Optics — NEET Class 12 revision
The full NEET Optics unit in one place: concept notes on reflection, refraction, lenses, mirrors and optical instruments, differentiate tables, the doubts students trip on, quick-recall facts — plus an interactive convex-lens ray diagram you can tap through.
Explore it visually
Tap through the interactive models for this unit — each part explained in plain English with an exam-accurate NEET note.
Key concepts
NEET includes mirrors and lenses, refraction at plane and spherical surfaces, lens maker's formula, total internal reflection, prisms, microscopes and telescopes, Huygens' principle, YDSE, single-slit diffraction, Brewster's law and Polaroids.
Sign convention
Use the New Cartesian sign convention consistently. Measure distances from the pole of a mirror or the optical centre of a lens; distances measured in the direction of incident light are positive.
- Object on the incident side → u is negative
- Real image → negative magnification (inverted)
Mirror formula
Spherical mirrors relate object and image distances to the focal length.
- 1/v + 1/u = 1/f
- m = −v/u
Thin lens formula
The lens formula has a different sign structure from the mirror formula — note the minus on 1/u.
- 1/v − 1/u = 1/f
- m = v/u
Power of a lens
Lens power describes how strongly a lens converges or diverges light. It is the reciprocal of focal length in metres, and powers add for thin lenses in contact.
- P = 1/f (f in metres; P in dioptres)
- Thin lenses in contact: P_eq = P₁ + P₂
Refraction & Snell's law
Light bends toward the normal entering a denser medium, away entering a rarer one.
- n₁ sin i = n₂ sin r
Refraction at a spherical surface
For paraxial rays refracting at a spherical boundary between two media, object distance, image distance and radius of curvature are related by a standard formula.
- n₂/v − n₁/u = (n₂−n₁)/R
Lens maker's formula
The focal length of a thin lens depends on the refractive index of the lens material and the curvatures of its two surfaces.
- In air: 1/f = (μ−1)(1/R₁ − 1/R₂)
Total internal reflection
Going from a denser to a rarer medium, beyond the critical angle light is completely reflected — the basis of optical fibres.
- Conditions: light travels from denser to rarer medium and i > i_c
- sin i_c = n₂/n₁ (n₁ > n₂)
- Applications include optical fibres and totally reflecting prisms
Prism & dispersion
A prism deviates and disperses light; violet bends most, red least, spreading white light into a spectrum.
- δ = i + e − A
- δ_min: n = sin[(A+δ_min)/2] / sin(A/2)
Optical instruments
A compound microscope uses two converging lenses to magnify a nearby small object. Astronomical telescopes use a large objective and an eyepiece; the objective may be a lens (refracting) or a concave mirror (reflecting).
- Compound microscope (normal adjustment): M ≈ (L/fₒ)(D/fₑ)
- Astronomical refracting telescope (normal adjustment): |M| = fₒ/fₑ
- Reflecting telescope: the objective is a concave mirror
Huygens' principle and wavefronts
Every point on a wavefront acts as a source of secondary wavelets. The new wavefront is the common tangent to these wavelets. Huygens' construction gives the laws of reflection and refraction.
- Wavefront is a surface of equal phase
- Huygens' principle reproduces reflection and Snell's law
Interference (YDSE)
Two coherent sources with a constant phase difference produce sustained interference. In Young's double-slit experiment the bright and dark fringes are equally spaced for small angles.
- Fringe width β = λD/d
- Bright: path difference = nλ
- Dark: path difference = (n+½)λ
Diffraction
A single slit spreads light into a pattern whose central maximum is twice as wide as the others.
- Minima: a sinθ = nλ
- Central maximum is twice as wide
Polarisation
Polarisation shows that light is transverse. Brewster's law relates the polarising angle to refractive index, and Polaroids are used to produce or analyse plane-polarised light.
- Brewster's law: μ = tan i_p
- At Brewster angle, reflected and refracted rays are perpendicular
- Polaroids produce/analyse plane-polarised light
Differentiate the focal-length signs
The sign of f tells you converging from diverging (Cartesian convention).
Lenses
| Convex lens | f positive — converging |
| Concave lens | f negative — diverging |
Mirrors
| Concave mirror | f negative — converging |
| Convex mirror | f positive — diverging |
Concepts students confuse
Tap the pairs that decide tricky NEET questions.
Lens formula vs mirror formula ★
Thin lens
- 1/v − 1/u = 1/f
- m = v/u
- Note the minus on 1/u
Mirror
- 1/v + 1/u = 1/f
- m = −v/u
- Note the plus on 1/u
Interference vs diffraction ★
Interference
- Two coherent sources
- Equally spaced fringes
- Equal brightness (ideal)
Diffraction
- One slit/aperture
- Unequal spacing
- Central max twice as wide, brightest
Real vs virtual image
Real
- Rays actually meet
- Can be caught on a screen
- Inverted (single element)
Virtual
- Rays only appear to meet
- Cannot be projected
- Erect
When does total internal reflection occur?
TIR happens
- Denser → rarer medium
- Angle > critical angle
- 100% reflected
TIR impossible
- Rarer → denser medium
- Below the critical angle
- Light refracts through
Common NEET doubts
Can I substitute distances into the lens/mirror formula without signs?
No — this is the single biggest source of lost marks here. Apply the Cartesian sign convention to every distance before substituting.
Are the lens and mirror formulae the same?
No. The lens formula is 1/v − 1/u = 1/f; the mirror formula is 1/v + 1/u = 1/f. Watch the sign of the 1/u term.
Does a convex lens always form a real image?
No. When the object is inside the focal length, a convex lens acts as a magnifier and the image is virtual, erect and enlarged.
Can total internal reflection happen going from rarer to denser?
No. TIR requires travel from a denser to a rarer medium and an incidence angle greater than the critical angle. Rarer-to-denser can never give TIR.
Do I measure the angle of incidence from the surface?
No. Always measure it from the normal to the surface, not from the surface itself.
In diffraction, are all the maxima the same width?
No. The central maximum is twice as wide as each of the secondary maxima, and much brighter.
Is the slit separation d in YDSE the same as the slit width a in diffraction?
No. d is the distance between the two slits in interference; a is the width of a single slit in diffraction. Mixing them up is a common slip.
Quick recall
Q. Write the thin lens formula and its magnification.
1/v − 1/u = 1/f, with m = v/u.
Q. Write the mirror formula and its magnification.
1/v + 1/u = 1/f, with m = −v/u.
Q. State Snell's law.
n₁ sin i = n₂ sin r.
Q. Critical angle for total internal reflection?
c = sin⁻¹(n₂/n₁), with n₁ > n₂.
Q. Fringe width in Young's double-slit experiment?
β = λD/d.
Q. Power of a lens?
P = 1/f, in dioptres when f is in metres.
Q. Condition for single-slit diffraction minima?
a sinθ = nλ.
Q. Which colour deviates most through a prism?
Violet (largest refractive index); red deviates least.
Revise the whole NEET syllabus — free.
This is one unit from the free ArivoliAI NEET Lab app — AI-guided learning, chapter-wise MCQs, timed mock tests and interactive Visual Labs across Physics, Chemistry and Biology, in Tamil, Hindi and English. Works offline, no account.
Get it on Google Play