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## Main Question or Discussion Point

I am slightly unsure about how the divergence can be increased by the use of either bi-concave or plano-concave lenses. I understand the general theory behind it but am having trouble putting numbers to it. e.g. if you have a laser beam with a diameter of 2mm and a divergence of 2mrad what focal length lens would be needed to increase its divergence to 5mrad/10mrad/etc? Any help would be greatly appreciated especially if it explains the calculations needed to work these things out.