Right. Some simple questions, I want to make sure I'm not making stupid mistakes before moving on. I have the questions and my answers, If you tell me that I got them incorrect, i'll run through my working or something (if there could be any..). 1/ How fast must you travel towards a red light (650nm) for it to appear green (525nm)? Can this just be treated as length contraction? My Answer: 1.78x10^8m/s. 2/ How great must the relative speed of two observers be for their time-interval measurements to differ by 1%? My answer: 4.2x10^7m/s. 3/ A distant galaxy is moving away from the earth such that each wavelength is shifted by a factor of two; what is the speed of the galaxy relative to us? can this just be treated via length contracion? 4/ A particle moves north at 0.8c, what is this particles speed as measured by an observed travelling south relative to the earth at 0.5c? 5/Two spaceships of proper length 100m are travelling towards each other, each at a speed of 0.8c, measured on earth. (i) how long is each ship as measured by the terrestial observer? (ii)how long is earch ship when measured from the rest-frame of the other? Thats all the questions I have just now. My idea is that, since I haven't done relativity for ages I should try to brush up on this simple stuff before my new course starts. tomorrow i'm hitting the library for a textbook, sigh.