Optics is not my best subject. I know the basics of how light behaves as it go through a lens. I tried to understand further but got more confused as I get deeper into this subject.

My goal is to make a flashlight that shines into a spot as much as possible (but not so much so as to leave the focal point a dot). I'm looking for a 50% reduction in beam size. The candidate flashlight is already producing spot characteristics without any optics.

I was wondering if a combination of a Plano concave and a double convex lens arranged in a tube that extends out of the flashlight would do the trick. If not, what combination/lens would work?

I don't have a bunch of lenses to play around with but I'm planning to purchase the required optics for the final flashlight beam. Thanks!

  • $\begingroup$ Hi, and welcome to Physics Stack Exchange! No need to apologize for grammar/english, your english is quite good. I edited slightly to improve clarity; let me know if this is not the meaning you intended :) $\endgroup$ Sep 11, 2021 at 17:53
  • $\begingroup$ Do you already have a beam of some limited angular aperture and want to further reduce the angle (then you need lenses), or is the source emitting in all directions and the task is to get a beam in some direction at all (mirror as suggested below)? $\endgroup$ Sep 12, 2021 at 7:13

1 Answer 1


I think the easiest way is not to use lenses, but a concave mirror, just like in the the front lights of a car. It is very easy to get one from a car junkyard.

  • $\begingroup$ Yeah, and you don't lose the light going backwards like you would with a lens. $\endgroup$
    – DKNguyen
    Sep 11, 2021 at 17:53
  • $\begingroup$ Since the OP is also interested in the concepts behind why this works, could you elaborate on how this might look/why it works? (Also, I do think this is a good idea, though it loses the advantage of compactness depending on what OP is trying to do.) $\endgroup$ Sep 11, 2021 at 17:54
  • $\begingroup$ I just wait for the OP to ask back $\endgroup$
    – trula
    Sep 12, 2021 at 14:12

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