Discovery of spectrum of light

Discovery of spectrum of light

 

From the earliest times, people have wondered about the nature of light.

By 300 BC, Greek scholars had begun to study and contemplate optical phenomena generating theories to explain vision, color, light, and astronomical phenomena. Many of those theories turned out to be wrong, but they did serve to inaugurate the science of optics.

During the second century AD, Ptolemy, a Greek astronomer based in Alexandria, Egypt, studied and wrote about many topics in science. He published five books about optics, but only one book has survived to the modern era. This series of works was dedicated to the study of color, reflection, refraction, and mirrors of various shapes.

Few other advances were made in optics until after 1000 AD. The Arab scholar Alhazan, a.k.a. Abu Ali Hasan Ibn al-Haitham, conducted the first serious study of lenses in Basra (Iraq). He studied refraction in lenses, and also carried out research on reflections from spherical and parabolic mirrors. His writings were the first to explain vision correctly, as a phenomenon of light coming into the eye, rather than the eye emitting light rays.

Roger Bacon, an English philosopher from the 13th century, postulated, but could not demonstrate, that the colors of a rainbow are due to the reflection and refraction of sunlight through individual raindrops.

NOTE: The term “light” is often extended to adjacent wavelength ranges that the eye cannot detect - to infrared radiation, which has a frequency less than that of visible light

and to ultraviolet radiation,which have a frequency

 greater than that of visible light.

 green light. If it was the prism that was

coloring the light, the green should come

out a different colour. The pure green light

remained green, unaffected by the

second prism.

             Newtons expirement on prism 




                                     William Herschel
                 The Discovery of Infrared Light

 Even before Newton’s famous experiments  (1665) with light people were using prisms to experiment with colour, and thought that somehow the prism colored the light. Newton obtained a prism, and set up his so that a spot of sunlight fell onto it. Usually, in such experiments a screen was put close to the other side of the prism and the spot of light came out as a mixture of colour. Newton realised that to get a proper spectrum you needed to move the screen a lot further away.

After moving the screen and achieving a beautiful spectrum he did his crucial experiment to prove that the prism was not colouring the light. He put a screen in the way of his spectrum, and this screen had a slit cut in it, and only let the green light go through.

Then he put a second prism in the way of the green light. If it was the prism that was colouring the light, the green should come out a different colour. The pure green light remained green, unaffected by the second prism

William Herschel (1738 - 1822) was one of the most important astronomers that ever lived. In 1800 he performed a famous experiment where he tried to measure the temperature of different colors of the spectrum by placing a thermometer on each colour. He found to his amazement that the hottest part of the spectrum was in a place where there was no colour at all. It was a spot beyond the red end of the spectrum. For the first time it was possible to talk about invisible light. This hot light became known as Infrared (below the red) because it was shown to have longer wavelength than visible light. Apart from its wavelength, Infrared has all the other properties of light.

Johann Ritter
The Discovery of Ultraviolet Light

After learning about William Herschel's discovery of infrared light, which he found beyond the visible red portion of the spectrum in 1800, Johann Ritter began to conduct experiments to see if he could detect invisible light beyond the violet portion of the spectrum as well. In 1801, he was experimenting with silver chloride, which turned black when exposed to light. He had heard that blue light caused a greater reaction in silver chloride than red light did. Ritter decided to measure the rate at which silver chloride reacted to the different colors of light. He directed sunlight through a glass prism to create a spectrum. He then placed silver chloride in each color of the spectrum and found that it showed little change in the red part of the spectrum, but darkened toward the violet end of the spectrum. Johann Ritter then decided to place silver chloride in the area just beyond the violet end of the spectrum, in a region where no sunlight was visible. To his amazement, this region showed the most intense reaction of all. This showed for the first time that an invisible form of light existed beyond the violet end of the visible spectrum. This new type of light, which Ritter called Chemical Rays, later became known as ultraviolet light or ultraviolet radiation (the word ultra means beyond).