Production & Detection Across the Spectrum

NCERT Table 8.1: The Whole Spectrum on One Card

The single most quotable asset of this chapter — every band's wavelength range, production and detection:

Type	Wavelength range	Production	Detection
Radio	> 0.1 m	rapid acceleration/deceleration of electrons in aerials	receiver's aerials
Microwave	0.1 m to 1 mm	klystron/magnetron valves, Gunn diodes	point contact diodes
Infrared	1 mm to 700 nm	vibration of atoms and molecules	thermopiles, bolometer, IR photographic film
Light (visible)	700 nm to 400 nm	electrons moving to lower atomic energy levels	the eye, photocells, photographic film
Ultraviolet	400 nm to 1 nm	inner-shell electrons moving between levels	photocells, photographic film
X-rays	1 nm to $10^{-3}$ nm	electron bombardment of metal targets	photographic film, Geiger tubes, ionisation chamber
Gamma rays	< $10^{-3}$ nm	nuclear reactions, radioactive decay	photographic film, Geiger tubes, ionisation chamber

(NCERT's UV/X-ray boundaries here differ slightly from the prose values — the overlap is real and intended; production decides borderline cases.)

Reading the Table Like a Physicist

Three organising patterns turn the table from memorisation into understanding:

1. Production energy climbs with frequency: circuit-scale electron sloshing (radio) → vacuum-tube electronics (microwave) → molecular vibrations (IR) → outer-atomic transitions (visible) → inner-shell transitions (UV) → violent electron deceleration (X) → nuclear processes (gamma). The more energetic the microscopic event, the higher the frequency it emits.
2. Detection mirrors energy too: aerials and diodes for gentle waves; heat sensors (thermopile, bolometer) for IR; eye/photocell/film for visible-UV; Geiger tubes and ionisation chambers for X and gamma, whose photons ionise matter.
3. All bands share one identity: speed c in vacuum, transverse fields, $\nu\lambda = c$. The 'types' are human labels for one phenomenon.

[JEE Tip] Conversion fluency wins marks: memorise $c = 3\times10^8$ m/s and practice $\nu = c/\lambda$ at speed: 1 m ↔ 300 MHz; 1 mm ↔ 300 GHz; 1 $\mu$m ↔ $3\times10^{14}$ Hz; 1 nm ↔ $3\times10^{17}$ Hz.

[NEET Important] 'Which is detected by thermopiles?' (IR), '…by point contact diodes?' (microwave), '…by Geiger tubes?' (X/gamma) — straight Table 8.1 rows, asked verbatim.

Solved Examples

Example 1: One-metre benchmark [NEET Numerical]

Find the frequency of a 1 m wave and name its band and detector.

Solution:

1. $\nu = c/\lambda = 3\times10^8$ Hz $= 300$ MHz.
2. $\lambda > 0.1$ m: radio (TV/UHF region), detected by receiver aerials.

Example 2: One-millimetre benchmark [NEET Numerical]

The same for a 1 mm wave.

Solution:

1. $\nu = \frac{3\times10^8}{10^{-3}} = 3\times10^{11}$ Hz $= 300$ GHz.
2. The microwave/infrared boundary — produced by vacuum tubes or molecular vibrations; detected by point-contact diodes or bolometers. Borderline cases go by production.

Example 3: One-micron and one-nano benchmarks [JEE Numerical]

Find the frequencies of 1 $\mu$m and 1 nm waves and their bands.

Solution:

1. 1 $\mu$m: $\nu = 3\times10^{14}$ Hz — infrared, just below the visible's red edge (700 nm).
2. 1 nm: $\nu = 3\times10^{17}$ Hz — the UV/X-ray overlap; an inner-shell atomic source would make it UV, a metal-target tube makes it X-ray.

Example 4: Detector matching [NEET pattern]

Match detectors to bands: thermopile, point contact diode, Geiger tube, photocell.

Solution:

1. Thermopile (and bolometer): infrared — heat detection.
2. Point contact diode: microwaves.
3. Geiger tube (and ionisation chamber): X-rays and gamma rays — ionising photons.
4. Photocell: visible and ultraviolet light. Pure Table 8.1.

Example 5: Sort the list [JEE Numerical]

Arrange in increasing frequency: 21 cm radiation, 550 nm light, 0.1 nm radiation, 10 $\mu$m radiation.

Solution:

1. Convert: 21 cm → $1.4\times10^9$ Hz (radio); 10 $\mu$m → $3\times10^{13}$ Hz (IR); 550 nm → $5.5\times10^{14}$ Hz (visible); 0.1 nm → $3\times10^{18}$ Hz (X-ray).
2. Order: 21 cm < 10 $\mu$m < 550 nm < 0.1 nm.
3. Wavelength down, frequency up — always.

Example 6: Inner vs outer shells [JEE pattern]

Why does UV come from inner-shell electron transitions while visible light comes from outer ones?

Solution:

1. Inner-shell electrons sit in deeper energy levels; transitions between them involve larger energy differences.
2. Larger energy means higher frequency radiation: beyond visible, into UV.
3. Push the energy scale further — to violent electron deceleration or nuclear levels — and you climb to X-rays and gamma: the production ladder in atomic terms.

Example 7: The 0.6 nm vs 1 nm discrepancy

NCERT's prose says UV extends to 0.6 nm; Table 8.1 says 400 nm to 1 nm. Contradiction?

Solution:

1. No — the bands genuinely overlap and have no sharp boundaries (NCERT says so explicitly).
2. A 0.8 nm wave from an inner-shell transition is UV; the same wavelength from a metal-target tube is an X-ray.
3. Classification is by production/detection, so quoted edges are rough markers, not fences.

Example 8: Identify from production [NEET pattern]

Name the radiation: (a) emitted by a radioactive nucleus, (b) from a klystron valve, (c) from a hot body's molecular vibrations, (d) from electrons striking a tungsten target.

Solution:

1. (a) Gamma rays. (b) Microwaves.
2. (c) Infrared. (d) X-rays.
3. Production is identity — the table's central lesson.

Example 9: Communication chain [JEE Numerical]

A satellite uplink uses 6 GHz. Find the wavelength, the band, and an appropriate detector.

Solution:

1. $\lambda = \frac{3\times10^8}{6\times10^9} = 5$ cm.
2. 0.1 m to 1 mm range: microwave — produced by klystrons/magnetrons, detected by point contact diodes.
3. Short wavelength = directable dish beams, the same radar logic.

Example 10: One phenomenon, seven names

What do all seven bands have in common, and in what do they differ?

Solution:

1. Common: transverse EM waves; speed c in vacuum; $\vec E \perp \vec B \perp \vec k$; $B_0 = E_0/c$; carry energy; need no medium.
2. Different: frequency/wavelength — and therefore production mechanisms, detectors, penetration and uses.
3. The spectrum is one piano with seven octave-labels, not seven instruments.