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I've just read Feynman's book:'QED:The strange theory of light and matter' and, in order to measure the probability that a photon will reflect or transmit through a partially reflective piece of glass, Feynman introduces an 'imaginary stopwatch'. Can you explain me:

-what do the hands of this stopwatch represent;

-why does Feynman oppose the sense of the first hand in the case of a reflection by two surfaces;

-how does he know the lenght of the hands when he measures the probability?

Thanks.