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Photons 1) Photons are the packets of energy (or energy particles) which are emitted by a source of radiation. They are travel in straight line. Photons 2) The energy of a photon, E= hv = hc/λ . 3) The photons emitted from a source, travel through space with the same speed (equal to the speed of light). 4) The frequency of a photon does not change when it travels through different medium but its wavelength changes in different medium as the velocity of a photon in different media is different. 5) The rest mass of photon is zero. 6) Its momentum is hv /c or h/λ. Photoelectric effect 1) It is the phenomenon of emission of electrons from the surface of metals, when light radiations of suitable frequency fall on them. The emitted electrons are called photoelectrons and the current so produced is called photoelectric  current . 2) Alkali metals: like lithium, sodium, potassium, cesium etc. show photoelectric effect with visible light, whereas metals like zinc, cadmium...