I need to clear some conceptual doubts in the Special theory of relativity. So could anyone explain them in context to the following questions: 1. The speed of light in glass is 2.0 x 108m/s. Does it violate the second postulate of SR in any way? 2. A uniformly moving train passes by a long platform. Consider the events 'engine crossing the beginning of the platform' and 'engine crossing the end of the platform'. Which frame (the train frame or the platform frame) is the proper frame for the pair of events? 3. An object may be regarded to be at rest or in motion depending on the frame of reference chosen to view the object. Because of length contraction at relativistic speeds the same rod may have two different lengths depending on the state of the observer. Am I right? 4. Mass of a particle depends on its speed. Does the attraction of the earth on the particle also depend on the particle's speed?