Name of phenomenon that causes laser spot to have a "texture"

[macpod]

1. Take a ~5mW laser, defocus the beam so it is about 1 ft wide when projected onto a surface a few feet away
2. Affix the laser on an immobile surface so the laser is aimed at a non-glossy wall.
3. While looking at the spot, slowly shift your head left and right

Do you notice that the texture of the light seems to change?...Almost as if you were looking through a dirty piece of glass?

What is this phenomenon known as? I'm assuming it occurs because the wall itself is textured and the light source is collimated (a flashlight does not exhibit the same effect). I've been searching online for information on this phenomenon but without the right search terms I've had little success.

[NobleGas]

The appearance is known as speckle and it's a direct result of the coherence of the laser beam. It's an inteference pattern resulting from the 'texture' of the reflecting surface, as you surmised, but the 'texture' is in the order of nm so not something you could easily measure.

[macpod]

Perfect, thank you. Looks like there are quite a few interesting applications/experiments to try with this

[ronhip]

A really simple experiment is to stand a bit back (maybe a dozen feet) from the expanded spot on the wall. Stand far enough back so the speckle is obvious. Then slowly move your head left and right. If the speckle moves in the opposite direction to your head, you're nearsighted. It the speckle moves the same direction, you're farsighted. If it also moves somewhat diagonally, you have some astigmatism.

The speckle happens IN YOUR EYE, not on the wall. You can prove this to yourself by making a pinhole in front of your eye with your fingers while you look at the speckle. The smaller the pinhole, the bigger the speckles become. Now, there's no way a pinhole can change the size of the dots if they were on the wall, so it must be happening in your eye. The speckle is light interfering on your retina. Light coming through one part of your iris, interferes with light coming through other parts of your iris.

The things you learn working at a science museum...

Pretty cool!

[VDX]

... another interesting experiment -- turn of the light, so the room is dark ... then let your eyes accomodate to the dim light for 10-15 minutes ... then "draw" free with your laserpointer.

What you will notice -- some 10cm behind the spot a much weaker spot will follow! ... it's the "reaction time" of your brain neurons, that simulate an "echo" with a specified delay

Viktor

[lasersbee]

A really simple experiment is to stand a bit back (maybe a dozen feet) from the expanded spot on the wall. Stand far enough back so the speckle is obvious. Then slowly move your head left and right. If the speckle moves in the opposite direction to your head, you're nearsighted. It the speckle moves the same direction, you're farsighted. If it also moves somewhat diagonally, you have some astigmatism.

The speckle happens IN YOUR EYE, not on the wall. You can prove this to yourself by making a pinhole in front of your eye with your fingers while you look at the speckle. The smaller the pinhole, the bigger the speckles become. Now, there's no way a pinhole can change the size of the dots if they were on the wall, so it must be happening in your eye. The speckle is light interfering on your retina. Light coming through one part of your iris, interferes with light coming through other parts of your iris.

The things you learn working at a science museum...

Pretty cool!

Good experimental info.. Gotta try that.
BTW I've heard that a small pinhole in air acts like a lens...

Jerry

[ronhip]

A pinhole does not really act as a lens, but since it "stops down" your eye, it increases your depth of field to a point where you can bring objects VERY close to your eye ball and have them remain in focus. Close objects look large, so it seems like it acts as a magnifying glass, but not really. Go here for an Exploratorium "Snack" about the "Pinhole Magnifier":

https://www.exploratorium.edu/snacks/pinhole-magnifier

[Eidetic]

A pinhole was the first photographic lens, and pinhole cameras are in use by hobbyists and those observing solar eclipses to this day. It's a diffractive lens, not a refractive lens.

[Photonbeam]

The speckles your brain "sees" are the actual rods and cones of the retina.

[ronhip]

The speckles your brain "sees" are the actual rods and cones of the retina.

I disagree since you can change the size of the speckles with a pinhole... See my post below. The speckles are an interference pattern on your retina.

Ron