Electromagnetic Spectrum

Q: What are the wavelength ranges of the bands in NCERT?

Radio above 0.1 m, microwaves 0.1 m to 1 mm, infrared 1 mm to 700 nm, visible 700 to 400 nm, ultraviolet 400 nm to 1 nm, X-rays 1 nm to 10⁻³ nm and gamma rays shorter. The boundaries are not sharp and neighbouring bands overlap.

Q: Why do microwave ovens use 2.45 GHz?

Microwaves of 2.45 GHz are absorbed strongly by the water molecules in food, which share the energy with neighbouring molecules and heat the food from within. Porcelain and glass containers absorb little at this frequency and stay cool, while metal containers can spark and melt.

PhysicsElectromagnetic WavesFor JEE aspirants

The electromagnetic spectrum is the whole family of electromagnetic waves arranged by frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All of them travel at $c = 3 \times 1 0^{8} m s^{- 1}$ in vacuum and differ only in wavelength, which decides how each band is produced, detected, absorbed and used. This page covers the order, NCERT wavelength ranges, sources and detectors, uses, photon energies, the Wien and atmosphere links, and a quick band-identification method. The electromagnetic spectrum gives sure one-mark questions in NEET and JEE Main.

On this page1The spectrum at a glance2Band table3Radio to

γ

: uses4Where each band comes from5Temperature and the atmosphere6Revision map

Key Formulas - Quick Reference

★ Must learn Increasing frequency (decreasing $λ$ ): radio $<$ microwave $<$ infrared $<$ visible $<$ ultraviolet $<$ X-rays $<$ $γ$ -rays
★ Must learn Every band: $c = ν λ$ ; photon energy $E = h ν = \frac{h c}{λ}$ , i.e. $E (eV) = \frac{1240}{λ ( nm )}$
★ Must learn NCERT ranges: radio $> 0.1 m$ ; microwave $0.1 m$ to $1 mm$ ; infrared $1 mm$ to $700 nm$ ; visible $700$ to $400 nm$ ; UV $400 nm$ to $1 nm$ ; X-rays $1 nm$ to $1 0^{- 3} nm$ ; $γ$ below $1 0^{- 3} nm$
Visible light: $4 \times 1 0^{14}$ to $7.5 \times 1 0^{14} Hz$ , photon energies $1.8$ to $3.1 eV$
★ Must learn Radio bands: AM $530$ to $1710 kHz$ , short wave up to $54 MHz$ , FM $88$ to $108 MHz$ , TV $54$ to $890 MHz$ ; microwave oven $2.45 GHz$
Wien's law: $λ_{m} T = 2.9 \times 1 0^{- 3} m K$ ( $0.29 cm K$ )
★ Must learn No sharp boundaries: where X-rays and $γ$ -rays overlap they are named by origin (electrons give X-rays, nuclei give $γ$ -rays)

1. The Spectrum at a Glance

Figure 1: One family, one speed

c

, eighteen decades of wavelength. Moving right,

ν

and photon energy

E = h ν

rise while

λ

falls. Visible light is a sliver from

700 nm

(red) to

400 nm

(violet), about

4 \times 1 0^{14}

7.5 \times 1 0^{14} Hz

All bands are the same kind of wave: transverse $E$ and $B$ travelling at $c$ in vacuum.
They differ only in wavelength (frequency), so they differ in how they are produced, detected and absorbed by matter.
The classification is rough, based on how the waves are made and detected, and the bands overlap.

Exam Trick

"Raging Martians Invaded Venus Using X-ray Guns": Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma rays, in order of increasing frequency and photon energy (decreasing wavelength). Infrared lies just beyond red, ultraviolet just beyond violet.

Figure 2: There are no sharp boundaries. Ultraviolet and X-rays overlap between about

10 nm

and

0.6 nm

; X-rays and

γ

-rays overlap below

0.1 nm

. In the overlap the name depends on the origin: electrons give X-rays, nuclei give

γ

-rays.

Key idea

Same speed

c

, different

λ

: radio longest (

> 0.1 m

γ

-rays shortest (

< 1 pm

); boundaries are approximate.

2. Band Table: Range, Production, Detection

Band	Wavelength	Frequency	Produced by	Detected by
Radio	$> 0.1 m$	$< 3 \times 1 0^{9} Hz$	rapidly accelerated electrons in aerials	receiver aerials
Microwave	$0.1 m$ to $1 mm$	$3 \times 1 0^{9}$ to $3 \times 1 0^{11} Hz$	klystron, magnetron, Gunn diode	point-contact diodes
Infrared	$1 mm$ to $700 nm$	$3 \times 1 0^{11}$ to $4.3 \times 1 0^{14} Hz$	vibrating atoms and molecules, hot bodies	thermopile, bolometer, IR film
Visible	$700$ to $400 nm$	$4.3 \times 1 0^{14}$ to $7.5 \times 1 0^{14} Hz$	electrons in atoms jumping to lower levels	eye, photocell, photographic film
Ultraviolet	$400 nm$ to $1 nm$	$7.5 \times 1 0^{14}$ to $3 \times 1 0^{17} Hz$	inner-shell (and outer) electron jumps, very hot bodies, the Sun	photocell, photographic film
X-rays	$1 nm$ to $1 0^{- 3} nm$	$3 \times 1 0^{17}$ to $3 \times 1 0^{20} Hz$	X-ray tube (fast electrons on a metal target), inner-shell electrons	photographic film, Geiger tube, ionisation chamber
$γ$ -rays	$< 1 0^{- 3} nm$	$> 3 \times 1 0^{20} Hz$	radioactive decay of nuclei, nuclear reactions	as for X-rays

2.1 Uses and key facts (one-liners)

Band	Remember for the exam
Radio	radio and TV broadcasting, mobile phones (UHF); long-distance radio uses short-wave bands
Microwave	radar for aircraft navigation, speed guns (balls, cars); microwave oven at $2.45 GHz$
Infrared	"heat waves": absorbed by water, $CO_{2}$ , $NH_{3}$ molecules; physiotherapy lamps; TV remotes (LEDs); greenhouse effect; night vision and crop monitoring from satellites; snakes sense IR
Visible	detected by the eye; the eye is most sensitive near the Sun's peak wavelength (about 500 to 550 nm); many insects see into the UV
Ultraviolet	Sun is the main source; ozone layer absorbs most of it; causes tanning (melanin) and sunburn; absorbed by ordinary glass; welders' goggles; LASIK eye surgery; germicidal lamps in water purifiers
X-rays	medical diagnosis, treatment of some cancers; damage living tissue, so avoid over-exposure
$γ$ -rays	destroy cancer cells (radiotherapy); highest energy photons

Figure 3: NCERT's numbers on one scale. Radio for broadcasting (AM, short wave, FM, TV, UHF phones); microwaves (GHz) for radar, speed guns and ovens. Wavelength

λ = \frac{c}{ν}

: FM at

100 MHz

3 m

, the oven's

2.45 GHz

12.2 cm

Microwave oven in one line: $2.45 GHz$ microwaves are absorbed strongly by water molecules in food (NCERT: the frequency matches the rotation frequency of water molecules); the faster-moving molecules share the energy and the food heats from within. Porcelain and glass vessels stay cool because their large molecules do not absorb at this frequency; metal vessels spark and can melt.

X-rays

Emitted by electrons: fast electrons hitting a metal target, or inner-shell electron jumps in heavy atoms. Typical photon energy keV. Imaging bones, crystal structure.

γ

-rays

Emitted by nuclei (radioactive decay, nuclear reactions). Typical photon energy MeV. Radiotherapy. Where the wavelength ranges overlap, the origin decides the name.

3. Where Each Band Comes From

The photon energy of a band matches the energy scale of the system that emits it, and the wavelength is often close to the size of the emitter:

Figure 4: The photon energy

E = h ν

matches the energy scale of the emitter: nuclei (MeV) give

γ

-rays, inner electrons (keV) X-rays, outer electrons (eV) UV and visible, molecular vibrations infrared, rotations microwaves, and antenna currents radio waves.

Nuclei ( $1 0^{- 14}$ to $1 0^{- 15} m$ ) emit $γ$ -rays of similar wavelength; heavy atoms' inner electrons emit X-rays.
An antenna radiates most efficiently at a wavelength comparable to its length (a quarter-wave or half-wave antenna).
Exception: visible light from atoms has a wavelength (hundreds of nm) far larger than the atom ( $\sim 0.1 nm$ ).
Infrared shakes whole atoms and molecules, not just electrons, so it raises internal energy and temperature: hence "heat waves".

Exam Trick

The 1240 rule identifies a band from an energy. $E (eV) \times λ (nm) = 1240$ (or $12400$ with Å). Visible spans $1.8$ to $3.1 eV$ : below $1.8 eV$ think infrared or longer, above $3.1 eV$ ultraviolet or shorter; a few keV is X-ray, MeV is $γ$ .

Quick Recall: tap to check

Which band is produced by klystron and magnetron valves?

Microwaves.

Name two detectors of infrared radiation.

Thermopile and bolometer (also infrared photographic film).

A photon has energy

10 eV

. Which band?

λ = \frac{1240}{10} = 124 nm

: ultraviolet.

4. Temperature and the Atmosphere

4.1 Hot bodies and Wien's law

Every hot body emits a continuous spectrum whose peak shifts to shorter wavelength as it gets hotter, $λ_{m} T = 2.9 \times 1 0^{- 3} m K$ , so $T = \frac{2.9 \times 1 0 ^{- 3}}{λ _{m}}$ :

Figure 5: Wien's law

λ_{m} T = 2.9 \times 1 0^{- 3} m K

(

0.29 cm K

). The Sun (

5800 K

) peaks at

500 nm

, in the middle of the visible band where our eyes are most sensitive; a

3000 K

filament peaks at

967 nm

, so most of its output is infrared.

Band	Peak $λ_{m}$	Temperature $T$	What it tells you
radio	$1 m$	$3 \times 1 0^{- 3} K$	radio waves are not thermal in practice: they come from circuits
microwave	$1 mm$	$2.9 K$	cosmic background radiation at $2.7 K$ peaks here
infrared	$10 μ m$	$290 K$	room-temperature bodies (and people) glow in the infrared
visible	$500 nm$	$5800 K$	the Sun's surface
ultraviolet	$100 nm$	$2.9 \times 1 0^{4} K$	very hot stars
X-rays	$0.1 nm$	$2.9 \times 1 0^{7} K$	only extreme sources (solar corona); on Earth X-rays come from X-ray tubes

JEE Advanced

Thermal photons carry energy of order $k T$ . The peak photon energy of black-body radiation is about $2.8 k T$ (peak in frequency) and $\frac{h c}{λ _{m}} \approx 5 k T$ (peak in wavelength). At $300 K$ , $k T = 0.026 eV$ : infrared. To emit visible photons ( $\approx 2 eV$ ) a body must be thousands of kelvin; X-rays need millions. The two peaks differ because the spectrum per unit wavelength and per unit frequency are different functions, so $λ_{m} ν_{m} \neq = c$ (it is about $0.57 c$ ).

4.2 What the atmosphere lets through

Figure 6: Two windows reach the ground: the optical window (near UV, visible, near IR) and the radio window (about

1 cm

20 m

). That is why optical and radio telescopes sit on the ground while X-ray and

γ

-ray astronomy needs satellites. Boundaries are approximate.

Figure 7: The atmosphere is transparent to incoming visible light but absorbs much of the outgoing infrared (greenhouse gases

CO_{2}

H_{2} O

). Without this the average surface temperature would be far lower, about

- 18^{\circ} C

instead of about

15^{\circ} C

Ozone layer (stratosphere) absorbs most solar ultraviolet; its depletion by CFCs (freon) is a global concern.
Greenhouse effect: visible sunlight is absorbed by the ground and re-radiated as infrared, which $CO_{2}$ and water vapour trap, keeping the Earth warm.
Ionosphere reflects short-wave radio (a few MHz to about $30 MHz$ ) back to Earth, giving long-distance broadcasts; TV and FM frequencies pass through, so long-distance TV needs satellites.

Key idea

Hotter bodies peak at shorter

λ

(

λ_{m} T = 2.9 \times 1 0^{- 3} m K

); only the optical and radio windows reach the ground; ozone blocks UV and greenhouse gases trap IR.

Quick Recall: tap to check

Why can X-ray astronomy be done only from satellites?

The atmosphere absorbs X-rays; only optical and radio waves reach the ground in useful amounts.

Peak wavelength of radiation from the human body (

310 K

\frac{2.9 \times 1 0 ^{- 3}}{310} = 9.4 μ m

, infrared.

Why is the ozone layer crucial for life?

It absorbs harmful solar ultraviolet radiation.

5. Identifying a Band and Revision Map

Convert whatever is given to a wavelength, then compare with the NCERT boundaries. Revise the whole concept with the mind map.

Figure 8: Band identification in two steps: convert to

λ

, then compare with the NCERT boundaries

0.1 m

1 mm

700 nm

400 nm

1 nm

1 pm

Figure 9: Mind map of this concept. Cover a branch, recall its three points, then check.

6. Solved Examples

Solved Example 1

X-rays of wavelength

1 0^{- 10} m

, red light of wavelength

6800 \overset{˚}{A}

and radio waves of wavelength

500 m

are all travelling in vacuum. Which physical quantity is the same for all three?

Solution:

All electromagnetic waves travel through vacuum at the same speed. Frequency, wavelength and photon energy all differ.

Answer: the speed, $c = 3 \times 1 0^{8} m s^{- 1}$ .

Solved Example 2

A radio can tune to any station between

7.5 MHz

and

12 MHz

. Find the corresponding wavelength band.

Solution:

$λ = \frac{c}{ν}$ : $\frac{3 \times 1 0 ^{8}}{7.5 \times 1 0 ^{6}} = 40 m$ and $\frac{3 \times 1 0 ^{8}}{12 \times 1 0 ^{6}} = 25 m$ .

Answer: $40 m$ to $25 m$ (short-wave radio).

Solved Example 3

Using

E = h ν

, find the photon energy (in eV) for typical wavelengths of each band, and relate the energy scales to the sources.

Solution:

Band	typical $λ$	$E = \frac{1240}{λ ( nm )} eV$	Source energy scale
radio	$1 m$	$1.24 \times 1 0^{- 6} eV$	currents in circuits (not atomic levels)
microwave	$1 cm$	$1.24 \times 1 0^{- 4} eV$	molecular rotation
infrared	$10 μ m$	$0.124 eV$	molecular vibration
visible	$500 nm$	$2.48 eV$	outer-electron levels
ultraviolet	$100 nm$	$12.4 eV$	outer and inner electrons
X-rays	$0.1 nm$	$12.4 keV$	inner-shell electrons
$γ$ -rays	$1 pm$	$1.24 MeV$	nuclear levels

Answer: the photon energy of each band matches the spacing of the energy levels of its source, from $μ eV$ for radio to MeV for $γ$ -rays.

Solved Example 4

Use

λ_{m} T = 0.29 cm K

to find the characteristic temperatures for typical wavelengths

1 mm

(microwave),

10 μ m

(infrared),

500 nm

(visible) and

0.1 nm

(X-ray). What do the numbers tell you?

Solution:

$T = \frac{2.9 \times 1 0 ^{- 3}}{λ}$ : microwave $2.9 K$ ; infrared $290 K$ ; visible $5800 K$ ; X-ray $2.9 \times 1 0^{7} K$ .

Answer: a body must be at about these temperatures for its thermal radiation to peak in that band. Room-temperature objects radiate infrared, the Sun is visible, and radio or X-rays on Earth are not thermal but come from circuits and X-ray tubes.

Solved Example 5

Name the part of the spectrum for each: (a)

21 cm

(atomic hydrogen in space) (b)

1057 MHz

(Lamb shift) (c)

2.7 K

(cosmic background radiation) (d)

5890

5896 \overset{˚}{A}

(sodium lines) (e)

14.4 keV

(a nuclear transition in

^{57} Fe

Solution:

(a) $λ = 0.21 m > 0.1 m$ ( $ν = 1.43 GHz$ ): radio (short-wavelength end, next to microwaves).

(b) $λ$ $= \frac{3 \times 1 0 ^{8}}{1.057 \times 1 0 ^{9}}$ $= 0.284 m$ : radio.

(d) $589 nm$ : visible (yellow).

(e) $λ$ $= \frac{1240}{14 400} nm$ $= 0.086 nm$ : in the X-ray wavelength range, but emitted by a nucleus, so it is a $γ$ -ray.

Answer: (a) radio (b) radio (c) microwave (d) visible (e) X-ray range, a $γ$ -ray by origin.

Solved Example 6

Give the reason in one line: (a) long-distance radio broadcasts use short-wave bands (b) long-distance TV needs satellites (c) optical and radio telescopes are on the ground but X-ray telescopes orbit the Earth (d) the ozone layer is crucial (e) without an atmosphere, would the Earth be warmer or cooler? (f) the basis of a predicted nuclear winter.

Solution:

(a) The ionosphere reflects short-wave radio back to Earth.
(b) TV frequencies (VHF, UHF) pass through the ionosphere instead of being reflected, and the Earth's curvature blocks line of sight, so satellites relay them.
(c) The atmosphere transmits visible and radio waves but absorbs X-rays.
(d) It absorbs harmful ultraviolet radiation.
(e) Cooler: no greenhouse effect to trap the infrared re-radiated by the surface.
(f) Dense smoke and dust would block sunlight, so the surface would cool sharply.

Solved Example 7

Which sequence is in order of increasing frequency?
(A) radio waves, infrared, visible, ultraviolet
(B) ultraviolet, visible, infrared, radio waves
(C) infrared, radio waves, visible, X-rays
(D) visible, ultraviolet, infrared,

γ

-rays

Solution:

Answer: (A). Frequency rises from radio through microwave, infrared, visible and ultraviolet to X-rays and $γ$ -rays. (B) is the reverse; (C) and (D) misplace one band.

Solved Example 8

The radiation used to kill germs in water purifiers and in LASIK eye surgery is
(A) infrared
(B) microwaves
(C) ultraviolet
(D) X-rays

Solution:

Answer: (C). Ultraviolet: its short wavelength can be focused into very narrow beams (LASIK) and it destroys microorganisms (water purifiers).

Solved Example 9

A photon has energy

2 keV

. Find its wavelength and frequency and name its band.

Solution:

$λ = \frac{1240}{2000} nm = 0.62 nm$ ; $ν$ $= \frac{c}{λ}$ $= \frac{3 \times 1 0 ^{8}}{0.62 \times 1 0 ^{- 9}}$ $= 4.8 \times 1 0^{17} Hz$ .

Answer: $0.62 nm$ , $4.8 \times 1 0^{17} Hz$ : X-rays (between $1 nm$ and $1 pm$ ).

Solved Example 10

Find (a) the wavelength of an FM station at

100 MHz

and the length of a quarter-wave antenna for it (b) the wavelength and photon energy of the

2.45 GHz

microwaves in an oven.

Solution:

(a) $λ = \frac{3 \times 1 0 ^{8}}{1 0 ^{8}} = 3 m$ ; quarter-wave antenna $\frac{λ}{4} = 0.75 m$ .

(b) $λ$ $= \frac{3 \times 1 0 ^{8}}{2.45 \times 1 0 ^{9}}$ $= 0.122 m$ ; $E$ $= h ν$ $= \frac{6.63 \times 1 0 ^{- 34} \times 2.45 \times 1 0 ^{9}}{1.6 \times 1 0 ^{- 19}}$ $= 1.0 \times 1 0^{- 5} eV$ .

Answer: (a) $3 m$ , $0.75 m$ ; (b) $12.2 cm$ , $1.0 \times 1 0^{- 5} eV$ (far too little to ionise or break bonds: microwaves only heat).

Practice Questions

Find the frequency of the $21 cm$ line of atomic hydrogen.Answer: $1.43 GHz$
A photon has energy $5 eV$ . Find its wavelength and band.Answer: $248 nm$ , ultraviolet
Find the wavelength of an AM station broadcasting at $1 MHz$ .Answer: $300 m$
At what wavelength does the human body ( $310 K$ ) radiate most strongly? Which band?Answer: $9.4 μ m$ , infrared
Name two detectors each for infrared radiation and for X-rays.Answer: infrared: thermopile, bolometer; X-rays: Geiger tube, ionisation chamber (or photographic film)
Arrange in increasing photon energy: X-rays, microwaves, visible light, infrared.Answer: microwaves $<$ infrared $<$ visible $<$ X-rays
Why are infrared waves called heat waves?Answer: water and other molecules absorb them readily; the whole molecules vibrate faster, so the internal energy and temperature rise

Common Mistakes to Avoid

Watch out

Thinking different bands travel at different speeds in vacuum. All travel at $c$ ; only $λ$ and $ν$ differ.
Placing microwaves beyond radio waves in wavelength. Radio waves are the longest ( $> 0.1 m$ ).
Swapping infrared and ultraviolet. Infrared is beyond red (longer $λ$ ); ultraviolet is beyond violet (shorter $λ$ ).
Deciding between X-rays and $γ$ -rays by wavelength alone. In the overlap the origin decides: electrons give X-rays, nuclei give $γ$ -rays.
Using $E = \frac{1240}{λ}$ with $λ$ in metres or ångströms. It needs nanometres ( $12400$ with ångströms).
Saying ordinary glass lets UV through. Glass absorbs most UV, which is why you do not tan behind a window.
Confusing radar with sonar. Radar uses microwaves (EM waves); sonar uses ultrasound (sound waves).
Thinking a microwave oven heats the vessel first. It heats the water in the food; porcelain and glass stay cool.

Frequently Asked Questions

What is the electromagnetic spectrum?

It is the complete range of electromagnetic waves arranged by frequency or wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All travel at $3 \times 1 0^{8} m s^{- 1}$ in vacuum. They differ only in wavelength, which changes how they are produced, detected and absorbed.

What is the order of the electromagnetic spectrum from low to high frequency?

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Wavelength decreases and photon energy increases in the same order. A common memory aid is Raging Martians Invaded Venus Using X-ray Guns.

What are the wavelength ranges of the bands in NCERT?

Radio above $0.1 m$ , microwaves $0.1 m$ to $1 mm$ , infrared $1 mm$ to $700 nm$ , visible $700$ to $400 nm$ , ultraviolet $400 nm$ to $1 nm$ , X-rays $1 nm$ to $1 0^{- 3} nm$ and gamma rays shorter. The boundaries are not sharp and neighbouring bands overlap.

How are X-rays different from gamma rays?

X-rays are produced by electrons, for example fast electrons striking a metal target or inner-shell transitions in heavy atoms, while gamma rays come from atomic nuclei in radioactive decay and nuclear reactions. Their wavelength ranges overlap, so in the overlap the origin, not the wavelength, decides the name.

Why are infrared rays called heat waves?

Infrared radiation is readily absorbed by water and many other molecules. It sets whole atoms and molecules vibrating faster, raising their internal energy and so the temperature of the material. That is why infrared lamps warm the body and why infrared is linked with the greenhouse effect.

Why do microwave ovens use 2.45 GHz?

Microwaves of $2.45 GHz$ are absorbed strongly by the water molecules in food, which share the energy with neighbouring molecules and heat the food from within. Porcelain and glass containers absorb little at this frequency and stay cool, while metal containers can spark and melt.

Which electromagnetic spectrum questions are common in NEET?

NEET asks the order of the bands, which band is used for a given purpose (radar, LASIK, water purifiers, physiotherapy, remote controls, cancer treatment), how each band is produced or detected, and simple wavelength, frequency or photon-energy conversions to identify a band.

How is the electromagnetic spectrum tested in JEE Main?

JEE Main gives a frequency, wavelength or photon energy and asks for the band, asks for the correct order of bands, links bands with sources and detectors, and occasionally combines the spectrum with photon energy, Wien's law or the speed of light in a medium.

Previous year questions on Electromagnetic Spectrum

7 questions from past papers, each with a step-by-step solution.

Ready to master Electromagnetic Waves?

Take a full mock test, practice concept-by-concept, and get an AI-powered rank prediction — all on Fundamenthol.

Practice questions →Take a mock test