# Electromagnetic Waves > Electromagnetic Waves for NEET-UG Physics — displacement current, wave properties, the EM spectrum with uses, ozone and greenhouse effect. - Canonical URL: https://prepelephant.com/topics/neet-ug/physics/electromagnetic-waves-ncert - Exam / course: NEET-UG · Subject: Physics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Electromagnetic Waves", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/electromagnetic-waves-ncert ## Direct answer Maxwell's insight was that a changing electric field generates a changing magnetic field and vice versa, so the two sustain each other and travel together as a transverse wave at c = 1/sqrt(μ0ε0) = 3 × 10^8 m/s in vacuum — light being one member of the family. The displacement-current repair to Ampere's law is the theoretical hinge, and the spectrum from radio to gamma, with its named bands and uses, is what NEET-UG actually examines. ## What you must remember - Displacement current I_d = ε0 dΦ_E/dt: a changing electric field is equivalent to a current, patching Ampere's law so the field between capacitor plates stays consistent; total current = conduction + displacement. - Wave shape: E ⊥ B ⊥ direction of propagation; E and B oscillate in phase with E0/B0 = c; the waves are transverse and need no medium. - Energy divides equally between the fields; intensity I = (1/2)ε0 E0^2 c; momentum carried is U/c, so radiation pressure is I/c on a perfect absorber and 2I/c on a perfect reflector. - Spectrum by wavelength (NCERT values): 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 to 1 nm; X-rays 1 nm to 10^-3 nm; gamma below 10^-3 nm. - Band uses: radio and TV broadcast; radar and microwave ovens; infrared remotes, night vision and physiotherapy; UV sterilisation and fluorescent lamps; X-rays imaging; gamma rays cancer therapy. - The ozone layer absorbs harmful solar UV; greenhouse gases (CO2 and water vapour) transmit visible light but trap outgoing infrared — the greenhouse effect. - Hertz produced and detected electromagnetic waves in 1887, confirming Maxwell's 1865 prediction; microwaves were studied by Jagadish Chandra Bose, whose millimetre-wave work is a matter of national pride in Indian physics courses. ## Why the capacitor forced Maxwell's hand Charge a capacitor through a wire. Ampere's law, in its original form, relates the magnetic field around any loop to the current piercing the surface the loop bounds — but choose the surface to pass between the plates, where no conduction current flows, and the law returns a different field for the same loop. Contradiction. Maxwell's repair: the changing electric field between the plates is itself a current, I_d = ε0 dΦ_E/dt, numerically equal to the conduction current in the wire, so every surface spanning the loop is pierced by the same total current and the magnetic field becomes well-defined again. The repair is not cosmetic — it is a new statement that a changing E field sources a B field, the mirror of Faraday's changing-B-sources-E. Put the two together and fields can propagate: an E that changes makes a B that changes makes an E... and Maxwell computed the speed of this self-sustening ripple as 1/sqrt(μ0ε0) = 3 × 10^8 m/s, the measured speed of light. Hertz's sparks twenty-two years later, and Bose's millimetre waves after, closed the case. ## Where students slip The ordering question — arrange the bands by wavelength or by frequency — flips the answer: wavelength order radio to gamma is descending, frequency order is ascending, and the exam deliberately mixes the two phrasings. Second, E0/B0 = c, so the magnetic field amplitude in SI units is tiny compared with E0; students who invert the ratio lose the check the formula provides. Third, displacement current is not carried by charges and exists even in vacuum — "no current can flow through empty space" is false in Maxwell's sense, and assertion-reason questions probe precisely this. Fourth, X-rays and gamma rays overlap in the table and are distinguished by origin, not by wavelength: fast-electron deceleration for X-rays, nuclear transitions for gamma. Fifth, the greenhouse mechanism: incoming visible passes the atmosphere, the re-radiated infrared cannot — students who say CO2 blocks sunlight have the direction backwards, and the ozone question is the mirror trap (it filters UV, not IR). ## Frequently asked questions ### What is displacement current? ε0 times the rate of change of electric flux — a changing electric field acting as a current, which Maxwell added to Ampere's law to keep the magnetic field between capacitor plates consistent with the field around the feeding wire. ### What is the ratio of electric to magnetic field amplitudes in an electromagnetic wave? E0/B0 = c, the speed of light; in vacuum this holds at every instant and place, with E, B and propagation mutually perpendicular. ### Which band of solar radiation does the ozone layer absorb? Harmful ultraviolet; the greenhouse gases CO2 and water vapour, by contrast, absorb outgoing infrared, which is a different layer and a different mechanism. ### Arrange the spectrum in increasing frequency. Radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays — the exact reverse of the wavelength ordering, which is how the question is usually disguised. ### What pressure does radiation exert on a perfect absorber? P = I/c, where I is the intensity; a perfect reflector experiences double, 2I/c, because the momentum of the light reverses on reflection.