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Photoelectric Effect

The phenomenon was discovered by hertz and was experimentally proved by Thomson and Millikan. Einstein added his The Photoelectric Effect is equation to support the phenomenon. The phenomenon is basically about light energy (photo) converting into electrical energy. 


 When light of suitable wavelength is incident on the metal plate electrons are emitted. These emitted electrons are called as photo electrons and the phenomenon is photoelectric effect.The apparatus consists of an evacuated quartz tube having photosensitive plate called emitter A and collector B. Plate A is connected to the negative terminal and plate B is connected to the positive terminal of a battery via rheostat R. 


The potential difference between the plates AB can be adjusted by changing the value of the rheostat. When light of suitable wavelength is incident on the plate A then electrons are emitted and reaches plate B, thus measurable current flows through the circuitFactors on which Photelectric Effect Depends:The amount of current flow (number of electrons) and the kinetic energy of the emitted electrons depend upon1) 


Applied Potential difference between the plates:For a given photo metal if the frequency and intensity of incident light kept constant and if the potential difference between the plates is increased then photo current also increases until it reaches a maximum (saturated) value. If the terminals are reversed and we increase the potential difference gradually then photo current decreases and at one point it becomes zero. 


This point where the current is zero is termed as stopping potential.2) Intensity of incident radiation:For a given photo metal if the frequency and applied voltage is kept constant with increasing intensity then we observe that photo current increases with increases in intensity with out change in the stopping potential3) Frequency of incident radiation:The intensity and voltage across the plates is kept constant for a photo metal with change in frequency of the incident light. The stopping potential is measured for different frequencies. 


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The graph is drawn between frequencies versus stopping potential. The frequency at which the photo current begins is called threshold frequency. This is the minimum frequency for photo electric effect to occur.Stopping Potential4) Material of the Photo metal:The experiment is repeated for different photo metal and their threshold frequencies are noted. The variation of frequency versus stopping potential is graphed and noticed that threshold frequency varies from metal to metal.Photoelectric Effect Applications:1) Used in converting solar rays to generate electricity2) Used in detect traffic law defaulters3) Used as light meters in cameras to measure the intensity.4) Used in controlling the temperature of furnaces