### Principle of Superposition of Waves

When two of more waves in the SAME MEDIUM meet, a new "resultant" wave is formed during this meeting The amplitude of the resultant wave is the algebraic sum of the amplitude of the two waves.

### Constructive Interference

These two waves approach each other crest to crest (or trough to trough). This is called constructive interference.

The new amplitude becomes 12cm.
5cm + 7cm = 12cm

The waves pass through each other unchanged.

For light waves, constructive interference makes the light brighter.
In sound waves, constructive interference makes the sound louder.

### Destructive Interference

These two waves approach each other crest to trough. This is called destructive interference.
Minus signs are used to indicate amplitudes of troughs.

The new amplitude becomes 2cm.
7cm - 5cm = 2cm
This is smaller than either waves amplitude.

After the waves pass through each other, they will once again be 7cm and -5cm.

For light waves, destructive interference makes the light dimmer.
In sound waves, destructive interference makes the sound softer.