# The hypotenuse of the triangle in the Michelson and Morley experiment

In Michelson and Morley experiment, light is travelling from light source to partially silvered glass plate. The plate splits an incoming beam of light, and the two resulting beams continue in mutually perpendicular directions to the mirrors. Now, while the light is on its way from the plate to the mirror, the apparatus moves a distance $$v t_1$$, so the light must traverse a longitudinal distance $$L+v t_1$$, at the speed $$c$$. But from the plate to another mirror the light travels a distance along the hypotenuse of a triangle.

It is clear for me that the mirror (in a transverse direction) also must traverse a longitudinal distance $$v t_1$$. But I don't understand why should the light travel a distance along the hypotenuse of a triangle following the mirror. At the splitting moment, the beam receives a perpendicular direction (along with the cathetus toward the mirror location point at the splitting moment). At what moment and why does the direction of the beam change letting the beam to reach the mirror? Why doesn't beam continue with the initial direction (at the splitting moment) and why doesn't beam miss the mirror?

Please, help me to understand :)

• This graphic is probably a bit misguiding. If you think of the photon in the interferometer, the reflection is always perpendicular, and the length of the path taken is always the same. – Mechanix Mar 9 at 21:18
• If you think of this reflection as someone throwing a ball up in his hand, this is also always perpendicular, but if this person sits in a moving train or sth, an observer that is outside of the moving train (for example standing on the platform) would see the ball taking a triangular motion. The idea of this graphic is to illustrate the path that the photons take, viewed from a reference frame moving with respect to the interferometer. – Mechanix Mar 9 at 21:24
• @Mechanix the image is showing two simultaneous frame of reference: solid lines represent the view of someone at rest WRT the interferometer. The dashed lines are the view of someone moving at constant speed to the left WRT the interferometer. – dmckee Mar 9 at 21:59
• I'm sure this question has been asked several times on the site already, but I don't have the links handy. It is, of course, a good question but the answer is startlingly simple: all observer agree on what physical events happen. – dmckee Mar 9 at 22:00
• @Mechanix, your explanation clarifies everything for me. Thanks a lot! – Nataliia Mar 10 at 5:46