An oscillating charge emits electromagnetic radiation at its oscillation frequency. The wave contains electric and magnetic fields perpendicular to each other and to its direction of travel. In vacuum, c=fλ, so a higher frequency means a shorter wavelength. Electromagnetic waves need no material medium. An incident wave can drive charges in a receiving antenna to oscillate at the wave frequency.
The electric-field oscillation direction defines linear polarisation. A straight receiving antenna responds most strongly when aligned with that electric field and ideally minimally when perpendicular to it. The propagation direction is a separate perpendicular direction. Radio, microwave, visible and X−ray radiation all obey the vacuum wave-speed relationship despite their different frequencies and uses.
Example 1
A transmitter operates at 100 MHz. Use c=3.00×108ms−1. Find wavelength.
f=1.00×108 Hz. λ=fc=3.00m.
Example 2
A wave travels east with its electric field oscillating vertically. How should a straight receiving antenna be aligned?
Vertically, parallel to the electric-field oscillation, for the strongest ideal response.
Watch out:Electromagnetic waves propagate through vacuum; sound requires a material medium.
Electromagnetic waves and polarisation practice | Year 12 Physics | Ascent Arcade