MadSci Network: Physics

Subject: How does the dot in red-dot gunsight appear visible relative to the target?

Date: Tue Jan 4 00:42:58 2000
Posted by Mark Burton
Grade level: nonaligned School: No school entered.
City: Los Angeles State/Province: CA Country: USA
Area of science: Physics
ID: 946968178.Ph

My son has a BB gun with a "tubeless red dot sight" on it.  He asked me 
how it worked, and after much thought, I'm still not sure.  

Basically, the sight works by having the light from a rearwardly-situated 
LED projected onto a lens.  Once the sight is zeroed in (by way of a 
movable base), wherever the red dot is viewed by the shooter is where the 
point of impact of the BB will occur.  

What is curious is that if one's head moves while viewing the dot, it will 
appear on a different part of the lens (i.e. closer to the edge or the 
center), yet it will still be aligned with the same area of the target.  

I'm told this type of sight is known as a "reflex sight" and works similar 
to an SLR camera or certain sighting devices in astronomy.  

Here are my questions:  1) How is it that the dot always appears visible 
in relation to the target?  2) Why does the dot appear at infinity, 
suspended over the target, instead of just a couple feet from the viewer's 

Any explanations you could provide us with would be greatly appreciated.

Thank you,

Mark Burton

Re: How does the dot in red-dot gunsight appear visible relative to the target?

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