# Confusion regarding Interference of light

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1. Apr 8, 2015

### Sab95

I have been reading optics for the past few months.
If light has the property of interference, when I look around, why don't I see any interference patterns.
does it mean all the waves around me are in phase ?
Another question is, why don't Cellphone signals interfere with each other ? they are mostly microwaves within a range of frequencies. Should'nt they interfere ?

2. Apr 8, 2015

### PWiz

I'm not expert on the 2nd part, so I'll just stick to the first question. A proper interference pattern is only observed with light from the same source. Light all around you is not coherent (i.e. it does not maintain a constant phase relationship). Two waves from different sources may be in phase at one instant but out of phase in the next nanosecond. The human eye cannot cope with such rapid changes, so no observable interference pattern is produced. (It would be pretty disastrous if it did :P )

3. Apr 8, 2015

### Sab95

Lol! Then. If I sit in a room with white walls. And I turn on a monochromatic source ? Since they are all coherent now.
Will I see interference after they reflect from the walls ?

4. Apr 8, 2015

### PWiz

Do you mean walls which are perfect reflectors (no absorption, hypothetical of course)?

5. Apr 8, 2015

### nasu

Who are "they"? You mean waves reflected by different walls?

Interference simply means superposition of two (or more) waves. It happens every time you have more than one wave in the same volume of space.
In order to see a pattern of maxima and minima some extra conditions must be satisfied. They have to do with ensuring a pattern that is stable enough to be observed with your instrument (it may be your eye).

In the case of cell phones, the microwaves do interfere, of course. And not only the ones used for cell phones, the radio waves and so on.
But yo still pick up only one radio station at a time because your receptor is designed to reject all frequencies but one (actually a narrow range of frequencies) which is amplified and decoded.

6. Apr 13, 2015

### Philip Wood

Light derived from the same source (or same region of a large source) and travelling by different routes to the same point can be coherent, but even then, you're up against the very small wavelength (~$0.5 \mu \text{m}$), which implies that, except in special set-ups (such as Young's), the points of constructive and destructive interference) are too close together to be noticed.

7. Apr 13, 2015