4.9.1 Superposition

• The principle of superposition states:

When two or more waves with the same frequency arrived at a point, the resultant displacement is the sum of the displacements of each wave

• The waves often travel in opposite directions because they're reflected at a boundary
• This principle describes how waves that meet at a point in space interact

• Superposition experiments include using sound, light and microwaves

Sound

• Superposition creates stationary, longitudinal sound waves in a resonance tube such as in an organ pipe or woodwind instruments such as a flute

Stationary waves in an organ pipe

• Superposition experiments with sound often use air columns or speakers
• If two loudspeakers are connected to the same signal generator, the superposition of the sound waves can be heard when walking along in front of the speakers
  • A loud sound is heard when the sound waves reinforce one another
  • A quiet or no sound is heard when the waves cancel each other out

The superposition of sound waves can be detected by the person walking past hearing a loud and soft sound in intervals

Light

• The superposition of light waves is demonstrated through:
  • Young's double-slit experiment
  • Diffraction grating

• The light waves are superposed when they reach a screen
  • This shows an interference pattern

• Monochromatic laser light is commonly used for these experiment to produce the clearest interference pattern on the screen
• The distance between the maxima and minima on the pattern varies with the frequency of the light (colour)

An example of an experiment that demonstrates superposition is light passing through a diffraction grating

Microwaves

• Similar to light and sound, microwaves also superpose to create regions where the microwaves reinforce or cancel each other out
• The interference of microwaves creates a standing wave inside a microwave oven, which is used to heat food
• Microwave superposition experiments normally include:
  • Two microwave transmitters
  • A microwave detector

• To produce a microwave stationary wave, a microwave reflector is often used too, with just one transmitter