<text><span class="style10">uantum Theory and Relativity (2 of 4)Photons</span><span class="style7">If light is directed onto a piece of metal in a vacuum, electrons are knocked from the surface of the metal. This is the </span><span class="style26">photoelectric effect</span><span class="style7">. For light of a given wavelength, the number of electrons emitted per second increases with the intensity of the light, although the energies of the electrons are independent of the wavelength.This discovery led the German physicist Albert Einstein (1879-1955) to deduce that the energy in a light beam exists in small discrete packets called </span><span class="style26">photons</span><span class="style7"> or </span><span class="style26">quanta</span><span class="style7"> . These can be detected in experiments in which light is allowed to fall on a detector, usually photographic film. This has led to the theory of the </span><span class="style26">dual nature of light</span><span class="style7">, which behaves as a wave during interference experiments (see diagram 2) but as a stream of particles during the photoelectric effect. Further work on this phenomenon has led to the acceptance of </span><span class="style26">wave-particle duality</span><span class="style7">, which is a fundamental principle in quantum physics. The way a system is described depends upon the apparatus with which it is interacting: light behaves as a wave when it passes through slits in an interference experiment, but as a stream of particles when it hits a detector (see diagram 3).</span></text>
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<text><span class="style10">. Interference.</span><span class="style7"> The waves passing through slits A and B and reaching the screen C will be either in phase or out of phase and will either reinforce or cancel each other. The result is a series of light and dark bands on the screen. Reinforcement occurs when the path difference is a whole number of wavelengths.</span></text>
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<text>ΓÇó TIMEΓÇó MOTION AND FORCEΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó ELEMENTS AND THE PERIODIC TABLE</text>