Laser Light Questions

[richardland]

I have just purchased a spirit level within which is an embedded red 1mw laser. I have 2 questions regarding it.

Q1 Why does reflected laser light look speckled? The size of the speckling seems independant of distance from the reflected surface. i.e to the eye the speckles look the same size whether viewed from near or far.

Q2 I placed the spirit level on a solid tarmac road and shone the light about 150m on to the trunk of a tree at night. There was zero wind and no traffic within one mile of the level however the beam of light was seemingly randomly moving about by about 0.5 cm on the surface of the tree trunk in a seemingly random pattern. Why would this be?

[4GS]

Q1: Don't know (sorry)

Q2 Slight earth tremors?

[kempie]

Q1 http://en.wikipedia.org/wiki/Speckle

[matt1e]

Q2 over150 metres its hard to keep your eye fixed on one small spot so I would say -no movement of ground,level or tree? must be your eye ball or dodgy level lol

[richardland]

Thanks kempie for the wiki reference. Very interesting and completely answers my question,

Hi 4GS , hmm interesting. Could be I suppose but the movement seems continuous though seemingly random.

Hi matt1e. Sorry I did not make myself clear. The laser level is on the ground shining it's light onto a tree 150m away (there is a slight slope to the place where the level is resting so the light at 150m distance hits the tree at about eye level). I walk to the tree and look at the spot which at this point is about 2cm in diameter. There is a definite random movement of the spot by around .5cm in all directions.

[fo3nix]

2) Stick the laser down with duck tape or similar, see if it makes any difference.

[gen2]

I assume your laser is not being blown by wind?

May I suggest that Qu.2 is caused by variations in the refractive index of the air above the road over that 150m distance.

During the day, tarmac heats up faster than surrounding grassy areas, and at night cools down more rapidly. Cold air is denser and so will refract light more. You are probably familiar with the visual effect in the daytime called 'heat shimmer' and your laser would show similar behaviour in daytime if it was strong enough to be seen. You are seeing the reverse at night as the air cools.

[richardland]

Thanks Gen2, yep that seems to make sense. I'll try it on different nights after different types of day and see if it makes any difference.

[fo3nix]

I'd also think gen2 is right with this (assuming no movement of the laser itself). This same phenomena also causes mirages, etc.