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1. Dual nature of the electromagnetic radiation. So the radiation has particle like
     properties and wave properties.
           establishment of electromagnetic wave spectrum for visible and invisible
2. Experiments on spectra of atoms. The results were explainable only by assuming that the electrons have quantized energy levels.

     Planck's quantum theory, photoelectric effect. Emission and absorption
     spectra. Line spectrum of Hydrogen and Balmer series.

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1) dual nature of matter
2)electromagnetic spectrum
3)photoelectric effect