Light Intensity: Is it Decreasing at 1/r2?

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Discussion Overview

The discussion centers on the behavior of light intensity as it relates to distance from a light source, specifically whether it decreases according to the inverse square law (1/r²). The scope includes theoretical considerations and practical applications, particularly in relation to different types of light sources, such as lasers.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions if the intensity of light decreases as 1/r² from a point source.
  • Another participant affirms that this is correct for a point source but notes that real light sources are not perfectly point-like, making this an approximation.
  • A participant inquires whether this principle applies to all types of light, including lasers.
  • It is suggested that the inverse square law holds true at significant distances from the source, particularly for lasers, but this is contingent on factors such as diffraction and the absence of absorption or other optical elements in the light's path.

Areas of Agreement / Disagreement

Participants express varying degrees of agreement regarding the application of the inverse square law to different light sources, with some acknowledging limitations and conditions that affect its validity.

Contextual Notes

Limitations include the assumption of point-like sources, the impact of diffraction for lasers, and the influence of environmental factors like absorption and optical elements.

hiltac
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Hello,
If we consider a source of light (in a point x), is it wright to say that the intensity decreases in 1/r2 ?

Thank you !
 
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If r is the distance to point x, it is right. Note that there are no exactly point-like sources of light, so for real sources this is only an approximation.
 
Does this apply to any light? Including, say, a laser?
 
Very far away from the source, yes, as you are always limited by diffraction. For a laser pointer (or all visible light lasers with a similar beam diameter), it becomes a good approximation after several hundred meters.

This assumes no absorption or other elements (like lenses) in the path, of course.
 

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