Explaining Length Contraction with the Light Clock Argument

[kehler]

We're just learning special relativity in class and my lecturer uses the light clock argument (two moving clocks - one parallel and one perpendicular) to explain length contraction.

I didn't quite get one thing about it. My notes say the light beam reflects at different times (i.e. they have different half-ticks) when the clocks are moving but reflects at the same time when the clocks are at rest, and this difference is due to the relativity of simultaneity.
But why doesn't this relavity of simultaneity principle also apply to full ticks?? From what I understand, the argument of length contraction is based on the fact that the clocks have to detect the beam at the same time.

I'm sure I must be understanding something wrongly here. Would appreciate if anyone could point it out to me :)

 

[Doc Al]

Realize that the half-tick events--when the light pulses hit the mirrors--take place at different locations, while the full-tick events happen at the same location. And, due to the relativity of simultaneity, events separated in space (along their direction of motion) that are simultaneous in the rest frame will happen at different times according to a moving observer.

But every observer in every frame agrees that the full-tick events--the two light pulses returning to the source--happen simultaneously.

 

[kehler]

Oh right, I see. Thanks Doc Al :)