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# Observing interference of waves

## Prerequisites for interference observation

The patterns that can be observed are limited by human visual processing capability. There are several prerequisites for the interference to be observable by the human eye:

1. The interfering waves must be coherent. If they are not, the waves would constantly shift from being in phase to being in anti-phase and there would be no fixed pattern of interference.

Recall that the frequency of most sources is high enough (much more than $1\text{ Hz}$) that any difference in the frequency of the waves would result in rapidly changing interference patterns.

These patterns change too fast to be perceivable by the human eye.

2. The interfering waves should approximately have the same amplitude. If this is not the case, the difference between regions of constructive and destructive interference would be too small to be perceived by the human eye.

3. For transverse waves, the interfering waves must be either unpolarised or polarised in the same plane.

This is because interference can only occur between transverse waves which cause oscillations in a particular plane (as oscillations in different planes are not affected by each other).

## Water wave interference

The two-source interference pattern is the simplest type of interference pattern produced by interacting waves.

Two vibrating dippers which produce circular waves of the same frequency can be set up in a ripple tank (a tank of water where water waves can be observed).

The waves interfere with each other and produce interference patterns depending on whether constructive or destructive interference occurs at particular points.

Two source ripple tank interference pattern. The black lines connect points of maximum amplitude.

## Microwave interference

The two-source interference pattern is the simplest type of interference pattern produced by interacting waves.

A metal double-slit (two openings cut into a continuous piece of metal) can be placed in front of a source producing a beam of microwaves.

When these microwaves pass through the slits, they diffract and form two coherent waves groups (or trains). These wave groups interfere with each other and form alternating regions of high and low amplitude microwaves.

Microwave double-slit interference pattern