It is well known, that in some experiments, light exhibits ‘wave’ property; like interference and diffraction; whereas in other experiments, like photoelectric effect, it shows ‘particle’ nature. So, the physicists currently think in terms of ‘wave-particle-duality’ of light, their mutual exclusiveness, and complementarily. Here, in this paper, this ‘wave-particle-duality’ is explained; that at the frequencies of light, very narrow-band filtering, and generation of purely monochromatic light, of one Hz bandwidth, are not technically possible; so there is always some bandwidth and ‘line-width’, due to which the waves form wave-packets, localized in space, as ‘particles’. Then this familiarity with waves is applied to understand the observations of ‘cosmological red shift’.
The present experimental tests have proved high precisely the validity of the Pauli exclusion principle (PEP) in usual cases. The future experiments should be combined widely with various theories of hidden and obvious violation of PEP. Author think that known experiments and theories seem to imply the violation at high energy. Some possible tests have been proposed in particle physics, nuclei at high energy and astrophysics, etc., in particular, the excited high-n atoms and multi-quark states. Moreover, the violation is possibly relevant to the nonlinear quantum theory, in which the present linear superposition principle may not hold. Finally, the possible violation at very low temperature is discussed. These experiments may be connected with tests of other basic principles, for example, the present wave property, possible decrease of entropy due to internal interactions in isolated systems, and PCT invariance, etc.