Mysterious Tiny Spots in a Laser Spot

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Discussion Overview

The discussion revolves around the phenomenon of tiny spots observed within a laser spot on a wall, with participants exploring the nature of these spots, their causes, and whether similar effects can be seen with other light sources like LEDs.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant inquires about the tiny spots seen in a laser spot, suggesting they might be related to the retina's rods and cones.
  • Another participant identifies the spots as "speckles," attributing them to interference fringes formed in the eye due to varying distances of light from different parts of the wall.
  • A further explanation is provided regarding the coherence of laser light, which allows for interference patterns to form, unlike non-coherent light.
  • A participant notes a similar speckle effect observed with an LED, questioning if LEDs can produce the same interference fringes as lasers.
  • It is mentioned that while LEDs and lasers are similar, LEDs are not as coherent, which may affect the visibility of such speckles.

Areas of Agreement / Disagreement

Participants express differing views on the coherence of light sources and the conditions under which speckles can be observed, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

Participants do not fully resolve the relationship between coherence and the visibility of speckles in different light sources, leaving some assumptions about the nature of light and interference unaddressed.

Who May Find This Useful

This discussion may be of interest to those exploring optics, light behavior, and visual perception, particularly in relation to coherent versus non-coherent light sources.

YellowTaxi
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Does anyone know what those tiny spots are you can see inside a laser spot on a wall. They seem arranged something like a honeycomb - hexagonally packed tightly against each other.

They seem to follow the motion of your head around when you see them too, so I guessed that maybe I'm seeing my own retina - the rods and cones inside the back of my own eye ? Is that what they are ?

cheers
 
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They are called speckles - there are interference fringes formed in your eye from different bits of wall that are a whole or half multiple of the wavelength difference in distance
 
The reason you seem them for a laser beam and not for an ordinary beam of light is that the laser light is coherent (the waves are all in step, so they can actually interfere with each other). In non coherent light, the waves are in random phases so you will not get addition and cancellation in any particular place.
We had to wait for someone to invent a good laser before we could make holograms of objects, which rely entirely on the coherence for the formation of a visible, photographable interference (diffraction) pattern.
 
Right, that's a bit complicated though..

I noticed I saw much the same thing (speckles) last night , there was an LED glowing in the corner of the room just before I went to bed, and it gave the same or a very similar effect to a laser spot speckle. So can LEDs cause the same fringes when you look straight into them ?
 
Yes, LEDs and lasers are pretty similar - the light is still monochromatic thought not as coherent.
 
thanks
 

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