Sorry for the seemingly silly question but what's the advantage of having a concave mirror in car flashlight? I understand how the physics works but I just don't get what the advantage of having this over just a light flashing out is.

  • $\begingroup$ You say you understand the physics so I'm not clear what you're confused about. Without the mirror, the light that is shone backwards towards the car is wasted. Does that answer your question? $\endgroup$
    – lemon
    Jun 7, 2016 at 10:04
  • $\begingroup$ Ah but what I'm saying is why is the light not shone forward? $\endgroup$
    – MystMan
    Jun 7, 2016 at 10:22
  • 2
    $\begingroup$ light is shone in all directions. $\endgroup$ Jun 7, 2016 at 10:24
  • $\begingroup$ Yh and what does that mean? $\endgroup$
    – MystMan
    Jun 7, 2016 at 10:32
  • $\begingroup$ when you have a light source (bulb) at the focus, it does not just shine only forward, but to all directions. so, to save energy, you have to have a mirror that reflects the light that is shone backward. $\endgroup$ Jun 7, 2016 at 10:33

1 Answer 1


Light from the focus, when reflected to a parabolic mirror, will all be reflected in parallel rays. enter image description here

  • $\begingroup$ Yh I understand the physics but why do we need a reflective surface, y can't we just shine the light forward? $\endgroup$
    – MystMan
    Jun 7, 2016 at 10:24
  • $\begingroup$ If you have just a point light, that is made to shone forward, with the same intensity per unit area when the light hits a surface (as when you have mirrors), then you would make the point so very bright, that looking at it will make you blind. $\endgroup$ Jun 7, 2016 at 10:29
  • $\begingroup$ Wow philip_0008 man your confusing me. You gave me two arguments, one says that you must collect the light that is shone backwards and the other one says that shining it forward would be too bright to look at. $\endgroup$
    – MystMan
    Jun 7, 2016 at 10:38
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    $\begingroup$ "shone" is the past tense of the verb "to shine". In a cheap flashlight, the light is emitted in all directions from the bulb: forward, up, down, backwards, and sideways. The mirror is there to collect the light which isn't traveling forward out of the bulb and forming it into a beam which is moving forward away from the light. I suppose you could make a special bulb where the light is emitted in only one direction, but then you wouldn't have a cheap flashlight any longer. $\endgroup$
    – user16622
    Jun 7, 2016 at 10:42
  • $\begingroup$ Ah that's perfect I understand now. Thanks user16622!!! $\endgroup$
    – MystMan
    Jun 7, 2016 at 10:50

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