Is Light Dispersed Due to Interference?

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Discussion Overview

The discussion revolves around the phenomenon of light dispersion, particularly in the context of how light from a flashlight appears duller when projected onto a distant wall. Participants explore various factors contributing to this effect, including interference, scattering, and the geometry of light propagation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether light dispersion is due to interference, particularly in the context of a flashlight's beam appearing duller at a distance.
  • Another participant suggests that light dispersion occurs because it cannot propagate perfectly straight, leading to inherent dispersion as it travels.
  • Scattering due to minute surface deformities on the wall is mentioned as a reason for the dull appearance of reflected light.
  • Specular reflection is discussed, noting that polished surfaces can reflect light more directly, contrasting with the scattering from rough surfaces.
  • A participant elaborates on the non-quantum factors affecting light from flashlights, such as the shape of the light source and the design of the flashlight, which contribute to the spread of light.
  • The geometry of light propagation is addressed, with the idea that the brightness diminishes as the area of the light cone increases with distance.
  • One participant expresses a prior belief that photon interference was responsible for light dispersion, indicating a shift in understanding based on the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether interference is the primary cause of light dispersion, with multiple viewpoints presented regarding the roles of scattering, reflection, and the physical characteristics of light sources.

Contextual Notes

Some assumptions about the nature of light propagation and the effects of surface texture on reflection are discussed, but these remain unresolved in terms of their implications for understanding light dispersion.

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Why does light dispurse? is it because of interference? for example, when you shine a flashlight at wall that is farther away, why is the appearance duller?
 
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When light comes out of the flashlight, it can't come out perfectly straight, so you'll always have some dispersion as it propagates in space, that is the first reason.

If light hits a wall, the minute deformities on its surface will cause the light to reflect in different directions, such that it becomes duller. This is scattered reflection, basically diffraction.

If the wall is polished so smooth that deformities are much smaller than the light's wavelength, you have specular reflection - the case with mirrors, which are polished metal.
 
demosthenes_001 said:
Why does light dispurse? is it because of interference? for example, when you shine a flashlight at wall that is farther away, why is the appearance duller?

For things like flashlights, there are non-quantum factors that play a significant role. Flashlights with a lightbulb have a filiament which radiates, more or less, in a spherical fashion, and then a mirror to redirect the light to the front. Because the light bulb is not a point source, it is impossible to redirect all of the light that hits the mirror in the same direction. Moreover, there is light that goes directly from the bulb through the lens of the flashlight which will always have a fairily wide cone.

Since this was posted in the QM forum, I will note that there are also quantum mechanics related limitations on how precise light can be, but they're AFAIK usually not noticable using flashlights. Moreover I don't know nearly enough about QM to make any informed comments about it.

Regarding distance and light dullness:
The flashlight produces a cone of light. If you assume that the amount of light is constant for a cut across this cone (usually a reasonable assumption) then it can be inferred that the light per area (brightness) is less as the sections of the cone get larger i.e. further from the flashlight. If the cone is not spread out - for example, if mirrors are used to focus the light - it is quite possible to have relatively bright light at fairily long distances. Similarly, laser lights are bright for long distances because they have very narrow cones.
 
Last edited:
thank you. I always thought that it was the photons interfering with each other that made light dispurse.
 

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