Relation Between Superposition Principle and Uncertainty Principle

The red curve is the sum of the blue curves. The larger the spread in wavelengths of the blue curves, the smaller the size of the resulting wavepacket (red curve). Drag rectangle (below the picture) back and fourth to change spread in wavelengths.

Moving and Spreading Wave-Packet

Notice that as the wave moves, the group velocity (velocity of the peak of the wave-packet) is larger than the phase velocity (velocity of one particular crest of the wave).  Also, notice that after a while, the front of the wave has shorter wavelength than the back of the wave. 

Two Stationary and Spreading Wave-Packets: 2-Slit Experiment