Page 305 - Blog Posts v1

E45 - When is a laser beam not a laser beam? When it enters the skin...
Laser light is unique due to three particular properties:
Monochromacity Divergence Coherence
Lasers generate a very narrow band of wavelengths, typically within a few nanometres around a central line. So, a ruby wavelength of 694nm will actually be a spread between 692 and 696nm, depending on how ‘good’ the resonator is. Poor resonators will have a wider spread.
Most laser beams are minimally divergent meaning that they spread out very slowly over distance. This is why we can fire them across rooms and annoy cats and dogs and small children. We say that such beams are highly collimated.
Finally, they are highly coherent. Coherence is often misunderstood but is actually quite simple. All light which emerges from very small single sources is coherent. The light waves can be spherical or planar – it doesn’t matter. The important point is that there is no interference from other light sources. Starlight is highly coherent since they appear to be from ‘point’ sources, to us.
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