EM Spectrum
From radio (km) → gamma (pm). Visible band 380–750 nm. Toggle bands.
Key Notes
The electromagnetic spectrum spans many decades of frequency (≈ 10³ to 10²² Hz) — all the same kind of wave, only λ and source differ.
Radio waves (≥ 10⁻¹ m): generated by oscillating LC circuits; used in AM/FM, TV, mobile, radar.
Microwaves (1 mm – 30 cm): produced by klystrons / magnetrons; used in radar, satellite communication, microwave ovens (resonate water at 2.45 GHz).
Infrared (700 nm – 1 mm): from hot bodies / molecular vibrations; used in thermal imaging, IR remote controls, night vision.
Visible light (400–700 nm): the narrow band the human eye detects; produced by electron transitions in atoms.
Ultraviolet (10–400 nm): from very hot sources / electron transitions; causes sunburn, fluorescence, sterilisation. Mostly absorbed by the ozone layer.
X-rays (0.01–10 nm): from sudden deceleration of high-energy electrons (bremsstrahlung) or inner-shell transitions; used in medical imaging and crystallography.
γ-rays (< 0.01 nm): from nuclear transitions and cosmic events — highest frequency, shortest λ, most penetrating, most ionising.
All EM waves travel at c in vacuum. They differ in λ and frequency — and therefore in HOW they interact with matter.
Formulas
Wave-speed relation
Connects frequency and wavelength for any EM wave in vacuum.
Photon energy
h = 6.626 × 10⁻³⁴ J·s; useful for predicting which photons can ionise atoms (UV+) vs only vibrate molecules (IR).
Wavelength shift in a medium
Frequency unchanged across media.
Important Points
There is NO gap between bands — they overlap. The boundaries (700 nm, etc.) are conventional, not physical.
Higher frequency ⇒ shorter λ ⇒ more energetic photons ⇒ more harmful (ionising).
The ozone layer (O₃) absorbs UV-B/UV-C — without it, surface life would face unsurvivable DNA damage.
Atmosphere is transparent only to two windows: visible/near-IR and radio. Astronomy in UV, X-ray, γ-ray is done from satellites.
Microwave ovens use 2.45 GHz because that frequency couples efficiently to water's rotational modes — not because of resonance (the absorption band is broad).
Visible light is a NARROW slice of the spectrum (< 1 octave) — yet it dominates everyday perception because of the Sun's blackbody peak and atmospheric transparency.
EM Spectrum notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.