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20.Telescope (a little bit of history)

  In 1608 Hans Lippershey, an eyeglass maker, submitted the first patent of telescope. But who made the first telescope, it is not clear. Later on, Galileo improved the design and started applying in Astronomy. This telescope had convex objective lens and concave eyepiece lens. In 1611 Kepler developed the design using convex lens in both objective and eyepiece and this design was more useful than the previous one developed by Galileo. Diagram of Galilean and Keplerian Telescope In 1655 Huygen was making powerful and enhanced Keplarian telescopes using compound eyepiece but those telescopes were proved inaccurate. In 1688 Isaac Newton built the first reflector telescope which consisted of a tiny flat diagonal mirror to reflect the incoming light on an eyepiece. Using achromatic lens, in 1733, Chester Moore Hall built a telescope. First Reflector Telescope built by Newton Diagram of Achromatic Lens When light of white color passes through a medium it splits into several colors. For exam
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1905 Nobel Prize in Physics

 

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Gold medal of Nobel Prize

Electron beam or Cathode rays are streams of electrons observed in discharge tubes. In 1888 Lenard had done his first work with cathode rays when he was working at Heidelberg under Quincke. Lenard investigated the view then held by Hertz that cathode rays were analogous to ultraviolet light and Lenard did an experiment to find out whether cathode rays would pass through a quartz window in the wall of a discharge tube like ultraviolet light.
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Ultraviolet Light

Lenard found that cathode rays wouldn't do that. But later in 1892, when he was working as an assistant to Hertz at the University of Bonn, Hertz called him to see the discovery he made that if a piece of uranium glass covered with aluminium foil put inside the discharge tube became luminous beneath the aluminium foil when the cathode rays struck it. Hertz suggested that by means of a thin plate of aluminium it would be possible to seperate two spaces, one in which the cathode rays are produced in ordinary way and the other in which one could observe them in a pure state. Hertz had never done this experiment. Because of being busy, he gave permission to Lenard to do it and it was then that he made the great discovery of the Lenard Window.
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Cathode Rays


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Schematic Diagram of Discharge Tube

After many experiments with aluminium foil of various thickness, in 1894, Lenard was able to publish his discovery that the plate of quartz that until then had been used to close the discharge tube, could be replaced by a thin plate of aluminium foil. Just thick enough to maintain the vaccum inside the tube but yet thin enough to allow the cathode rays to pass out. Thus it became possible to study the cathode rays outside the discharge tube and also the fluorescence the caused. Lenard concluded from the experiments that the cathode rays propagated through the air for a distance of the order of a decimetre and that they travel in a vaccum for several metres without being weakened. Although Lenard at first followed Hertz in believing that the cathode rays propagated in the ether. Later, he abandoned this view as a result of the work of Jean Perrin in 1895, Sir J.J. Thomson in 1897 and W. Wein in 1897, which proved the corpuscular nature of the cathode rays. Later Lenard extended the work of Hertz on the photoelectric effect which expised the limitations of the wave theory of light and led to the era of Quantum Mechanics.

The 1905 Nobel Prize in Physics was awarded to Philipp Eduard Anton Von Lenard for his work on cathode rays which played a vital role in developing modern science.
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