Why can I see light from TEM00 laser in direction of propagation?

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Discussion Overview

The discussion revolves around the visibility of light from a TEM00 laser in the direction of propagation, specifically exploring the classical electromagnetic (EM) explanation for this phenomenon. Participants examine the nature of laser light, the behavior of electric and magnetic fields, and the interaction with the human retina.

Discussion Character

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

Main Points Raised

  • One participant notes that while there is no electric or magnetic field in the direction of propagation, the intensity of the laser is greatest along that direction, questioning what excites the retina.
  • Another participant argues that laser light should be viewed as a coherent state rather than a stream of photons, emphasizing that both electric and magnetic fields are transverse.
  • A different participant asserts that the electric field is responsible for exciting the retina, regardless of its direction relative to propagation.
  • One participant questions whether all light behaves similarly, with field vectors perpendicular to the direction of propagation, and seeks to understand what distinguishes the TEM00 laser.
  • Another participant explains that the eye's aperture plays a role in detecting light, suggesting that alignment with the laser's direction affects light detection.

Areas of Agreement / Disagreement

Participants express differing views on the nature of laser light and its interaction with the retina, with no consensus reached on the classical explanation for visibility in the direction of propagation.

Contextual Notes

The discussion includes assumptions about the behavior of electric and magnetic fields in relation to laser light and the interaction with biological systems, which may not be fully resolved.

Aziza
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In my lab I have a TEM00 laser, so there is no electric or magnetic field in the direction of propagation (call it z direction). However, I know that the intensity of the laser is greatest when my eye is along z direction...why? If there is no electric field along this direction, then what is exciting the molecules in my retina??

I know that the modern explanation is that the laser is actually a stream of photons, but I am trying to model laser light propagation using classical EM, so I am looking for the classical explanation
 
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First of all the laser light is not a stream of photons but, if you wish to express it in terms of quantum electrodynamics, a coherent state, i.e., a superposition of photon states with arbitrary photon numbers.

Second, as the name of the mode says, both the electric and the magnetic field are transverse. But there is a field, and that excites your retina.

Third, I hope you haven't really looked directly into the laser light! This is no fun for your eyes!
 
The electric field is still doing the exciting, there's no requirement for this field to be in the same direction as the direction of propagation. The forces on charges due to the wave will be in the polarisation direction.

This is the nature of the plane wave solution to Maxwell's equations
 
Isn't all light just like this, with the field vectors perpendicular to the direction of propagation? What's different about the TEM00 laser?
 
Because your eyes have an aperture that the light must pass through before it is detected by the retina. Obviously if the aperture is aligned in the direction of the laser, you will detect the most light.

Claude.
 

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