PiTheory

PiTheory JEE Physics Practice & Revision Pack · Class 12 · Chapter 9 · JEE Main

Ray Optics and Optical Instruments

Chapter-wise practice and revision for JEE Main: concise notes on mirrors, refraction and total internal reflection, lenses and the lens maker's formula, lens–mirror combinations, the prism, and optical instruments, concept figures, and 51 selected previous-year JEE Main questions covering the major concepts and question patterns, each with a step-by-step solution.

51 selected questions6 topics4 concept figuresPreview · first 3 of 51 questions

How to study this chapter

  1. Look at the topic's figures, then read its notes (5 to 10 minutes). They give the major equations and methods the questions use.
  2. Solve the topic's questions with a timer: 2 minutes for Easy, 3 to 4 for Medium and Hard.
  3. Check the solution. Read the Point to note line even if you were right: it is the idea you will reuse.
  4. Mark each question Got it or Revise. Come back to the Revise list after two days.

Where the marks come from

Main idea of each of the 206 previous-year JEE questions in the PiTheory bank

Lenses54
Refraction, TIR40
Mirrors38
Combinations32
Prism30
Instruments12

Contents

  1. 1Spherical and plane mirrorsQ1–Q9
  2. 2Refraction, apparent depth and total internal reflectionQ10–Q18
  3. 3Thin lenses and refraction at a curved surfaceQ19–Q28
  4. 4Lens and mirror combinations, silvered lensesQ29–Q37
  5. 5Prism, minimum deviation and dispersionQ38–Q46
  6. 6Eye, microscopes and telescopesQ47–Q51
  7. ✓Last-day checklist
01

Topic 1 · Q1–Q9

Spherical and plane mirrors

38 of the 206 JEE PYQs in the bank test this as their main idea

Look first

CFOI1/v + 1/u = 1/f, m = −v/u
Two rays fix the image.

Quick notes

$$\frac1v+\frac1u=\frac1f,\qquad f=\frac R2,\qquad m=-\frac vu=\frac{f}{f-u}$$

Distances from the pole, positive along the incident light (so real objects have $u<0$). Concave $f<0$, convex $f>0$.

  • Real image: inverted, $m<0$; virtual: erect, $m>0$. Convex mirror always gives a virtual, diminished, erect image.
  • Object and image on the same side for a real image: their distances subtract to give the gap.
  • Image speed along the axis: $v_I=m^2v_O$ (for a lens the same).
  • Small object along the axis: image length $\approx m^2\times$ length; for long rods image each end.
  • Plane mirror moved by $x$: image moves $2x$. Rotated by $\theta$: reflected ray turns $2\theta$.
  • Mirror focal length does not depend on the surrounding medium.
Trap. A car's rear-view mirror is convex.

Practice: first 3 of 9 questions

Q1JEE Main 2025 (23 Jan, Shift 2)Easy

A concave mirror of focal length $f$ in air is dipped in a liquid of refractive index $\mu$. Its focal length in the liquid will be

  1. A$\mu f$
  2. B$f$
  3. C$\dfrac{f}{\mu-1}$
  4. D$\dfrac{f}{\mu}$
Answer and solution

Answer: Option B: $f$

Concept. A mirror's focal length $f=\dfrac R2$ comes from the law of reflection alone. Reflection does not depend on the medium, so $f$ does not change.

Answer. $f$ (Option B).

Point to note. Medium does not matter.

Q2JEE Main 2026 (24 Jan, Shift 2)Easy

Distance between an object and three times magnified real image is 40 cm . The focal length of the mirror used is ______ cm .

  1. A-20
  2. B$-15/2$
  3. C-10
  4. D-15
Answer and solution

Answer: Option D: -15

Concept. A real image from a mirror is inverted, so $m=-\dfrac vu=-3$ gives $v=3u$, and both lie in front of the mirror.

Step 1. Distances. $|v|-|u|=2|u|=40$ cm, so $u=-20$ cm and $v=-60$ cm.

Step 2. Mirror formula. $\dfrac1f=\dfrac1v+\dfrac1u=-\dfrac1{60}-\dfrac1{20}=-\dfrac4{60}$, so $f=-15$ cm.

Answer. Option D.

Point to note. $v=3u$.

Q3JEE Main 2025 (4 Apr, Shift 1)Medium

The distance between an object and its image (magnified by $-\dfrac13$) is $30\ \text{cm}$. The focal length of the mirror used is $\left(\dfrac x4\right)\ \text{cm}$, where the magnitude of the value of $x$ is ______.

Numerical answer type

Answer and solution

Answer: 45

Concept. $m=-\dfrac13$: real, inverted, diminished, so $v=\dfrac u3$ on the same side.

Step 1. Distances. $|u|-\dfrac{|u|}{3}=30\Rightarrow u=-45$ cm, $v=-15$ cm.

Step 2. Focal length. $\dfrac1f=-\dfrac1{15}-\dfrac1{45}=-\dfrac4{45}$, so $f=-\dfrac{45}{4}$ cm.

Answer. $|x|=45$.

Point to note. $v=u/3$.

48 more questions and 5 more topics in the full chapter

  1. 2Refraction, apparent depth and total internal reflection9 questions with solutions
  2. 3Thin lenses and refraction at a curved surface10 questions with solutions
  3. 4Lens and mirror combinations, silvered lenses9 questions with solutions
  4. 5Prism, minimum deviation and dispersion9 questions with solutions
  5. 6Eye, microscopes and telescopes5 questions with solutions
  6. ✓Last-day checklist

The complete Ray Optics and Optical Instruments chapter is in the Class 12 Pack and the Complete Pack, as a PDF on WhatsApp. Want to see a whole chapter first? Laws of Motion and Current Electricity are free in full.

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