When I point a laser at a wire , a circle of light appears on a wall behind it . why this happens.

What is the circle of light and why does it appear?

Could anyone give some tips on what I should search for or any references?

wire have diameter 3.17 mm

PS.I don't good at English,,, sorry

Wire perpendicular to the projection screen Wire perpendicular to the projection screen

Laser under on the wire Then born circle of light Laser under on the wire Then born circle of light

  • 1
    $\begingroup$ You're going to have to give a lot more information if you want to get anything out of this question. Add some context. $\endgroup$ – Brionius Feb 26 '15 at 14:04
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    $\begingroup$ I think Johnny Cash wrote a song about this. $\endgroup$ – Jiminion Feb 26 '15 at 14:19
  • $\begingroup$ Give more information about the 'wire'. What material is it? What is its diameter? $\endgroup$ – Chris Mueller Feb 26 '15 at 14:28
  • $\begingroup$ @Jiminion So love is the reason this happens? $\endgroup$ – Jim Feb 26 '15 at 14:39
  • $\begingroup$ I don't understand physics.stackexchange.com/questions/161924/… T^T $\endgroup$ – Sarunya Wattananukit Feb 26 '15 at 14:48

The wire is a cylindrical reflector. The laser light that hits the top of the wire is reflected upwards; the light that hits the side is reflected sideways.

This is simple reflection - no need for a diffraction explanation.


Light has properties of waves and particles and doesn't necessarily travel in "straight lines".

There is an interesting experiment (I will look up the original scientists who did it and post that info here) where they set up basically a cardboard circle in front of a wall and then shine a flashlight at it.

Interestingly enough, there is a shadow in the shape of a circle on the wall behind the cutout, but there is a bright spot in the middle of the shadow. How did the light get there? Well, the answer is, it doesn't move in a straight line.

I think your laser is doing the same thing, but the effect is probably exacerbated by the reflective properties of the metal wire that you are shining the laser at.


Please see the very informative comments below for more information about the experiment. It is referred to as the Arago/Fresnel/Poisson spot.

  • $\begingroup$ You're thinking of the Arago/Fresnel/Poisson spot. The history is kind of great--Fresnel proposes wave theory, Poisson points out this spot should be there and that's ridiculous so there's clearly no such waves, and Arago discovers that there is indeed a spot. en.wikipedia.org/wiki/Arago_spot $\endgroup$ – zeldredge Feb 26 '15 at 15:08
  • $\begingroup$ The "interesting experiment" you refer to is known as "Poisson's spot" - Poisson ridiculed the notion of light as a wave, and predicted the outcome of the experiment ("like that could ever happen, hahahaha"). Arago did the experiment - but the spot was named (much to his chagrin) after Poisson. $\endgroup$ – Floris Feb 26 '15 at 15:09
  • $\begingroup$ @SirElderberry - I just realized I was typing my comment at the same time that you did... Good link, and yes, terrific story. $\endgroup$ – Floris Feb 26 '15 at 15:10

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