Mysterious Tiny Spots in a Laser Spot

[YellowTaxi]

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

 

[mgb_phys]

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

 

[sophiecentaur]

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.

 

[YellowTaxi]

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 ?

 

[mgb_phys]

Yes, LEDs and lasers are pretty similar - the light is still monochromatic thought not as coherent.

 

[YellowTaxi]

thanks