On a new piece of paper, place two mirrors parallel and 18 cm
apart in the y direction. Draw the locations of the mirrors on the
paper. Then the mirrors move in the x direction for 24 cm. Draw the
final locations of the mirrors on the paper. The light travels from
the center of one mirror to the center of the other mirror. Draw
the path of the light. The speed of light is still even in
this reference frame.
How much time does it take?
This is the time measured when the mirrors move (with respect to
you). This time is called the dilated time.