Far from being a solely quantum mechanical phenomenon, interference occurs with all kinds of waves, including water waves and sound waves. Interference is the process in which two or more waves of the same kind come together, and they can add so as to reinforce one another or they can combine so as to cancel one another. For example, the crests of two water waves can add together to give a very large wave, or the crests of one wave can combine with the troughs of another to give a much smaller wave. The latter effect is intentionally employed with sound waves in noise-canceling headphones. Typically, interfering waves reinforce in some places and cancel in others.

You can see interference of light even without using a modern device such as a laser. All you need is a hair and a far-away light such as a street lamp. The light needs to be far enough away that it nearly looks like a point of light. Simply hold the hair close to one eye as you look at the light and then slowly move the hair so that it is between your eye and the light. You should observe two spires of light shooting out perpendicular to the hair, due to the diffraction of the light at the hair.

If you look carefully at these spires, you will notice that they are dashed, due to the interference of the light passing around one side and the other side of the hair. The dark patches correspond to places where the waves are destructively interferingcanceling each other outwhereas the bright spots correspond to places where the waves are adding together constructively.

As mentioned in the main article, explaining this effect does not require the notion of photons or quantum mechanics; considering light to be a classical (that is, nonquantum) wave suffices. However, if you were to repeat the experiment sending one photon at a time at the hair, the same pattern would be built up, photon by photon. That result reveals that each photon interferes with itself and each photon is in a quantum superposition of passing on one side of a hair and the other.

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