Light is moving up and down ( y direction ), while the ref frame is considered to have a velocity in the x direction , hence no component of velocity in y direction, hence only c , velocity of light is considered.
Thank you so much, There is a new idea. In Michelson's experiment in 1881 to search for the carrier of light - ether. It is necessary to replace the light source with a maser. With the help of this experiment on transport, it is possible to determine its speed relative to the dominant gravitational field of the Earth. Maser - invented in England in 2018. Maser - microwave amplification by stimulate demission of radiation.,
