60}$

$v = \mathbf{-60}$ cm

The image is real (negative $v$ , same side as object), inverted, and magnified ( $m = -v/u = -(-60)/(-30) = -2$ ).

PYQ 2 — NEET 2023

Problem: The critical angle for glass-air interface is 42 degrees. What is the refractive index of glass?

Solution:

$\sin C = \frac{n_{\text{air}}}{n_{\text{glass}}} = \frac{1}{n}$

$n = \frac{1}{\sin 42°} = \frac{1}{0.669} \approx \mathbf{1.49}$

PYQ 3 — NEET 2022

Problem: In Young's double slit experiment, if the slit separation is halved, the fringe width:

(A) Halves (B) Doubles (C) Remains same (D) Quadruples

Solution:

$\beta = \lambda D / d$ . If $d$ becomes $d/2$ :

$\beta' = \lambda D / (d/2) = 2\lambda D/d = 2\beta$

Fringe width doubles.

Answer: (B) Doubles

Expert Strategy

Week 1: Ray optics — mirror formula, lens formula, sign convention. Do 20-25 numerical problems until sign convention becomes automatic. Practice with concave/convex mirrors and converging/diverging lenses.

Week 2: Refraction, TIR, prisms. Snell's law applications, critical angle calculations. For prisms, know the minimum deviation formula.

Week 3: Wave optics — Young's double slit is the main topic. Know the fringe width formula and what happens when you change $\lambda$ , $D$ , or $d$ .

💡 Expert Tip

For NEET, ray optics problems are 3-4x more common than wave optics. If short on time, prioritise mirror/lens formula and refraction over interference/diffraction. But don't skip YDSE entirely — one question is almost guaranteed.

Common Traps

⚠️ Common Mistake

Trap 1 — Mirror formula has $+$ between $1/v$ and $1/u$ ; lens formula has $-$ . Writing the wrong sign in the formula gives completely wrong answers. Mirror: $1/v + 1/u = 1/f$ . Lens: $1/v - 1/u = 1/f$ .

⚠️ Common Mistake

Trap 2 — TIR only occurs when light goes from denser to rarer medium. Light going from air to glass CANNOT undergo TIR. It must go from glass to air (higher to lower refractive index). NEET tests this condition explicitly.

⚠️ Common Mistake

Trap 3 — Power of a lens is in dioptres when f is in metres. If $f = 20$ cm $= 0.2$ m, then $P = 1/0.2 = 5$ D. Students who forget to convert cm to m get the wrong power by a factor of 100.

⚠️ Common Mistake

Trap 4 — Fringe width is inversely proportional to slit separation. Increasing $d$ decreases fringe width (fringes become closer together). Increasing wavelength or screen distance increases fringe width.