NDA Physics · Teaching notes

Light and Optics — NDA Physics

Light and Optics is the biggest and most diagram-heavy chapter in NDA Physics — 97 PYQs across 2017–2026, and the chapter where a single sign convention makes or breaks a numeric answer. It teaches in six movements that follow the way light behaves when it meets a surface: (1) Reflection and mirrors — light bouncing back: the laws of reflection, plane mirrors, and the spherical-mirror formula with its image-formation rules for concave and convex mirrors; (2) Refraction, speed of light, and total internal reflection — light bending as it changes medium: Snell's law, refractive index as n = c/v, the critical angle, and the everyday effects (mirage, optical fibre, raised pool bottom); (3) Lenses and the lens formula — refraction through a curved piece of glass: the lens formula, power in dioptres, the lens maker's equation, magnification, and lenses in combination; (4) Prisms and dispersion — splitting white light: deviation through a prism and why violet bends most while red bends least, plus the rainbow; (5) The human eye and optical instruments — the recall layer: accommodation, the defects (myopia, hypermetropia, presbyopia, cataract) and their corrections, and the microscope and telescope; (6) Light phenomena and the spectrum — the wave layer: scattering (why the sky is blue and sunsets red), the electromagnetic spectrum ordered by wavelength, polarization, and the speed of light. The marquee subtopic is Light Phenomena and Spectrum (29 q) — mostly recall — but the marks that separate students live in the sign-convention numerics of mirrors and lenses. Get the sign convention right, drill the formula, learn the table, walk out with the marks.

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Subtopic notes

PYQ weightage by concept

28 concepts · 97 PYQs — where the marks actually sit, so you know what to drill first

Reflection and Mirrors18 PYQs · 19%

Concept	PYQs	Share
Plane mirror images	5	5%
Concave mirror — image formation	5	5%
Convex mirror — always virtual, erect, diminished	4	4%
Mirror formula and magnification	2	2%
Light rays and the laws of reflection	1	1%
Spherical mirrors — pole, focus, centre, and R = 2f	1	1%

Refraction, Speed of Light, and Total Internal Reflection17 PYQs · 18%

Concept	PYQs	Share
Refractive index — n = c/v	6	6%
Refraction and Snell's law	4	4%
Everyday refraction effects	3	3%
Mirage and the optical fibre	3	3%
Total internal reflection and the critical angle	1	1%

Lenses and the Lens Formula12 PYQs · 12%

Concept	PYQs	Share
Power of a lens — the dioptre	3	3%
Lenses in contact — powers add	3	3%
Convex and concave lenses	2	2%
Lens magnification and image formation	2	2%
Lens formula and the sign convention	1	1%
Lens maker's equation	1	1%

Prisms and Dispersion8 PYQs · 8%

Concept	PYQs	Share
Dispersion — why violet bends most	4	4%
Refraction through a prism and deviation	2	2%
The rainbow	2	2%

The Human Eye and Optical Instruments13 PYQs · 13%

Concept	PYQs	Share
Microscope and telescope	6	6%
Eye defects and their corrections	5	5%
The human eye and accommodation	2	2%

Light Phenomena and the Electromagnetic Spectrum29 PYQs · 30%

Concept	PYQs	Share
The electromagnetic spectrum	10	10%
Scattering — blue sky and red sunset	6	6%
Wave nature of light and related devices	6	6%
Speed of light and straight-line travel	4	4%
Colours of light and the spectrum	3	3%

Formula & revision sheet

11 formulas · 3 reference tables · 37 gotchas across all subtopics — the exam-eve cheat-sheet

Reflection and Mirrors

Formulas (2)

Spherical mirrors — pole, focus, centre, and R = 2f · Focal length and radius of curvature $f = \dfrac{R}{2}$
Mirror formula and magnification · Mirror formula and magnification $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}, \qquad m = -\dfrac{v}{u}$

Watch out for (9)

Measure angles from the normal, not the surface
→ Light rays and the laws of reflection
Virtual + erect + same-size — and only laterally inverted
→ Plane mirror images
Half your height — distance does not matter
→ Plane mirror images
R = 2f, so f = R/2 — not f = 2R
→ Spherical mirrors — pole, focus, centre, and R = 2f
Object at F gives the image at infinity, not between F and P
→ Concave mirror — image formation
Only inside F does a concave mirror give a virtual image
→ Concave mirror — image formation
A convex mirror NEVER inverts
→ Convex mirror — always virtual, erect, diminished
Sign convention is the whole game
→ Mirror formula and magnification
Magnification sign tells you real vs virtual
→ Mirror formula and magnification

Refraction, Speed of Light, and Total Internal Reflection

Formulas (3)

Refraction and Snell's law · Snell's law of refraction $n_1 \sin\theta_1 = n_2 \sin\theta_2$
Refractive index — n = c/v · Refractive index and speed $n = \dfrac{c}{v} \quad\Rightarrow\quad v = \dfrac{c}{n}, \qquad \dfrac{v_1}{v_2} = \dfrac{n_2}{n_1}$
Total internal reflection and the critical angle · Critical angle $\sin\theta_c = \dfrac{1}{n}$

Watch out for (7)

Frequency is the invariant — not speed or wavelength
→ Refraction and Snell's law
Normal incidence still slows the light
→ Refraction and Snell's law
Speed is the INVERSE of refractive index
→ Refractive index — n = c/v
Twinkling = refraction; blue sky / red sunset = scattering
→ Everyday refraction effects
TIR only goes denser → rarer
→ Total internal reflection and the critical angle
Both conditions, not just a big angle
→ Total internal reflection and the critical angle
Mirage is TIR, not simple reflection or dispersion
→ Mirage and the optical fibre

Lenses and the Lens Formula

Formulas (5)

Lens formula and the sign convention · Lens formula $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$
Power of a lens — the dioptre · Power of a lens $P = \dfrac{1}{f\,(\text{in metres})}$
Lens maker's equation · Lens maker's equation $\dfrac{1}{f} = (n - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$
Lenses in contact — powers add · Combination of thin lenses in contact $P = P_1 + P_2, \qquad f = \dfrac{1}{P}$
Lens magnification and image formation · Lens magnification $m = \dfrac{h'}{h} = \dfrac{v}{u}$

Watch out for (6)

A concave lens has no real-image setting
→ Convex and concave lenses
Lens uses 1/v − 1/u; mirror uses 1/v + 1/u
→ Lens formula and the sign convention
Convert cm to metres before computing power
→ Power of a lens — the dioptre
Sign the radii — convex faces are not both positive
→ Lens maker's equation
Add powers, not focal lengths
→ Lenses in contact — powers add
Lens m = v/u (no minus); mirror m = −v/u
→ Lens magnification and image formation

Prisms and Dispersion

Watch out for (3)

Deviation is refraction, not reflection
→ Refraction through a prism and deviation
Violet bends most because its speed in glass is LOWEST
→ Dispersion — why violet bends most
Primary rainbow = ONE internal reflection (the inner bow)
→ The rainbow

The Human Eye and Optical Instruments

Formulas (1)

Microscope and telescope · Telescope magnification (normal adjustment) $M = \dfrac{f_o}{f_e}$

Reference tables (1)

Eye defects and their corrections4 rows

Defect	Problem	Correction
Myopia (short / near-sightedness)	Cannot see DISTANT objects clearly; image of a distant object focuses BEFORE the retina; far point is finite	Concave (diverging) lens Myopia = sees near clearly, far blurred. Power P = −1/(far point in m).
Hypermetropia (long / far-sightedness)	Cannot see NEAR objects clearly; image focuses behind the retina	Convex (converging) lens
Presbyopia	Age-related loss of accommodation; both near and far affected	Bifocal lens
Cataract	Eye lens becomes cloudy/opaque	Surgery (lens replacement) — not a spectacle lensQ

Myopia → concave, Hypermetropia → convex, Presbyopia → bifocal, Cataract → surgery. The match-list pairing is tested almost every year.

Watch out for (5)

The eye is a CONVERGING system, not a diverging one
→ The human eye and accommodation
Myopia → concave; hypermetropia → convex (don't swap)
→ Eye defects and their corrections
Cataract is surgery, not a lens
→ Eye defects and their corrections
Microscope wants a SHORT objective; telescope wants a LONG one
→ Microscope and telescope
Newtonian telescope = mirrors only
→ Microscope and telescope

Light Phenomena and the Electromagnetic Spectrum

Reference tables (2)

The electromagnetic spectrum7 rows

Wave / band	Typical wavelength	Use / note
Radio waves	> 1 m	Longest wavelength; broadcasting, communication
Microwaves	mm to cm	Radar, microwave ovens; LONGER wavelength than light
Infrared	~700 nm to 1 mm	Heat waves; absorbed strongly by water
Visible light	≈ 400–700 nm	The only band the eye detects
Ultraviolet (UV)	≈ 10–400 nm	Detects forgery in currency notes; higher energy than visibleQ
X-rays	≈ 0.01–10 nm (≈ 1 Å)	Smallest wavelength of the common four; medical imaging X-ray ≈ 1 nm ≈ 1 Å — the standard tested value. Smallest wavelength among radio/UV/visible/X-ray.
Gamma rays	< 0.01 nm	Highest energy of all

Memorise the ORDER (radio longest → gamma shortest) and the X-ray value (≈ 1 nm ≈ 1 Å). Sound is not on this list — it is mechanical, not electromagnetic.

Colours of light and the spectrum7 rows

Fact	Value
Primary colours of light	Red, Green, Blue (RGB) These ADD to white. Distinct from the primary pigments (paints).
Red + Green light gives	Yellow
Blue + Green light gives	Cyan
Red + Blue light gives	Magenta
Red + Green + Blue gives	White
First obtained sunlight's spectrum with a prism	Isaac NewtonQ
Order of colours in white light	VIBGYOR (Violet → Red)

The three primary colours of light are Red, Green, Blue; red + green = yellow; Newton first dispersed sunlight with a prism.

Watch out for (7)

Light speeds UP leaving a denser medium
→ Speed of light and straight-line travel
Shorter wavelength = higher energy; UV beats visible
→ The electromagnetic spectrum
EM waves are NOT elastic and DO travel in vacuum
→ The electromagnetic spectrum
Sky/sunset colour = scattering; twinkling/early-sunrise = refraction
→ Scattering — blue sky and red sunset
Primary colours of LIGHT are R, G, B — not R, Y, B
→ Colours of light and the spectrum
Only polarization proves transverse nature
→ Wave nature of light and related devices
The eye responds to the ELECTRIC field
→ Wave nature of light and related devices