sher@rochester.UUCP (David Sher) (03/03/85)
To you net.physics people this article is a result of a discussion of light sails and how they work. The question I am addressing is does being reflected by a light sail change the wavelength of the reflected light. This is a tricky problem because it is not well defined. (I am taking this from rememberances of a modern physics class I took 3 years ago so I am not authoritative). Light only has a wavelength relative to an observer. (or a frame I guess). There was an interesting problem I was given in the afore mentioned physics class which was given an observer for which a beam of light with wave length (relative to the observer) lambda is reflected from a mirror moving with relativistic velocity v what is the wave length of the reflected light. I believe it is not the same as the original light except when v is a small fraction of c (whats an epsilon between friends). Try throwing mirors around and see for your self :-). -David Sher
karn@petrus.UUCP (03/04/85)
It would seem to me that photons reflected off a sail would indeed appear to be shifted in wavelength to an observer watching both the outgoing solar radiation and the reflected radiation. This would be due to the doppler shift caused by the relative motion of the sail with respect to the sun. The effect is equivalent to that which makes a police radar function. Energy is still conserved. Phil