Explain the Adding Waves?

When two waves come from the same light source, the light waves are coherent, meaning that the phase difference between the waves is constant in time. If these two coherent waves meet at a point in space, the result is equal to the sum of the two waves. This phenomenon is also called the superposition of waves. If the two waves are almost equal sine waves, the result is a sine wave about twice as big.

This is a case of constructive interference, where the result is bigger than either of the original waves. If the waves are sine and negative sine, the result is almost complete cancellation of the wave at this point. This situation is a case of destructive interference, where the result is smaller than either of the original waves being added.