What is the principle of sight?

 

The light from the target enters the human eye through the spectroscope, and the human eye can see the real image of the target. And the light of the lighting system irradiating the reticle is transmitted to the concave surface of the spectroscope, and then the concave surface reflects the light into parallel light to enter the human eye. At this time, the human eye can not recognize the real position of the reticle, only can see its virtual image, that is, the red dot, and think that the image is in the infinite distance.

 

The virtual image of the reticle and the real image of the target seen by the human eye can be superimposed to aim. And because the concave surface of the spectroscope reflects the parallel light into the human eye, the human eye can see the virtual image in any direction.