Optics

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Lenses in Contact

8 MCQs4 PYQs1 revision cardWorked example

Easy Recall (3)Direct Application (3)Calculation (2)

Magnification

Easy Recall (3)Direct Application (3)Calculation (1)Concept Trap (1)

Magnifying Powers

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Microscope

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Mirror Formula

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Polarization Brewster

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Easy Recall (3)Direct Application (3)Calculation (2)

Polaroid

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Easy Recall (2)Direct Application (2)Concept Trap (2)Calculation (2)

Power of Lens

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Reflection Light

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Refraction Light

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Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Refraction Prism

Easy Recall (3)Direct Application (3)Calculation (1)Concept Trap (1)

Thin Lens Formula

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Total Internal Reflection

Easy Recall (3)Direct Application (3)Concept Trap (1)Calculation (1)

Wavefront Huygens Principle

8 MCQsWorked example

Easy Recall (3)Concept Trap (1)Direct Application (3)Calculation (1)

Past-paper questions from this unit

21 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025, 2026. Answers verified against NTA official keys.

By year in our set: 2020 (3) · 2021 (3) · 2022 (4) · 2023 (2) · 2024 (3) · 2025 (4) · 2026 (2)

Lesson: Refraction Light

NEET 2026

Consider three media P, Q and R with refractive indices 1, 1.25, and 1.5, respectively. The medium Q having a thickness of 5 cm is placed between extended media P and R as shown in the figure. An object O is placed at the center of medium Q. If viewed from medium P near the normal direction, the apparent depth of O is h₁. For similar observation from medium R, the apparent depth is h₂. The value of |h₁ − h₂|, in cm, is :

NTA Answer: Option 2(final)

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Lesson: Lenses in Contact

NEET 2026

The lens combination as shown in the figure, consists of two lenses, L₁ and L₂, of the focal lengths +10 cm and −10 cm, respectively. The position of the image formed is :

120 cm to the left of the concave lens

260 cm to the left of the concave lens

330 cm to the right of the concave lens

460 cm to the right of the concave lens

NTA Answer: Option 2(final)

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Lesson: Lenses in Contact

NEET 2025

In a certain camera, a combination of four similar thin convex lenses are arranged axially in contact. Then the power of the combination and the total magnification in comparison to the power (p) and magnification (m) for each lens will be, respectively

1p4 and m4

24p and 4m

3p4 and 4m

44p and m4

NTA Answer: Option 4(final)

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Lesson: Microscope

NEET 2025

A microscope has an objective of focal length 2 cm, eyepiece of focal length 4 cm and the tube length of 40 cm. If the distance of distinct vision of eye is 25 cm, the magnification in the microscope is

1250

2100

3125

4150

NTA Answer: Option 3(final)

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Lesson: Polaroid

NEET 2025

The intensity of transmitted light when a polaroid sheet, placed between two crossed polaroids at 22.5° from the polarization axis of one of the polaroids, is (I₀ is the intensity of polarised light after passing through the first polaroid):

1I₀/16

2I₀/2

3I₀/4

4I₀/8

NTA Answer: Option 4(final)

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An unpolarized light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster’s angle. Then

NEET 2025

1Transmitted light is completely polarized with angle of refraction close to 30°

2Reflected light is completely polarized and the angle of reflection is close to 60°

3Reflected light is partially polarized and the angle of reflection is close to 30°

4Both reflected and transmitted light are perfectly polarized with angles of reflection and refraction close to 60° and 30°, respectively

NTA Answer: Option 2(final)

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Lesson: Interference YDSE

NEET 2024Revised key

If the monochromatic source in Young’s double slit experiment is replaced by white light, then

1Interference pattern will disappear.

2There will be a central dark fringe surrounded by a few coloured fringes.

3There will be a central bright white fringe surrounded by a few coloured fringes.

4All bright fringes will be of equal width.

NTA Answer: Option 3(revised_final)

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An unpolarised light beam strikes a glass surface at Brewster's angle. Then

NEET 2024Revised key

1The reflected light will be partially polarised.

2The refracted light will be completely polarised.

3Both the reflected and refracted light will be completely polarised.

4The reflected light will be completely polarised but the refracted light will be partially polarised.

NTA Answer: Option 4(revised_final)

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A small telescope has an objective of focal length 140 cm and an eye piece of focal length 5.0 cm. The magnifying power of telescope for viewing a distant object is:

NEET 2024Revised key

134

228

317

432

NTA Answer: Option 2(revised_final)

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Lesson: Total Internal Reflection

NEET 2023

Light travels a distance x in time t₁ in air and 10x in time t₂ in another denser medium. What is the critical angle for this medium?

1sin⁻¹(10t₂/t₁)

2sin⁻¹(t₁/(10 t₂))

3sin⁻¹(10 t₁/t₂)

4sin⁻¹(t₂/t₁)

NTA Answer: Option 3(final)

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Lesson: Interference YDSE

NEET 2023

For Young’s double slit experiment, two statements are given below: Statement I : If screen is moved away from the plane of slits, angular separation of the fringes remains constant. Statement II : If the monochromatic source is replaced by another monochromatic source of larger wavelength, the angular separation of fringes decreases. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are false.

2Statement I is true but Statement II is false.

3Statement I is false but Statement II is true.

4Both Statement I and Statement II are true.

NTA Answer: Option 2(final)

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Lesson: Lens Makers Formula

NEET 2022

A biconvex lens has radii of curvature, 20 cm each. If the refractive index of the material of the lens is 1.5, the power of the lens is

1Infinity

2+2 D

3+20 D

4+5 D

NTA Answer: Option 4(final)

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Lesson: Fringe Width YDSE

NEET 2022

In a Young’s double slit experiment, a student observes 8 fringes in a certain segment of screen when a monochromatic light of 600 nm wavelength is used. If the wavelength of light is changed to 400 nm, then the number of fringes he would observe in the same region of the screen is

112

NTA Answer: Option 1(final)

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Lesson: Refraction Light

NEET 2022

A light ray falls on a glass surface of refractive index 3 , at an angle 60°. The angle between the refracted and reflected rays would be

1120°

230°

360°

490°

NTA Answer: Option 4(final)

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Lesson: Refraction Light

NEET 2022

Two transparent media A and B are separated by a plane boundary. The speed of light in those media are 1.5 × 10⁸ m/s and 2.0 × 10⁸ m/s, respectively. The critical angle for a ray of light for these two media is

1tan⁻¹ (0.750)

2sin⁻¹ (0.500)

3sin⁻¹ (0.750)

4tan⁻¹ (0.500)

NTA Answer: Option 3(final)

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Lesson: Lenses in Contact

NEET 2021

A convex lens 'A' of focal length 20 cm and a concave lens 'B' of focal length 5 cm are kept along the same axis with a distance 'd' between them. If a parallel beam of light falling on 'A' leaves 'B' as a parallel beam, then the distance 'd' in cm will be

130

225

315

450

NTA Answer: Option 3(final)

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Lesson: Refraction Prism

NEET 2021

Find the value of the angle of emergence from the 3 prism. Refractive index of the glass is . 60°

190°

260°

330°

445°

NTA Answer: Option 2(final)

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Lesson: Lenses in Contact

NEET 2021

A point object is placed at a distance of 60 cm from a convex lens of focal length 30 cm. If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of 40 cm from it, the final image would be formed at a distance of 60 cm 40 cm

120 cm from the plane mirror, it would be a virtual image

220 cm from the lens, it would be a real image

330 cm from the lens, it would be a real image

430 cm from the plane mirror, it would be a virtual image

NTA Answer: Option 1(final)

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Assume that light of wavelength 600 nm is coming from a star. The limit of resolution of telescope whose objective has a diameter of 2 m is :

NEET 2020

16.00×10⁻⁷ rad

23.66×10⁻⁷ rad

31.83×10⁻⁷ rad

47.32×10⁻⁷ rad

NTA Answer: Option 2(final)

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The Brewsters angle ib for an interface should be :

NEET 2020

1ib = 908

208 < ib < 308

3308 < ib < 458

4458 < ib < 908

NTA Answer: Option 4(final)

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Lesson: Fringe Width YDSE

NEET 2020

In Young’s double slit experiment, if the separation between coherent sources is halved and the distance of the screen from the coherent sources is doubled, then the fringe width becomes :

1one-fourth

2double

3half

4four times

NTA Answer: Option 4(final)

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Exam traps and common mistakes in this unit

Lesson: Fringe Width YDSE

Trap

YDSE: D vs d inversion

Category: Similar Terms

Student writes β = λd/D instead of λD/d. Consequences: doubling D should INCREASE fringe width, not decrease.

When it triggers

YDSE fringe width question with changes to slit separation or screen distance.

How to avoid

β = λD/d. D is screen distance (large); d is slit separation (small). β scales with D/d. Mnemonic: 'D for distance increases width'.

Lesson: Lenses in Contact

Trap

Lens combination: cumulative sign and reciprocal errors

Category: Negative Marking

Sequential 1/F = 1/f1 + 1/f2 across multiple lenses. Each f sign error or reciprocal-skip propagates. Common with diopter conversions (P = 1/f in metres).

When it triggers

Multi-lens optical system; asks for equivalent focal length or power.

How to avoid

Standard procedure: (1) note signs of each f (concave negative); (2) sum 1/f_i; (3) take reciprocal at end. Skip the final reciprocal and the answer is off by orders of magnitude — always present as distractor.

Lesson: Mirror Formula

Trap

Optics sign convention error

Category: Sign Convention

Student inverts the sign of u, v, or f in mirror/lens formulas. Convention: distances measured from pole/optical centre; positive in direction of light propagation.

When it triggers

Mirror or lens problem requiring formula application.

How to avoid

Cartesian sign convention: pole at origin; light travels in +x direction; distances measured along axis are signed. Concave mirror f<0 on convention where light incident from left (or vice versa per text). Apply ONE convention throughout.

Lesson: Refraction Prism

Trap

Prism deviation: cumulative trig/inverse-trig error

Category: Negative Marking

Multi-step prism problem combining angle of incidence, refraction (Snell at each face), and minimum deviation formula. Each sin/sin⁻¹ inversion or angle-mode (deg vs rad) confusion compounds.

When it triggers

Prism problem with refractive index, prism angle A, angle of incidence i.

How to avoid

Use sequential angle bookkeeping: at first face, n sin(r1) = sin(i1); at second face, similar; r1 + r2 = A. Compute deviation δ = (i1 + i2) − A. Plug into minimum-deviation formula only when symmetric (i1 = i2).

Lesson: Fringe Width YDSE

Mistake

Inverts D/d ratio in fringe width formula.

Root cause: formula misuse

Correction

β = λD/d. D = screen distance (large), d = slit separation (small). β ∝ D/d. Larger D → wider fringes.

Lesson: Lenses in Contact

Mistake

Forgets to take reciprocal at end of 1/F = Σ1/f calculation; reports Σ1/f as F.

Root cause: arithmetic

Correction

Procedure: (1) compute Σ1/f_i with signed f values; (2) take reciprocal at end to get F. Skipping the final reciprocal gives 1/F (in 1/m = dioptres) instead of F (in metres).

Lesson: Mirror Formula

Mistake

Inverts sign of u, v, or f in optics formulas.

Root cause: sign error

Correction

Use Cartesian convention consistently: pole/optical-centre at origin, light travels in +x direction. Object usually has u < 0 (left of pole). Concave mirror f < 0 (most conventions). Apply ONE convention throughout.

Mistake

Uses sin(i_B) = n instead of tan(i_B) = n.

Root cause: formula misuse

Correction

Brewster's law: tan(i_B) = n (NOT sin). Different from Snell's critical angle sin(θ_c) = 1/n. Brewster ~57° for glass (n≈1.5); critical ~42°.

Lesson: Polaroid

Mistake

Drops square on cosine in Malus's law I = I_0 cos²(θ).

Root cause: formula misuse

Correction

Malus: I = I_0 cos²(θ). For θ=60°: I = I_0 × (1/4), not I_0 × 0.5.

Formulas in this unit

Astronomical telescope magnification (normal)

M = - \frac{f _{o}}{f _{e}}

Magnification of astronomical telescope in normal adjustment (final image at infinity).

Symbol	Quantity	SI Unit
f_o	objective focal length	m
f_e	eyepiece focal length	m

Valid when

Normal adjustment (image at infinity)
Object at infinity

Lesson: Fringe Width YDSE

YDSE fringe width

β = \frac{λ D}{d}

Fringe width in Young's double-slit experiment. Wavelength lambda, slit separation d, screen distance D.

Symbol	Quantity	SI Unit
beta	fringe width	m
lambda	wavelength	m
D	screen distance	m
d	slit separation	m

Valid when

D >> d
Coherent monochromatic source
Small angles

Lesson: Lens Makers Formula

Lens maker's formula

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

Focal length from refractive index and surface curvatures.

Symbol	Quantity	SI Unit
f	focal length	m
n	refractive index	-
R1, R2	radii of curvature	m

Valid when

Thin lens
Both surfaces specified with sign convention
Lens in air

Lesson: Lenses in Contact

Lenses in contact

\frac{1}{F} = \sum \frac{1}{f _{i}}

Equivalent focal length for thin lenses in contact. Powers add.

Symbol	Quantity	SI Unit
F	equivalent focal length	m
f_i	individual	m

Valid when

Thin lenses in contact
Same optical axis

Lesson: Magnification

Mirror formula

\frac{1}{v} + \frac{1}{u} = \frac{1}{f}

Spherical mirror formula. Sign convention: distances from pole, positive in direction of incident light.

Symbol	Quantity	SI Unit
v	image distance	m
u	object distance	m
f	focal length	m
m	magnification	-

Valid when

Spherical mirror, paraxial rays
Sign convention used consistently

Brewster's law

tan i_{B} = n

When light is incident at Brewster's angle on a dielectric, the reflected light is fully plane-polarised.

Symbol	Quantity	SI Unit
i_B	Brewster's angle	rad
n	refractive index	-

Valid when

Light from rarer to denser medium (plane interface)
Linearly polarised reflected component perpendicular to plane of incidence

Lesson: Polaroid

Malus's law

I = I_{0} cos^{2} θ

Intensity through a polariser depends on angle theta between light's polarisation and polariser axis.

Symbol	Quantity	SI Unit
I	transmitted intensity	W/m^2
I0	incident intensity	W/m^2
theta	angle	rad

Valid when

Linearly polarised input
Ideal polariser

Lesson: Power of Lens

Thin lens formula

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

Thin lens formula. Power P = 1/f (in dioptres if f in metres).

Symbol	Quantity	SI Unit
v	image distance	m
u	object distance	m
f	focal length	m

Valid when

Thin lens
Paraxial rays
Sign convention used

Lesson: Refraction Light

Snell's law

n_{1} sin θ_{1} = n_{2} sin θ_{2}

Refraction at boundary between two media of different refractive indices.

Symbol	Quantity	SI Unit
n1, n2	refractive indices	-
theta1, theta2	angles from normal	rad

Valid when

Plane interface
Monochromatic light

Lesson: Refraction Prism

Prism minimum deviation formula

n = \frac{s i n (( A + δ _{m} ) /2 )}{s i n ( A /2 )}

At minimum deviation (i1 = i2), refractive index from prism angle A and minimum deviation delta_m.

Symbol	Quantity	SI Unit
n	refractive index	-
A	prism angle	rad
delta_m	minimum deviation	rad

Valid when

At minimum deviation only
Symmetric ray path

Lesson: Total Internal Reflection

Critical angle (TIR)

sin θ_{c} = \frac{n _{2}}{n _{1}}

Beyond this angle of incidence, total internal reflection occurs.

Symbol	Quantity	SI Unit
theta_c	critical angle	rad
n1	denser medium index	-
n2	rarer medium index	-

Valid when

Light going from denser to rarer medium
Plane interface

NEET question patterns in this unit

Telescope magnification M = -f_o/f_e. Tube length L = f_o + f_e.

Direct ApplicationEasy

Common distractors

inverts fo fe

Uses f_e/f_o instead of f_o/f_e

Lesson: Fringe Width YDSE

YDSE fringe width β = λD/d. Compute new fringe given changes in λ, D, d.

Multi StepMedium

Common distractors

inverts D d relation

Uses d/D instead of D/d

Lesson: Lenses in Contact

Lenses in contact: 1/F = sum(1/f_i) or P = sum(P_i). Concave is negative.

Multi StepMedium

Common distractors

forgets sign of concave

Treats concave f as positive