Optics Formula Sheet — NEET Physics

Every key Optics formula, definition and fact for NEET Physics in one place — with common examiner traps and worked examples. Free to read; blurt from memory, then check your gaps.

Syllabus — topics coveredNCERT · 5 sub-topics

Reflection & refraction, spherical mirrors & lenses, mirror & lens formulae
Total internal reflection, lens maker’s formula, power & combination of lenses
Prism, microscope & telescope
Huygens’ principle, interference, Young’s double slit experiment
Diffraction (single slit), polarisation, Brewster’s law, polaroids

Reflection & Spherical Mirrors

Mirror formula

\frac{1}{v} + \frac{1}{u} = \frac{1}{f}; f = \frac{R}{2}

u =

object dist.,

v =

image dist.,

f =

focal length,

R =

radius; Cartesian sign convention from the pole

Magnification

m = - \frac{v}{u} = \frac{h ^{'}}{h}

m > 0

erect/virtual;

m < 0

inverted/real;

∣ m ∣ > 1

magnified

Concave mirror ray diagram: object beyond C forms a real inverted diminished image between F and C — Object beyond C $\to$ real, inverted, diminished.

Comparative: mirror image nature

Mirror / object	Image
Plane	virtual, erect, same size
Convex (any)	virtual, erect, diminished
Concave, beyond F	real, inverted
Concave, within F	virtual, erect, magnified

🚫 Examiner Trap · Mirrors & sign convention

(1) Measure all distances from the

pole

, positive along incident light; real distances come out

negative

. (2)

f < 0

for concave,

f > 0

for convex. (3)

m = - v / u

— the minus sign matters for image orientation. (4) Convex & plane mirrors

never

give a real image.

Refraction & Total Internal Reflection

Snell's law

n_{1} sin θ_{1} = n_{2} sin θ_{2}; n = \frac{c}{v} = \frac{λ _{v a c}}{λ _{m e d}}

rarer

\to

denser bends

toward

the normal; frequency is unchanged on refraction

Apparent depth

\frac{real depth}{apparent depth} = n; shift = t (1 - \frac{1}{n})

t =

slab thickness; object looks raised when viewed from the rarer side

Refraction at a spherical surface

\frac{n _{2}}{v} - \frac{n _{1}}{u} = \frac{n _{2} - n _{1}}{R}

single surface;

R > 0

if centre is on the outgoing side

Left: refraction bending toward the normal entering a denser medium. Right: total internal reflection beyond the critical angle — Refraction vs total internal reflection.

Critical angle (TIR)

sin θ_{c} = \frac{1}{n} = \frac{n _{r a rer}}{n _{d e n ser}}

only

denser \to rarer

, for

i > θ_{c}

🚫 Examiner Trap · Refraction & TIR

(1) On refraction

frequency stays

the same; speed and

λ

change. (2) TIR needs light going

denser \to rarer

AND

i > θ_{c}

— not the reverse. (3) Higher n

\Rightarrow

smaller

θ_{c}

(diamond sparkles,

θ_{c} \approx 2 4^{\circ}

). (4) Apparent depth

<

real depth; the shift is

t (1 - 1/ n)

Lenses & Lensmaker's Formula

Lens formula

\frac{1}{v} - \frac{1}{u} = \frac{1}{f}; m = \frac{v}{u}

note the

minus

sign (vs mirror's plus);

m > 0

erect

Lensmaker's formula

\frac{1}{f} = (n - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})

n =

lens/medium index ratio;

f > 0

convex (converging),

f < 0

concave (diverging)

Convex lens ray diagram: object beyond 2F forms a real inverted image between F' and 2F' — Convex lens: real inverted image (object beyond 2F).

Power & combination

P = \frac{1}{f ( m )} (dioptre); \frac{1}{F} = \frac{1}{f _{1}} + \frac{1}{f _{2}} - \frac{d}{f _{1} f _{2}}

in contact (

d = 0

P = \sum P_{i}

Comparative: lens types

	Convex	Concave
$f$ , $P$	$+$	$-$
Action	converging	diverging
Real image?	yes (object beyond $f$ )	never

🚫 Examiner Trap · Lenses

(1) Lens uses

1/ v - 1/ u

(

minus

); mirror uses

1/ v + 1/ u

(plus) — don't mix. (2) f depends on the

surrounding medium

: a glass lens is weaker in water and can even change converging

\leftrightarrow

diverging. (3) For lenses in contact powers ADD; with a gap subtract

d / f_{1} f_{2}

. (4) A diverging lens always gives a virtual, erect, diminished image.

Prism, Dispersion & Optical Instruments

Prism

A = r_{1} + r_{2}, δ = i_{1} + i_{2} - A; δ = (n - 1) A (small A)

A =

prism angle,

δ =

deviation,

r, i =

refraction/incidence angles

Minimum deviation

n = \frac{sin \frac{A + δ _{m}}{2}}{sin \frac{A}{2}}

δ_{m}

i_{1} = i_{2}

r_{1} = r_{2} = A /2

(symmetric passage)

A prism deviating a ray and dispersing white light into colours, violet deviated most — Violet deviates most; angular dispersion $(n_{v} - n_{r}) A$ .

Dispersive power

ω = \frac{δ _{v} - δ _{r}}{δ _{y}} = \frac{n _{v} - n _{r}}{n _{y} - 1}

property of the material only

Comparative: microscope vs telescope

Instrument	Magnifying power	Tube $L$
Simple microscope	$1 + \frac{D}{f}$	—
Compound micro.	$\frac{L}{f _{o}} \cdot \frac{D}{f _{e}}$	$\approx v_{o} + f_{e}$
Telescope (normal)	$\frac{f _{o}}{f _{e}}$	$f_{o} + f_{e}$

🚫 Examiner Trap · Prism & instruments

(1)

δ

minimum

for symmetric passage (

i_{1} = i_{2}

); at any other angle

δ

is larger. (2) Dispersive power

ω

depends on material only, not on A. (3)

Telescope

: large-f objective, small-f eyepiece (

M = f_{o} / f_{e}

);

microscope

: BOTH focal lengths small. (4) Bigger M for a telescope means

f_{o} ≫ f_{e}

, opposite to brightness needs.

Wave Optics — Huygens & Interference

Huygens & coherence

▸Every point on a wavefront is a source of secondary wavelets; their envelope is the new wavefront.
▸Interference needs two $coherent$ sources (constant phase difference, same frequency).

Fringe width (YDSE)

β = \frac{λ D}{d}; y_{n}^{b r i g h t} = \frac{nλ D}{d}, y_{n}^{d a r k} = \frac{( 2 n - 1 ) λ D}{2 d}

d =

slit separation,

D =

screen distance; bright:

Δ x = nλ

, dark:

Δ x = (2 n - 1) λ /2

Young's double-slit setup with slit separation d, screen distance D and equally spaced bright fringes — Equally spaced fringes of width $β = λ D / d$ .

Intensity

I = I_{1} + I_{2} + 2 I_{1} I_{2} cos ϕ; \frac{I _{ma x}}{I _{min}} = \frac{( I _{1} + I _{2} ) ^{2}}{( I _{1} - I _{2} ) ^{2}}

ϕ =

phase difference; equal slits

\Rightarrow I_{ma x} = 4 I_{0}

I_{min} = 0

🚫 Examiner Trap · Interference / YDSE

(1) Sources must be

coherent

— two independent bulbs never interfere. (2) In a medium

β \to β / n

(the wavelength shrinks). (3) A thin sheet (

μ, t

) in one path shifts the pattern by

(μ - 1) t D / d

but

doesn’t change

β

. (4) Intensity adds as

(I_{1} \pm I_{2})^{2}

, not

I_{1} \pm I_{2}

— amplitudes add, not intensities.

Diffraction & Resolving Power

Single-slit minima

a sin θ = nλ (n = 1, 2, \dots)

a =

slit width; this gives

minima

(dark), the opposite of YDSE bright condition

Central maximum width

width = \frac{2 λ D}{a}; angular = \frac{2 λ}{a}

central max is

twice

as wide as the side maxima

Single-slit diffraction intensity pattern: a wide bright central maximum with weaker side maxima — Central max twice as wide as side maxima.

Resolving power (Rayleigh)

θ_{min} = \frac{1.22 λ}{D}

D =

aperture diameter; smaller

θ_{min} \Rightarrow

better resolution

Comparative: interference vs diffraction

	Interference	Diffraction
Sources	two coherent	one slit (parts)
Fringe spacing	equal	central widest
Intensities	all equal	fall off from centre

🚫 Examiner Trap · Diffraction & resolution

(1) For a single slit,

a sin θ = nλ

gives

minima

(in YDSE the same form gives maxima) — easy to confuse. (2) The central maximum is

twice

as wide and far brighter than side maxima. (3) Narrower slit

\Rightarrow

WIDER pattern (

\propto 1/ a

). (4) Diffraction sets the

resolution limit

of all instruments.

Polarisation

Polarisation: Confining the

E

vibrations of a transverse wave to one plane. Unpolarised light through one polaroid becomes plane-polarised with

half

the intensity (

I_{0} /2

Malus' law

I = I_{0} cos^{2} θ

θ =

angle between polariser and analyser axes;

I_{0} =

intensity after the polariser

Unpolarised light through a polariser then an analyser, transmitted intensity I0 cos squared theta — Polariser then analyser at angle $θ$ .

Brewster's law

tan θ_{p} = n

θ_{p}

reflected light is fully polarised; reflected

⊥

refracted (

θ_{p} + θ_{r} = 9 0^{\circ}

)

Key facts

▸Only $transverse$ waves polarise — proves light is transverse (sound can't).
▸Crossed polaroids ( $θ = 9 0^{\circ}$ ) block all light.
▸Uses: polaroid sunglasses, LCDs, 3-D glasses, stress analysis.

🚫 Examiner Trap · Polarisation

(1) First polaroid halves the intensity (

I_{0} /2

) for

unpolarised

input; Malus

I_{0} cos^{2} θ

applies

after

it. (2) Crossed polaroids (

9 0^{\circ}

) give zero — but a third at

4 5^{\circ}

in between lets some through. (3) Brewster:

tan θ_{p} = n

, and the reflected ray

⊥

refracted ray. (4) Only transverse waves polarise — a key proof that light is transverse.

More NEET Physics formula sheets

Frequently Asked Questions

What are the most important Optics formulas for NEET?

This Optics formula sheet covers all the high-yield Physics formulas, definitions and facts you need for NEET, across Reflection & refraction, spherical mirrors & lenses, mirror & lens formulae, Total internal reflection, lens maker’s formula, power & combination of lenses, Prism, microscope & telescope, Huygens’ principle, interference, Young’s double slit experiment, Diffraction (single slit), polarisation, Brewster’s law, polaroids — each shown with the key result and, where useful, a worked example.

Is this Optics formula sheet free?

Yes — the full chapter formula sheet is free to read online, no login or payment required.

How should I revise Optics for NEET?

Blurt the Optics key points from memory, then check against this sheet to find your gaps — and practise a few previous-year questions on the chapter to make sure you can apply them under time pressure.

Also useful: all formula sheets · NEET previous-year papers · most important chapters.