Interference

Interference is the phenomenon of superposition of two coherent waves that reinforce or cancel each other depending on their phase difference.

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How to measure it in class

With FizziQ and a single sound source connected to two loudspeakers, students can observe sound interference in real time.

Steps:

Connect two loudspeakers to the same source (a smartphone playing a pure tone of a given frequency, for example 1,000 Hz, via a splitter or the two channels of a stereo output) and place them about 50 cm apart, facing the observation area. Use FizziQ’s sound library to generate the tone. Since the two loudspeakers are fed by the same signal, they emit coherent waves (essential for observing stable interference): two independent smartphones would not be suitable, because their clocks drift and the phase difference between the two sounds fluctuates constantly.
With a smartphone running FizziQ in Sound level mode, move slowly in front of the two loudspeakers, parallel to the line joining them (or following an arc of a circle around them), at a roughly constant distance.
Observe that the sound level varies: it increases in some zones (constructive interference) and decreases in others (destructive interference).
Interpret: the zones of reinforcement correspond to the places where the path difference is a multiple of the wavelength. The zones of silence correspond to an odd multiple of the half-wavelength. On the perpendicular bisector of the two sources, the path difference is zero and no alternation is observed: it is by moving sideways, parallel to the line of the sources, that you sweep through varying path differences and that the loud and quiet zones follow one another.

Scientific activities on this topic

Several experiments easily performed with a smartphone, tablet, or computer allow you to observe and understand interference.

Learn more

A bit of history

In 1801, Thomas Young performed his famous slit experiment, which demonstrated the wave nature of light. By illuminating two narrow slits, he observed alternating bright and dark fringes. This result can only be explained by interference and contradicted Newton’s corpuscular theory.

Thin-film interference

The colors of a soap bubble or an oil slick come from interference. Light is reflected on the two faces of the thin film. Depending on the thickness and the viewing angle, some wavelengths interfere constructively (visible colors) and others destructively.

Modern applications

Interferometry is used in astronomy (VLBI), in metrology (Michelson interferometer) and in gravitational wave detection (LIGO). Anti-reflective coatings on glasses and camera lenses exploit destructive interference.

Complementary experiments

The experiment A bubble without noise explores the destructive interference used in active noise-cancelling headphones.

Formula

Condition for constructive interference: δ = k × λ (k integer)

Condition for destructive interference: δ = (k + 1/2) × λ

Meaning:

δ: path difference between the two waves (m)

λ: wavelength (m)

k: integer (0, 1, 2, …)

The resulting amplitude of two waves of the same amplitude A is:

A_resulting = 2A × |cos(φ/2)|

φ: phase difference (rad)

Application examples

Noise-cancelling headphones use destructive interference to suppress ambient noise
The colored reflections on a soap bubble are due to interference of reflected light
In a concert hall, some seats may have poor sound quality due to destructive interference
The iridescence on an oil slick results from light interference in the thin film
Wi-Fi networks can have dead zones due to interference between waves reflected by walls

FAQ

Q: What conditions are needed to observe interference? A: Two waves of the same frequency and a stable phase relationship between them (coherence) are required. For sound, two loudspeakers fed by the same signal are sufficient.

Q: Does interference destroy energy? A: No. Total energy is conserved. It is redistributed in space: it is concentrated in the zones of constructive interference and absent from the destructive zones.

Q: Can interference be observed with light? A: Yes, but a coherent source such as a laser is needed. Young’s slit experiment shows alternating bright and dark fringes.

Q: Why are beats related to interference? A: Beats occur when two sounds of close frequencies interfere. The phase difference varies slowly, creating alternations of constructive and destructive interference perceived as volume pulsations.

Q: How can destructive interference be avoided in a room? A: Acousticians position the loudspeakers and use absorbent materials to minimize parasitic reflections that create unwanted interference.

Phase Shift - Wavelength - Acoustic Beat