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Newton, Herschel and Ritter
The Discovery of the Spectrum of Light
Hands On Activity: Repeat Newton’s Prism Experiments and Herschel's and Ritter's Experiments

Famous Inventors

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  • A triangular prism, dispersing light
    A triangular prism, dispersing 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. This is the attitude employed by the editors of this page.

    Newton’s Prism Experiments

    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.

    Newton's Prism Experiment
    Newton's Prism Experiment

    In another Experiment, after getting a spectrum with his prism, he placed another prism upside-down in the way of the light spectrum after passing the first prism. The band of colors combined again into white sunlight. For a demonstration of this experiment click here.

    In these experiments, Newton had proved that white light was made up of colors mixed together, and the prism merely separated them - he was the first person to understand the rainbow.

    To repeat Newton's experiments check out the following links:
    Optics - Jose Wudka
    Newton and the Colour of Light - The College of Optometrists, London
    Newton´s Prism Experiment With Light - KSU Physics Education Research Group
    Newton's Blue Light Experiment - Florida State University
    Prism - Eric Weisstein's World of Physics
    Newton’s Prism Experiments and Theory of Color - Ether Wave Propaganda
    Newton’s Prism Experiment and Goethe’s Objections - Gernot Hoffmann

    William Herschel:
    The Discovery of Infrared Light

    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.

    William Herschel's Infrared Experiment
    William Herschel's Infrared Experiment

    To repeat Herschel's experiment check out the following links:
    The Herschel Experiment - Cool Cosmos
    The Herschel Experiment - Angela C. Des Jardins
    Laboratory Investigation on Visible Light and Infrared Radiation
    The Herschel Experiment - JPL

    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).

    BEWARE: Since this experiment involves the use of chemicals, it should be performed under the supervision of teachers or adults familiar with safety procedures.

    To repeat Ritter's experiment check out the following links:
    The Ritter Experiment - Cool Cosmos
    Discovering the Electromagnetic Spectrum - NASA
    Procedures for Seeing the Invisible
    Detecting invisible radiation - Australia School Innovation in Science

    Background Resources
    The Electromagnetic Spectrum - KryssTal
    Optics Annotated Timeline - Molecular Expressions
    Multiwavelength Milky Way: The Nature of Light - NASA
    Color Science- Dan Bruton
    Electromagnetic Spectrum - NASA

    Isaac Newton Books

    My Dog Kelly

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    Last updated: June 2013
    Copyright © 2003-2013 Julian Rubin