Ray Optics for NEET: Unit 16 Formulas, Signs and Traps
Ray optics is the part of physics that treats light as straight rays and works out where mirrors, lenses and prisms send them. For NEET it is the first half of Unit 16, Optics: reflection at spherical mirrors, refraction at plane and curved surfaces, thin lenses, total internal reflection, the prism, and the microscope and telescope. This guide takes ray optics for NEET phrase by phrase, gives every formula with the sign rule that makes it work, uses NCERT's own numbers for critical angles, and ends with a revision plan you can start this week.
What is ray optics in the NEET syllabus?
Ray optics in NEET is the list printed at the start of Unit 16, before the line that begins "Wave optics". The Syllabus for NEET UG 2026 from the National Medical Commission, the file the official NEET site links for the 2026 exam, puts it in two dense sentences. Split into phrases, here is what each one expects from you.
| Syllabus phrase (Unit 16) | What you should be able to do |
|---|---|
| Reflection of light, spherical mirrors, mirror formula | Use 1/v + 1/u = 1/f with signs, and f = R/2 |
| Refraction of light at plane and spherical surfaces | Use Snell's law and n₂/v - n₁/u = (n₂ - n₁)/R |
| Thin lens formula and lens maker formula | Use 1/v - 1/u = 1/f and 1/f = (n - 1)(1/R₁ - 1/R₂) |
| Total internal reflection and its applications | Use sin ic = 1/n and explain optical fibres and diamonds |
| Magnification | Use m = -v/u for mirrors and m = v/u for lenses |
| Power of a lens | Use P = 1/f with f in metres, in dioptres |
| Combination of thin lenses in contact | Add powers: P = P₁ + P₂ |
| Refraction of light through a prism | Use the minimum deviation formula and Dm = (n - 1)A |
| Microscope and astronomical telescope (reflecting and refracting) and their magnifying powers | Use the simple, compound and telescope formulas |
The printed syllabus spells "mirror formula" as "morror formula". It is a typo, not a new topic.
Is ray optics still in NEET 2027? As far as anyone can say today, yes. The NEET (UG) 2026 Information Bulletin answers its own FAQ with "there is no change in the syllabus of NEET (UG) 2026 with respect to the previous year", and the NEET public notices page showed no 2027 notice when we checked on 28 October 2026. If the 2027 syllabus moves anything in Unit 16, our NEET 2027 deleted syllabus guide is where we will flag it.
How many marks can ray optics decide?
There is no official number. The bulletin fixes the paper, not the chapter split: Physics has 45 questions worth 180 of the 720 marks, every correct answer earns plus four, every wrong one costs one mark, and a blank scores zero. NTA has never published how many questions each unit gets.
What you can say is how the chapter behaves. Ray optics questions are short calculations where a single wrong sign flips the answer to a distractor that the setter has placed on purpose. So the chapter rewards care far more than speed, and a guess here is expensive: a wrong option moves you 5 marks away from a right one. Our NEET negative marking guide shows when a guess is worth the risk, and our NEET Physics questions guide explains how all 45 questions are scored.
Why does the sign convention decide every answer?
Because the formulas in ray optics are only true with signs attached. NCERT uses the Cartesian sign convention, and the NCERT Class 12 Physics chapter on ray optics states it in four rules:
- Measure every distance from the pole of the mirror or the optical centre of the lens.
- Distances in the same direction as the incident light are positive.
- Distances against the incident light are negative.
- Heights above the principal axis are positive, heights below it negative.
Draw the light coming in from the left every time and these rules settle themselves. A real object in front of a mirror or lens then always has a negative u. A concave mirror has a negative f, a convex mirror a positive f. A convex lens has a positive f, a concave lens a negative one.
Here is the habit that saves marks: put the signs in when you substitute, never at the end. Write u = -15 cm, not u = 15 cm with a mental note. Most ray optics errors are not physics mistakes at all. They are a minus sign that went missing between the question and the formula.
Ray optics formulas, each with its condition
Fewer than they look. Every formula below is in the NCERT chapter, and the right hand column is the part that most formula lists drop: when each one holds.
| Quantity | Formula | Holds when |
|---|---|---|
| Reflection | angle i = angle r | Any surface, at the point of incidence |
| Snell's law | n₁ sin i = n₂ sin r | Any pair of media |
| Mirror equation | 1/v + 1/u = 1/f | Paraxial rays, Cartesian signs |
| Focal length of a mirror | f = R/2 | Approximately, for small apertures |
| Mirror magnification | m = h′/h = -v/u | Negative m means an inverted image |
| Single curved surface | n₂/v - n₁/u = (n₂ - n₁)/R | Light going from medium 1 into medium 2 |
| Thin lens equation | 1/v - 1/u = 1/f | Thin lens, Cartesian signs |
| Lens maker formula | 1/f = [(n₂ - n₁)/n₁](1/R₁ - 1/R₂) | Thin lens of index n₂ in a medium of index n₁ |
| Lens magnification | m = v/u | Same sign rule as above |
| Power | P = 1/f | f in metres, P in dioptres |
| Lenses in contact | P = P₁ + P₂ + P₃ | Thin lenses touching each other |
| Critical angle | sin ic = 1/n | Light going from the denser to the rarer medium |
| Prism | n = sin[(A + Dm)/2] / sin(A/2) | At minimum deviation Dm |
| Thin prism | Dm = (n - 1)A | Small prism angle A |
| Simple microscope | m = 1 + D/f, or D/f for image at infinity | D = 25 cm, the near point |
| Compound microscope | m ≈ (L/fo)(D/fe) | L between the focal points |
| Telescope | m = fo/fe | Final image at infinity |
Look at the two magnification formulas side by side. The mirror has a minus sign and the lens does not, because the mirror sends the light back the way it came. Our NEET physics formula sheet puts the key optics lines next to all 20 Physics units; use it for the wide view and this page for ray optics in depth.
Mirrors and lenses: two worked examples
Two short examples show the routine. Both are our own numbers.
Concave mirror. An object stands 15 cm in front of a concave mirror of focal length 10 cm. With signs, u = -15 cm and f = -10 cm.
- 1/v = 1/f - 1/u = -1/10 + 1/15 = -1/30, so v = -30 cm.
- m = -v/u = -(-30)/(-15) = -2.
The image is 30 cm in front of the mirror, real (v is negative, on the same side as the object), inverted (m is negative) and twice the size of the object.
Convex lens. An object stands 30 cm from a convex lens of focal length 20 cm. Now u = -30 cm and f = +20 cm.
- 1/v = 1/f + 1/u = 1/20 - 1/30 = 1/60, so v = +60 cm.
- m = v/u = 60/(-30) = -2.
Same size and orientation as before, but on the far side of the lens, where a screen can catch it. That matches the photo at the top of this page: a glass ball acts like a strong converging lens and shows the tree upside down.
Power works the same way. A convex lens of focal length 25 cm has P = 1/0.25 = +4 D. Put a concave lens of focal length 50 cm, or -2 D, in contact with it and the pair has P = +2 D, which is a converging combination of focal length 50 cm.
When does total internal reflection happen?
Total internal reflection happens when light travelling in a denser medium meets a rarer one at an angle of incidence greater than the critical angle. Then no light is refracted out and all of it reflects back inside. The critical angle depends only on the refractive index: sin ic = 1/n for a medium against air.
NCERT's Table 9.1 gives four values, and they make the pattern easy to see.
Diamond's small critical angle means light that enters a well cut stone reflects inside it again and again before it escapes, which is the sparkle. Crown glass has a critical angle below 45 degrees, so a right angled glass prism can turn light through 90 or 180 degrees by total internal reflection; NCERT uses this for totally reflecting prisms. Optical fibres work on the same rule: a glass core coated with a material of lower refractive index carries light along even when the fibre bends.
You can check the table yourself. For water, 1/1.33 is about 0.752, and the angle whose sine is 0.752 is about 48.75 degrees. Two facts sit behind every number in ray optics: light in a vacuum travels at exactly 299,792,458 m/s, the value NIST lists as exact, and a refractive index is simply how many times slower light goes in the medium. Light in water, at n = 1.33, moves at about three quarters of that speed.
How does a prism deviate light?
A prism bends light twice, once at each face, and the total bend is the angle of deviation. As you increase the angle of incidence, the deviation first falls, reaches a minimum Dm, and then rises again. At minimum deviation the ray passes through the prism symmetrically, and that gives the formula NEET uses most:
n = sin[(A + Dm)/2] / sin(A/2)
where A is the angle of the prism. For a thin prism the sines are close to the angles, and the formula shrinks to Dm = (n - 1)A. A glass prism of n = 1.5 and A = 5 degrees deviates light by only 2.5 degrees, which is why NCERT notes that thin prisms do not deviate light much.
The syllabus line says "refraction of light through a prism" and no more. Dispersion gets a one-line definition in the NCERT summary, as the splitting of light into its colours, but the NEET syllabus does not name it separately, so learn the definition and do not go hunting for long derivations.
Microscope and telescope magnifying power
Learn three formulas and the one number behind them, D = 25 cm, the least distance of distinct vision.
- Simple microscope: m = 1 + D/f when the image is at the near point, and m = D/f when it is at infinity. NCERT notes that a magnification of six needs a convex lens of focal length 5 cm, since 1 + 25/5 = 6.
- Compound microscope: m = mo × me, roughly (L/fo)(D/fe), where L is the distance between the focal points of the objective and the eyepiece. Both lenses have short focal lengths.
- Astronomical telescope: m = fo/fe in normal adjustment. Here the objective has the long focal length. A telescope with fo = 100 cm and fe = 5 cm gives m = 20.
The syllabus asks for both reflecting and refracting telescopes, and NCERT gives the reason modern telescopes use mirrors: a mirror has no chromatic aberration and can be supported across its whole back. Its examples are worth one read: the largest lens objective in use is about 1.02 m across at the Yerkes Observatory, the largest telescope in India is a 2.34 m Cassegrain reflector at Kavalur in Tamil Nadu, and the pair of Keck telescopes in Hawaii have 10 m reflectors.
Which Unit 20 experiments belong to ray optics?
Unit 20, Experimental Skills, asks for "familiarity with the basic approach and observations" of a list of experiments. Two of them are ray optics:
| Unit 20 item | What it measures | What NEET can ask |
|---|---|---|
| 13. Focal length of a convex mirror, a concave mirror and a convex lens, using the parallax method | f of each | How parallax is removed, and the sign of f |
| 14. Plot of angle of deviation against angle of incidence for a triangular prism | The deviation curve | Its shape, with a single minimum at Dm |
Questions on Unit 20 often ask what you would see rather than what you would calculate, such as the U-shaped curve of deviation against incidence. Read the observations, not only the formula.
Eight ray optics traps that cost marks
These are the places where a correct method still lands on the wrong option.
- Dropping the sign of u. A real object always has a negative u in the Cartesian convention. Put the minus sign in when you substitute.
- Mixing up the two magnification formulas. Mirror: m = -v/u. Lens: m = v/u.
- Power in centimetres. P = 1/f needs f in metres. A 25 cm lens is 4 D, not 0.04 D.
- A lens in water. The lens maker formula uses the index of the lens relative to its surroundings. NCERT's own example: a glass lens of n = 1.5 with f = 20 cm in air has f of about 78.2 cm in water.
- Total internal reflection the wrong way round. It only happens from denser to rarer, never from air into glass.
- Covering half a lens. The full image still forms, because every part of the lens focuses rays from the whole object. It is only dimmer.
- Telescope versus microscope. The telescope's objective has the long focal length; the microscope's objective has the short one.
- Minimum deviation without symmetry. The prism formula holds only at minimum deviation, where the ray inside the prism runs parallel to the base for an isosceles prism.
How should you revise ray optics for NEET?
Give ray optics one focused week, then keep it alive with short reviews. Here is a plan that fits around school or a dropper's day.
- Day 1: read the unit against the syllabus. Use the phrase table at the top of this guide and tick each line only when you can explain it out loud.
- Day 2: signs. Twenty mirror and lens problems where you write every u, v, f and R with its sign before solving.
- Day 3: refraction and total internal reflection. Snell's law, the single curved surface and the critical angle, including the four values in NCERT's Table 9.1.
- Day 4: lens maker and power. Lenses in different media and combinations in contact, all in dioptres.
- Day 5: prism, instruments and Unit 20. The minimum deviation formula, the three magnifying power formulas, and the observations for experiments 13 and 14.
- Day 6 and after: mixed and timed. Solve old NEET Physics questions on this chapter under a timer and log each wrong answer by trap number. Turn each trap into a card, as our NEET flashcards guide shows.
Past papers are the best timed practice, and our NEET previous year question papers guide shows how to use them without burning through them too early. For where this chapter sits in the whole Physics plan, see our NEET 2027 Physics preparation strategy. The same chapter-guide format covers current electricity for NEET and modern physics for NEET.
On our sister site, futureMBBS keeps a NEET syllabus overview with all three subjects on one page, a NEET 2027 chapter-wise weightage guide for setting your revision order, and a plain explainer on why NEET leans on the NCERT book. For the plan beyond one chapter, its guide on how to prepare for NEET is a sensible next read.
Want to know whether ray optics is actually costing you marks? NEETsheet's chapter-wise Physics sets and full-length mocks show which sign or formula you keep getting wrong, so your next revision session starts there. Begin at neetsheet.in.
Sources
Every syllabus line and number above comes from these documents, opened and read on 28 October 2026.
- National Medical Commission, UGMEB, public notice of 22 December 2025 with the Syllabus for NEET UG 2026, linked from neet.nta.nic.in for the 2026 examination (Physics Unit 16, Optics, and Unit 20, Experimental Skills): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/01/202601081066816297.pdf
- National Testing Agency, Information Bulletin NEET (UG) 2026 (pattern: Physics 45 questions and 180 of 720 marks; plus four, minus one and zero marking; syllabus FAQ stating no change for 2026): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/02/202602231394640855.pdf
- National Testing Agency, NEET (UG) public notices, checked on 28 October 2026 for any NEET (UG) 2027 notice: https://neet.nta.nic.in/public-notices/
- NCERT, Physics textbook for Class 12 Part II, Chapter 9, Ray Optics and Optical Instruments, reprint 2026-27 (sign convention, mirror, lens and prism formulas, Table 9.1 critical angles, Example 9.7 lens in water, microscope and telescope figures): https://ncert.nic.in/textbook/pdf/leph201.pdf
- NCERT, Physics textbook for Class 12 Part II, prelims and contents, reprint 2026-27: https://ncert.nic.in/textbook/pdf/leph2ps.pdf
- NIST, CODATA value of the speed of light in vacuum (299,792,458 m/s, exact): https://physics.nist.gov/cgi-bin/cuu/Value?c
Frequently Asked Questions
Yes, as far as anyone can say today. Ray optics is the first half of Unit 16, Optics, in the NMC syllabus for NEET UG 2026, and the 2026 bulletin says the syllabus did not change from the previous year. No NEET (UG) 2027 syllabus had been published by 28 October 2026.
Ray optics is Chapter 9, Ray Optics and Optical Instruments, in the NCERT Class 12 Physics textbook Part II. It covers mirrors, refraction, total internal reflection, lenses, the prism and the microscope and telescope, which matches the ray optics list in Unit 16 of the NEET syllabus.
NCERT uses the Cartesian sign convention. All distances are measured from the pole of the mirror or the optical centre of the lens; distances along the incident light are positive and those against it are negative. Heights above the principal axis are positive and heights below it are negative.
There is no official number. NTA publishes the subject pattern, 45 Physics questions for 180 marks in NEET (UG) 2026, but not a chapter-wise split. Treat any fixed count you see online as an estimate and prepare the whole ray optics list in Unit 16.
Diamond has a refractive index of 2.42 and a critical angle of only 24.41 degrees, against 41.14 degrees for crown glass, in NCERT's Table 9.1. Light inside a cut diamond meets its faces above that angle again and again, so it is totally internally reflected many times before it escapes.
