Two waves and one detector

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Two equal-amplitude waves give four times one-wave intensity at equal phase, zero at opposite phase, and twice one-wave intensity at a stable relative phase of pi over two.

Explore: two waves and one detector

Keep the first wave fixed. Predict how the amplitude and relative phase of the second wave change the detector reading.

At one point in a common transparent medium: two co-propagating plane waves with the same frequency and polarization. Field units are relative to the first wave’s peak amplitude E₁; intensity units are relative to its intensity I₁.

The detector averages over optical cycles while the relative phase stays fixed.

Signed electric fields at one observation point

Wave 1 · solidWave 2 · dashedField sum · thick

Two optical periods are shown; this is a static time trace, not an optical-speed animation. A negative field is a direction of the electric field, not negative intensity.

Optical-cycle-averaged detector intensity

Separate intensity sum / I₁
Interference term / I₁
Detected intensity / I₁

All axes stay fixed as you change the controls. Field scale: −3 to +3 E₁. Intensity scale: 0 to 9 I₁. The detector bars show averaged intensity, not an instantaneous squared field or total beam power.