Lesson 4 of 12 · 8 min
Wave nature of electromagnetic radiation
NCERT §2.3.1
The street lamp's yellow, the rocket's red and the radio playing film songs next door are all the same kind of wave. What makes them different?
The lesson in notes
In short
Maxwell (1870) showed that an accelerating charge produces oscillating electric and magnetic fields that travel as electromagnetic waves; the two fields are perpendicular to each other and to the direction of travel.
Electromagnetic waves need no medium and all travel through vacuum at c = 3.0 × 10⁸ m s⁻¹ (2.997925 × 10⁸ m s⁻¹ more exactly).
Frequency ν (unit hertz, Hz = s⁻¹) is the number of waves passing a point per second; c = νλ.
Wavenumber ν̄ = 1/λ is the number of wavelengths per unit length; its SI unit is m⁻¹ but cm⁻¹ is the common unit in spectroscopy.
The regions differ only in wavelength: radio about 10⁶ Hz (broadcasting), microwave about 10¹⁰ Hz (radar), infrared about 10¹³ Hz (heating), visible about 10¹⁵ Hz, ultraviolet about 10¹⁶ Hz (part of sunlight).
Visible light runs from violet at 400 nm (7.5 × 10¹⁴ Hz) to red at 750 nm (4.0 × 10¹⁴ Hz); longer wavelength means lower frequency.
Interference and diffraction are wave behaviours, and the wave picture explains them.