How does the image moves by 2v when the mirror moves with speed v towards the object?

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    $\begingroup$ Hello @Hell-Volhard-Zelinsky and welcome to Physics SE! Please read the two links in the banner above (duplicated here and here). The policy does not apply to homework questions per se, but to specific kinds of questions as outlined in the posts above. The more conceptual you can make the question, the more likely it is to be reopened by the community. $\endgroup$
    – Chris
    Sep 8, 2022 at 6:18

1 Answer 1


"by geometry we can prove that image doesn't move. " seems strange. Geometry says the mirror image is as far behind the mirror as the source is in front of the mirror, if you move the mirror half way to the wall, the the distance of source and the image are both half, so the image moves with 2v , it has nothing to do with the angele of the source to the mirror. (I will not look at foreign sources where I would have to accept cookies, to see what you did for calculation. So please put your thoughts directly in here)

  • $\begingroup$ I have edited the question what I meant was light patch and not image. 'i.imgur.com/jqPeyti.png' this will directly show the image and wont ask for cookies and all $\endgroup$ Sep 6, 2022 at 12:02
  • $\begingroup$ again, geometric says the reflected patch moves to the left when the mirror moves to the wall. Why not just sketch the reflected ray if the mirror is half way to the wall? the velocity of the moving patch depend on the angle you direct your ray. a point source makes an increasing patch, so do you talk about a light point source or a ray in one direction? $\endgroup$
    – trula
    Sep 6, 2022 at 20:48
  • $\begingroup$ Triangles formed by normal and wall with incident and reflected rays respectively will be congruent, and although the angle is increasing, its due to the side adjacent to angle i.e. normal and not because of side opposite to it i.e. SP where S is source and P is point where normal cuts the wall. Have you checked that image? If I attach it in the question then question will never get reopened, I guess even now it won't get reopened. Also check how I proved light patch to be not moving i.imgur.com/tk7Or96_d.webp?maxwidth=760&fidelity=grand (no trackers, cookies I checked with browser) $\endgroup$ Sep 7, 2022 at 5:14
  • $\begingroup$ if you see my calculation, the patch length doesn't depend upon speed it just depends upon horizontal distance d1 and d2 which do not change by time here. $\endgroup$ Sep 7, 2022 at 5:19
  • $\begingroup$ I now see what you do, you look at the patch, if all the mirror is used to reflect the point source. Then the patch gets larger if you move closer, the borders of the patch move. But your calculation says how a single ray's point on the wall travels, so this are different things, $\endgroup$
    – trula
    Sep 7, 2022 at 20:42

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