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- Ph.- outer electron emission substance under the influence of electromagnetic radiation. Observed in gases, liquids and solids. External Ph. described by Einstein $$h \nu=A+\dfrac{m v_{\max }^{2}}{2}$$ where $h \nu$ — energy photon $A$ — work out and $\dfrac{m v_{\max }^{2}}{2}$ — the maximum kinetic energy of the electron, $h$ — Planck’s constant. Value $\nu_{0}=\dfrac{A}{h}$ called red border Ph.
- Internal Ph.- redistribution of electron energy states in semiconductors and dielectrics, which is under the influence of electromagnetic radiation. Ph. internal manifested in the appearance of photoconductivity or foto-emf. Laws external Ph. installed in 1888-1890 Russian physicist A.P. Stoletov (1839-1896).