Waves: Why No Interference Pattern with 2 Flashlights?

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Discussion Overview

The discussion centers around the question of why shining two flashlights on a wall does not produce an interference pattern. It explores concepts related to wave interference, the nature of light sources, and the conditions necessary for observable interference patterns.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the expectation of seeing an interference pattern from two flashlights.
  • One participant explains that the light from a flashlight consists of many different wavelengths, which leads to an average mix of bright and dark spots, preventing a clear interference pattern.
  • Another participant acknowledges the role of uniform wavelength in producing specific interference patterns but argues that it is not strictly necessary, citing that white light can also create interference patterns under certain conditions.
  • Factors such as diffraction are noted as significant in preparing light waves for interference, with distinctions made between Fraunhofer and Fresnel diffraction regarding their requirements for monochromatic sources.
  • Participants express interest in clarifying the type of interference or experiment being considered.

Areas of Agreement / Disagreement

There is no consensus on the expectations surrounding interference patterns from flashlights, with multiple viewpoints presented regarding the conditions necessary for such patterns to emerge.

Contextual Notes

The discussion highlights the complexity of light behavior and the conditions under which interference patterns can be observed, indicating that assumptions about light sources and their wavelengths are critical to understanding the phenomenon.

noobphysicist
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You shine two flashlights on a wall. Why don’t you see an interference pattern?
 
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Why would you expect to see an interference pattern?
 
noobphysicist said:
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.
 
noobphysicist said:
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?
 

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