Fletcher Hays

2022-06-25

How exactly can a combination of monochromaticity and coherent light make a light not want to spread out? When it's all one wavelength and it's all amplified it should make the light brighter and thus more energetic. I can't seem to understand why the light doesn't spread normally.

pheniankang

If you take a light source such as a hot filament (not a laser) and put the filament one focal length away from a parabolic reflector, then you will get an accurately collimated beam if the filament is small compared to the focal length. This is the sort of thing that happens in searchlights (though depending on type they might use an arc or some other source rather than a filament).
To get this collimation, however, the source has to be small in length and width, and this means it is hard to get a bright collimated beam this way. Also you have to position the source accurately relative to the reflector. So it is hard in practice to get as good collimation as you can quite easily from a laser, but it is not impossible. In this sense lasers are no better collimated than other light sources could in principle be if they were engineered carefully. It is just that the collimation is a whole lot easier to achieve for a laser without compromising on brightness.
The ultimate limit to collimation of a light beam is called the diffraction limit. For a beam of width $w$ and wavelength $\lambda$ the diffraction limit is approximately given by the angle
$\theta \simeq \frac{\lambda }{w}$
(you can multiply this by a $1.22$ if you like). This is angular spread of the beam. This limit comes from the wave nature of light, so no matter what the source (laser or not), a beam of width $w$ will, at a large distance $L$ from the start point, have spread out to a width at least
$\theta L=\frac{\lambda }{w}L.$
No light source can do better than this. Most light sources do a lot worse. Lasers often get close to it because their internal manufacture typically requires accurate mirrors at each end of the laser cavity, and the amplification process favours one particular spatial configuration of the light.

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