Fig.1 The Plan of his experiment. Fig.2. Heinrich Rudolf Hertz Ger. 1857 –1894 In 1887 Hertz noticed that when two oppositely charged metal rods were exposed to ultraviolet light , there was an electric spark. Wilhelm Hallwachs, one of his coleagues established the photoelectric effect using a very simple experiment.
Fig. 3. Hallwachs experiment. He used a gold leaf electroscope for this. When a positively charged electroscope was exposed to bright light , it got discharged. When a negatively charged one was used, that did not happen. In 1901 Max Planck had proposed that electromagnetic energy may be emitted only in small lumps which he called quanta. This is somewhat similar to the existence of atoms in matter. Fig.4 Philip Lenard 1862-1947.(Ger) In 1902 Philip Lenard (Ger) . studied how the kinetic energy of the emitted electrons varied with the intensity of light. He used a very powerful light that can change the intensity. (brightness) The emitted electrons could be prevented from reaching the collector by giving the latter a negative charge. The necessary charge to oppose the electrons gave a measure of the energy in the emitted electrons. Fig.5. Plan for the experiment of Leonard. Lenard discovered the following.
When Lenard received the Nobel prize for his research in 1905, Albert Einstein came up with an explanation for this phenomenon. The Nobel Prize in Physics 1921 was awarded to Albert Einstein "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".Fig.6. Energy of photo emissions plotted against the freqency of the light rays used.What does it show?
(This is the same as =6.62607 × 10−34 J.s.) |
Physics >