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Post details Post   (Contract pics)
simon3
2011-08-16 13:17:29
"Laser Beam" from simon3 Contract pics
Picture: Laser Beam (Contract pics)

Lasers in the mist
On the top of Stumpa Dúloigh jackill did his usual thing of assessing where the highest point actually is. MountainViewers have tended to use the Abney Level as a way of assessing this. In this case the highest point appears to be some metres from the junction of two fences. jackill was assessing the distance from this junction to the highest point where I was with the camera. He has a handy little laser rangefinder gadget for this which showed the fence junction to be 21m from the highest point.

The laser light shows up nicely, being scattered by the misty air. Shows up that is except when it was getting near to the camera - something that begs explanation. As anyone knows who takes photos in the Irish mountains, mistiness is a huge issue. In this case the mistiness is Mie scattering caused by water droplets. These are rather large compared to the scattering caused by small particles that causes blue skies (Rayleigh scattering). Mie scattering also has another important quality. Most of the scattered light goes forward close to the original line of travel. So when viewing the droplets in the beam from a distance you see them because you and they are nearly in line. However, nearer, the angle is different and you can't see the beam.

It's all beautifully explained here:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

And the relevance? Well knowing about the two types of mist and what, if anything, you can do about them is really useful for upland photographers. For example, because of forward Mie scatter the sun or the moon tend to get a whitish halo around them when there are large particles in the air. The same doesn't happen for Rayleigh scattering - you don't see a blue halo around the sun. This laser beam is a great example of the physics.
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