Electromagnetic waves from an aerial – E ⊥ B, c = 1/√(μ₀ε₀), c = fλ, polarisation
PhysicsOscillations & WavesAges 17–18
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Sign in to playA rotatable 3D model: charges oscillating in a half-wave dipole aerial radiate an electromagnetic wave whose electric field E and magnetic field B are perpendicular to each other and to the direction of travel, oscillate in phase and travel at c = 1/√(μ₀ε₀). Change the frequency to see the wavelength λ = c/f and the wave's place in the spectrum, turn the receiving aerial to see the signal follow cos θ, and put a block of refractive index n in the path to slow the wave to v = c/n.
Lesson: Electromagnetic waves: production by accelerating charges, E ⊥ B, c = 1/√(μ₀ε₀), c = fλ, polarisation, transmitting and receiving aerials
What it shows
An alternating current makes charges accelerate along a dipole aerial, and accelerating charges radiate electromagnetic waves. The electric field E is parallel to the aerial, the magnetic field B is perpendicular to E, and both are perpendicular to the direction of travel, so the wave is transverse. E and B oscillate in phase with B₀ = E₀/c, and Maxwell's theory gives the speed c = 1/√(μ₀ε₀). A dipole emits plane-polarised waves, so a receiving aerial turned through θ picks up only E cos θ. In a material of refractive index n the wave slows to v = c/n and its wavelength shrinks to λ/n but the frequency stays the same.
How to use
Drag the Frequency f slider and watch the wavelength bracket, the aerial length λ/2 and the red marker on the spectrum strip change. Turn the Receiver angle θ slider and compare the green trace on the oscilloscope with the yellow one. Raise Refractive index n to place a block in the path. Tick Radiation pattern to see where the aerial sends most energy. Restart sends a fresh wave front from the aerial; drag the 3D view to rotate it or switch to 2D.
Parameters you can change
- Frequency f 100–1000 MHz
- Angle between receiving aerial and E 0–90 °
- Refractive index n of the block (1 = no block) 1–2
- View 3D, rotatable, 2D (oblique projection)
- Show radiation pattern
Questions to explore
- When you double the frequency, what happens to the wavelength and to the length of the half-wave aerial?
- Why does a receiving aerial at 90° to the transmitting aerial pick up no signal?
- When the wave enters a block with n = 1.5, which quantities change and which one stays the same?
