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The chemical that reacts to the photons is known as Visual Purple . This arrangement has several advantages : first , by conservation of angular momentum the photon pair emitted in the cascade has total angular momentum 0 , and if the photons are collected in cones of small aperture along the z and - z directions the total orbital angular momentum is small , with the consequence that the total spin ( or polarization ) angular momentum is close to 0 and therefore the polarizations of the two photons are tightly correlated ; second , the photons are in the visible frequency range and hence susceptible to quite accurate polarization analysis with standard polarization analyzers ; and third , the stochastic interval between the time of emission of the first photon and the time of emission of the second is in the range of 10 nsec ., which is small compared to the average time between two productions of pairs , and therefore the associated photons 1 and 2 of a pair are almost unequivocally matched . " After this first sorting stage , the photons are ... The wavelength of the photons is chosen so that they will interact only with atoms that are moving toward the photons .