NDA Physics · Light and Optics

Light Phenomena and the Electromagnetic Spectrum

Light is an electromagnetic wave that travels in straight lines at ~3 × 10⁸ m/s. The EM spectrum runs from radio (longest wavelength) to gamma (shortest); scattering explains the blue sky and red sunset; polarization proves light is transverse; and the primary colours of light are red, green, blue.

All Light and Optics notes NDA Physics strategy

Why this matters

The biggest subtopic — 29 PYQs, overwhelmingly EASY recall, so the highest-yield block of marks in the chapter. The recurring tests are: the speed of light and the 8-minute Sun fact, the wavelength ordering of the EM spectrum (X-ray smallest, microwave/radio largest), scattering (blue sky, red sunset, Tyndall), the primary colours, and the wave facts — polarization, the eye responding to the electric field, and that sound is NOT an EM wave.

Concept 1 of 5: Speed of light and straight-line travel

Light is the fastest thing there is — about 3 × 10⁸ m/s (3 lakh km/s) in vacuum, and it takes roughly 8 minutes to reach us from the Sun. In a uniform medium it travels in perfectly straight lines, which is why a sunbeam through dusty air looks like a straight ray. When light leaves a denser medium (water, glass) for air, it speeds back up.

Definition

Light is an electromagnetic wave travelling at $c \approx 3 \times 10^8$ m/s (= 3 lakh km/s ≈ 300 million m/s) in vacuum/air.

Sun to Earth: light takes about 8 minutes.
Rectilinear propagation: in a uniform medium light travels in straight lines (a dusty sunbeam is visible because dust scatters the straight-travelling light into our eyes).
Light speeds up when it passes from a denser medium into a rarer one (e.g. water → air), and slows down going the other way; its speed in any material is less than in vacuum.

Worked example

A statement claims 'light speeds down as it leaves a water surface and enters the air'. Is it correct?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q75Moderate

Example 1 · Light and Optics · Light Phenomena and Spectrum

Which one of the following statements is not correct for light rays ?

Light speeds UP leaving a denser medium

Going from water/glass into air, light enters a rarer medium and speeds up. The trap statement claims it 'speeds down' — wrong. Also note c is 3 lakh KILOmetres per second, not metres per second.

Drill 3 more on speed of light and straight-line travel

Concept 2 of 5: The electromagnetic spectrum

All electromagnetic waves are the same kind of thing — oscillating electric and magnetic fields travelling at the speed of light, able to move through vacuum. They differ only in wavelength (and so frequency and energy). Going from longest wavelength to shortest: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Shorter wavelength means higher frequency and higher photon energy.

Definition

EM waves share these properties: they are not elastic (need no medium), they travel in vacuum, their electric and magnetic fields are mutually perpendicular, and they move at $c$ . Ordering by decreasing wavelength (increasing energy): Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma.

X-rays: wavelength ≈ 0.01–10 nm (≈ 1 Å = 0.1 nm); smallest wavelength among radio/UV/visible/X-ray.
Energy: X-ray photon > UV photon > visible photon (shorter wavelength ⟹ more energy). Visible wavelength > X-ray wavelength.

Sound is NOT an electromagnetic wave — it is a mechanical wave needing a medium.

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.

Practice this conceptself-check · 6 quick reps

The same idea in a real exam question:

NDA · Sep 2022 · Q65Moderate

Example 2 · Light and Optics · Light Phenomena and Spectrum

Which one of the following wavelengths corresponds to the wavelength of X-rays?

Shorter wavelength = higher energy; UV beats visible

UV photons have MORE energy than visible photons (UV has the shorter wavelength). The trap statement 'UV energy is less than visible' is false. X-ray > UV > visible in energy.

EM waves are NOT elastic and DO travel in vacuum

EM waves need no medium (not elastic) and travel through vacuum. Their speed is 3 lakh KM/s. A statement saying they are elastic or move at '3 lakh metres per second' is wrong.

Drill 9 more on the electromagnetic spectrum

Concept 3 of 5: Scattering — blue sky and red sunset

When sunlight hits tiny particles and air molecules, they re-radiate it in all directions — that's scattering. Short wavelengths (blue) scatter much more than long ones (red), so the daytime sky is blue. At sunrise and sunset the light travels through much more atmosphere, the blue is scattered away, and the Sun looks red. The Tyndall effect is the same scattering by larger colloidal particles.

Definition

Scattering is the redirection of light by particles/molecules in its path; shorter wavelengths scatter more strongly.

Blue sky: blue (short wavelength) is scattered far more than red by air molecules.
Red Sun at sunrise/sunset: sunlight passes through more atmosphere, blue is scattered away, leaving the red/orange to reach us.
Tyndall effect: scattering of light by colloidal particles (visible beam through a colloid/fog).
The visible dusty sunbeam is dust scattering light into the eye.

(Contrast: the twinkling of stars and the early sunrise are atmospheric refraction, not scattering.)

Worked example

Why does the Sun appear reddish at sunrise and sunset but white overhead at noon?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2020 · Q141Moderate

Example 3 · Light and Optics · Light Phenomena and Spectrum

The Sun appears reddish during sunrise and sunset. The phenomenon in optics which is responsible for this appearance of the Sun is

Sky/sunset colour = scattering; twinkling/early-sunrise = refraction

The blue sky and red sunset are SCATTERING. The twinkling of stars and seeing the Sun before it rises are atmospheric REFRACTION. NDA tests both in the same paper — keep them apart.

Drill 5 more on scattering — blue sky and red sunset

Concept 4 of 5: Colours of light and the spectrum

White light is made of seven colours (VIBGYOR), as Newton first showed with a prism. But the eye builds all colours from just three primary colours of LIGHT — red, green and blue — which add to make white. Mixing red and green light gives yellow. (This is colour by addition of light, different from mixing paints.)

Definition

Key colour facts tested by NDA:

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.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q67Easy

Example 4 · Light and Optics · Light Phenomena and Spectrum

Which of the following are the primary colours of light ?

Primary colours of LIGHT are R, G, B — not R, Y, B

Red, Green, Blue are the primary colours of light (they add to white). Red-Yellow-Blue are pigment/paint primaries. The question almost always means light, so the answer is RGB.

Drill 2 more on colours of light and the spectrum

Concept 5 of 5: Wave nature of light and related devices

Light shows wave behaviour — and one of those behaviours, polarization, can only happen for transverse waves, which proves light is transverse. The eye responds to the electric-field part of the wave. A few devices and one-liners round out the topic: a solar cell turns light into electricity, and infrared is the 'heat' part of the spectrum that water absorbs strongly.

Definition

Polarization restricts a wave's vibrations to one plane — only possible for transverse waves. So polarization is the phenomenon that proves light is a transverse wave (refraction, diffraction and interference happen for longitudinal waves too).
The human eye is sensitive to the electric field component of an EM wave (not the magnetic field, not infrared).
Solar cell: converts light energy into electrical energy.
Infrared = heat waves, and water absorbs infrared strongly (vibrational resonance) — so IR is the correct explanation for why water heats up under sunlight.
For totally reflecting surfaces, radiation force $\propto$ area; halving the area halves the force.

Worked example

Which optical phenomenon proves that light is a transverse wave: refraction, diffraction, interference, or polarization?

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

NDA · Sep 2023 · Q134Moderate

Example 5 · Light and Optics · Light Phenomena and Spectrum

Which one of the following optical phenomena supports that the light is a transverse wave?

Only polarization proves transverse nature

Refraction, diffraction and interference all happen for longitudinal waves (like sound) too, so they cannot prove light is transverse. Polarization is the unique discriminator.

The eye responds to the ELECTRIC field

Of an EM wave's two fields, it is the electric field that the eye (and detectors generally) respond to — not the magnetic field, and certainly not the infrared band.

Drill 5 more on wave nature of light and related devices

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

Test yourself on Light and Optics

20 past NDA questions from this chapter, timed at 20 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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