# What causes the specific pattern of sparkly light from a laser pointer

I have a fairly cheap green laser pointer that produces a fairly big spot of light (actually two spots most of the time!). I'd guess the size of the spot is about 1/100th of the distance from the end of the pointer to the surface it hits (e.g. after 1 metre the size is approximately 1 cm).

If the pointer is steady and I sit still I see a pattern of sparkly light (on the wall, table top, etc.). Although it sparkles, each tiny point stays in the same spot within the circle of laser light. If I move my head left or right the pattern moves with me but disappears at the edge of the spot, while a "new" part of the pattern appears on the other edge. What causes this effect?

I wear contact lenses and I've noticed exactly the same thing whether I'm wearing them or not, no matter the distance. Because each point emitted by the laser should be a true point source it should not be possible for it to appear blurry. My personal observations agree with this.

• Just to clarify: when you say "move to the left or right", do you mean translation or rotation?
– user42076
Mar 28, 2015 at 12:29
• @NoEigenvalue If the laser light is travelling in the Z axis then up/down is the Y axis and left/right is the X axis. I mean moving my head in the X axis. Mar 28, 2015 at 12:44

What the others have already told you is that the pattern forming is a speckle pattern: each bump in the surface acts as a source for a spherical wave, these waves then interfere to produce this pattern. (The black dots and circles in the image below.)

In the wikipedia-article there is also mentioned the concept of "objective speckle pattern": when the reflected laserlight falls on another (smooth) surface after being shined on a rough surface, then the pattern on the smooth surface is, as wikipedia puts it, "the same regardless of how it is imaged, just as if it were a painted pattern".

Now you can imagine that your eyes are in the plane of a imaginary smooth surface where the light reflected from the rough wall is hit on. On this "wall" the pattern (the green dots) stays the same. When you move your eyes parallel to this plane, a part of the pattern disappears on the one end while a new one appears on the other end.

• Shouldn't the slightest tilt of the eyes make the pattern change dramatically? Mar 29, 2015 at 7:39
• @JensSchauder Yes, normally it should. If it changes though then it should also change gradually, that means that you will see a part of the pattern disappear while a new part appears.
– user42076
Mar 29, 2015 at 11:52
• You can divide your "tilt" in two components: one parallel to this imaginary plane I constructed above and one away from this "wall". As it turns out, when you observe the speckle-pattern from a far enough distance, you observe the pattern in the Fraunhofer region. In this region the speckles still have a lateral dimension while they are infinitely extended in the z-direction. That means that the pattern changes and the speckles become bigger, but since your head only moves a little in the z-direction you hardly notice the latter effect.
– user42076
Mar 29, 2015 at 11:52