When we look at a light source from a distance in suitable darkness, why do we see discrete light spikes coming out of source like Fig. b of above picture instead of Fig. a?
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$\begingroup$ Do you mean spikes like this? If so, your question is addressed in How does Fraunhofer diffraction depend on the orientation of the sides of a lens?. $\endgroup$– John RennieCommented Feb 7, 2016 at 6:44
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2$\begingroup$ Who is this "we" you speak of? I don't see that unless my glasses are dirty. $\endgroup$– Carl WitthoftCommented Feb 7, 2016 at 14:13
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$\begingroup$ Perhaps more direct than Johns link is: physics.stackexchange.com/questions/35935/… $\endgroup$– dmckee --- ex-moderator kittenCommented Feb 7, 2016 at 17:20
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Your cristaline is not a lense made of homogeneous glass, it is made of cells. These are mostly transparent, but still there is a difference at their borders, and theses are aligned at large scale, forming a structure. This structure makes a slight variation of the optical properties, but it does diffract a bit of the incomming light, and at huge intensity you can see the resulting pattern.
It is even more intense at night when you see a bright spot, since your iris is then more widely open (exposing more of the structure).
