https://www.hsgd-optics.com/optical-spherical-lens/double-concave-lens.html Double Concave ...    More ...

https://www.hsgd-optics.com/optical-spherical-lens/double-concave-lens.html          
 
Double Concave Lens
Double concave lens                are bi-concave lenses with concave surfaces on both sides. Double concave lens are designed for applications such as beam expansion, image reduction and light projection. In addition, these lenses are ideal for increasing the focal length of an optical system.     
 
A double concave lens/mirror has a negative focal length, thereby causing collimated incident light to diverge. The double concave lens/mirror has an equal radius of curvature on both sides of the lens. They are generally used to expand light or increase focal length in the existing system, such as beam expander and projection systems.     
 

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