Waves: Why No Interference Pattern with 2 Flashlights?

[noobphysicist]

You shine two flashlights on a wall. Why don’t you see an interference pattern?

 

[TumblingDice]

Why would you expect to see an interference pattern?

 

[Nugatory]

You shine two flashlights on a wall. Why don’t you see an interference pattern?

The light from a flashlight is a mix of many different wavelengths. Interference puts the dark and bright spots at different places for different wavelengths, so on average you end up with an equal mix of bright and dark everywhere. To get a visible interference pattern you need light sources that produce light of a uniform wavelength.

 

[TumblingDice]

You shine two flashlights on a wall. Why don’t you see an interference pattern?

I asked why you expected to see interference to understand if you were having a problem understanding a specific type of interference. What Nugatory said is true in a general sense, and yet other factors come into play. The key mechanism in interference patterns are the 'destructive' and 'constructive' interactions between the waves as they cancel or reinforce each other as they meet.

Uniform wavelength can assist in producing specific interference patterns, but isn't required. White light can produce interference patterns, too, given conditions to set up the waves to regularly destruct/construct. Soap bubbles and oil films produce interference patterns.

Diffraction plays a big part in prepping light waves to allow interference patterns. Fraunhofer diffraction is more restricted requiring monochromatic sources. Fresnel diffraction is more relaxed, and can also create interference patterns. More here:
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/diffracon.html#c1

Is there a particular kind of interference or experiment you are wondering about?

 

[pmacias]

Interference patterns are typically observed when two or more coherent sources of waves, such as light or sound, interact with each other. In the case of two flashlights shining on a wall, the light waves produced by each flashlight are not coherent, meaning they do not have a consistent phase relationship with each other. This is because the light waves from each flashlight are produced by independent sources and have different frequencies and wavelengths. Therefore, when these two sets of light waves reach the wall, they do not interact in a way that produces an interference pattern. Instead, the light from each flashlight will simply add together, resulting in a brighter spot on the wall where the two beams overlap and no interference pattern will be observed. In order to produce an interference pattern, the light waves must have a consistent phase relationship, which can be achieved through the use of a single coherent light source or by splitting and recombining a single light source using a device such as a diffraction grating.