[Physics] see three ‘beams’ when I look to a distant light source at dark

diffractionopticsvisible-lightvision

In the night, when I look at a distant light source (for example a street lamp a few hundred meters away) I do not simply see a spot but rather a spot which is surrounded by 'light beams'.

So instead of seeing the image on the left-hand side of the picture below I see the one on the right-hand side.

enter image description here

I know a similar effect also comes up by using camera lenses, even though I am not sure if the reason is exactly the same. Below, for example, is a picture with four 'beams' (Source: https://astromallorca.org/astrofotografias/dsc_0015/).

enter image description here

So, why do I see with my own eyes three beams whereas with a camera there appear to be more?

Best Answer

From physical point of view:

  • One reason is diffraction spike by the mounts (support vanes) of optical reflector of reflecting telescope. Four-fold mounts give four-fold diffraction pattern. However, three-fold mounts give six-fold diffraction pattern since we can see intensity but not the phase of the diffraction pattern. This can be reduced by other design.

  • Photographers can add star effect filters (which contains etched gridlines) on their cameras for shooting Christmas lights and so on.

    enter image description here

From medical point of view:

  • Another reason may be due to astigmatism, glare or starbursting of someone's eye.

    enter image description here

  • See the link for the image here and vision simulations here and the causes of starbursting appears in this site.

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