Why do astronomers like green laser pointers?

  • When I've been to Astronomy-nights for the public at the local observatory, they always used green lasers to point things out.

    Why is there such a preference for green lasers?

    What's wrong with red lasers? (Which seem cheaper and more readily available.)

    I would guess it is because the human eye sees green better than red in low light conditions. Though the pointer usage of a few local astronomers isn't exactly proof that they all prefer green.

    @zibadawatimmy - that's the shortest and best answer. Green lasers are easily visible at powers that are not dangerous. A red or blue laser would require much more power to be as visible as green, and the more power you put into the beam the more easily things could go wrong. That's all there is to it.

    Several answers have been provided below, some quite long, most quite incorrect. The best one was given by @BanzaiAstronomy.

    this question is not about astronomy

    From the FAQ: *"The purpose of this site is to provide expert level answers to questions on: Setting up, using and maintaining your astronomy related equipment"*

    @Walter I'm sorry, but saying this is off topic is like saying a question asking how a Bayer lens works is off topic. A green laser pointer is a piece of astronomical kit and questions about it should be allowed.

    @abelenky A laser pointer is not astronomical equipment. Lasers are also used in astronomical equipment, for example in artificial guide stars for adaptive optics, but this was not the question.

    @abelenky I haven't seen even a single question here that fits that criterion.

  • Mostly general purpose laser pointers are used for pointing things at smaller distances eg. Diagram or Equations in Powerpoint presentations, so the power of such laser pointers is quite limited/restricted to 5mw ( Class 3A or IIIa) or 10mW in some regions.
    Because of the low power and small aperture of laser pointers if you point them through empty space, you can only see them impact spot where it hits a surface.

    So to point stars/planets as in astronomy club during observation sessions these low power laser pointers are not useful as there is no surface where the beam can hit & you can see the spot.

    Some higher-powered laser pointers project a visible beam via scattering from dust particles or water droplets along the beam path.
    Higher-power ( Class 3B or IIIb lasers: greater than 5mW) and higher-frequency (Green or Blue) lasers may produce a beam visible even in clean air because of "Rayleigh Scattering" from air molecules, especially when viewed in moderately-to-dimly lit conditions. The strong wavelength dependence of the scattering (~λ^ −4 ) means that shorter (green & blue) wavelengths are scattered more strongly than longer (red) wavelengths.

    [ Rayleigh scattering is the mainly the elastic scattering (in lay term collision) of light or other electromagnetic radiation by particles much smaller than the wavelength of the light.
    Rayleigh scattering results from the electric polarizability of the particles (locally separating positive & negative charges by small distance apart inside the atom. i.e. temporarily separating centre of positive charge and centre of negative charge). The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency (hence the green colour). The particle therefore becomes a small radiating dipole (or small sources of light) whose radiation we see as scattered light (the beam passing from pointer to star pointed). ]

    The intensity of such scattering increases when these beams are viewed from angles near the beam axis. (That's the reason, if you are standing close to the person pointing stars in astronomy club, you can see the beam bright and clearly whereas the people standing away can barely see it. So always stand near the presenter But avoid contact with your eye. :-) )

    The apparent brightness of a spot from a laser beam depends on the optical power of the laser, the reflectivity of the surface, and the chromatic response of the human eye. For the same optical power, Green Laser light will seem brighter than other colours because the human eye is most sensitive at low light levels in the green region of the spectrum (wavelength 520–570 nm). The most sensitive pigment, rhodopsin, has a peak response at 500 nm. Sensitivity decreases for redder or bluer wavelengths.

    Further, Green laser pointers are of moderate power (sufficient for Rayleigh Scattering in clean air), they are compact and relatively cheaper than blue ones (even though human eye is less sensitive to blue colour, blue laser are a little bit more expensive than green one.)

    Avoid their contact with your eyes and don't point them at aircraft.
    People have been given up to five years in jail for aiming a green laser at an aircraft.

    You gave a very long and partially wrong answer to a really simple problem. Scattering that makes laser beams visible is not via the Raleigh mechanism, it's simple scattering on dust motes and water droplets, so it's independent on wavelength. The reason why green lasers are preferred is simply because they require less power to produce the same visual impression. A red or blue laser would need to be more powerful to be as visible as green, and more power can be unsafe. Green is very visible at safe powers, that's all there is to it.

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Content dated before 7/24/2021 11:53 AM

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