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The Discovery of Infrared Light by William Herschel
Before I started researching this topic I was only aware of Herschel as the discoverer of the planet Uranus in 1781.
It turns out that he made at least one other important discovery, that if infrared light, in 1800.
Herschel's discovery is described at this website in the following way:
He was interested in learning how much heat passed through the different colored filters he used to observe the Sun and noticed that filters of different colors seemed to pass different levels of heat. Herschel thought that the colors themselves might contain different levels of heat, so he devised a clever experiment to investigate his hypothesis.
Herschel directed sunlight through a glass prism to create a spectrum - the "rainbow" created when light is divided into its colors - and measured the temperature of each color. He used three thermometers with blackened bulbs (to better absorb the heat) and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as control samples. As he measured the temperatures of the violet, blue, green, yellow, orange and red light, he noticed that all of the colors had temperatures higher than the controls and that the temperature of the colors increased from the violet to the red part of the spectrum. After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all.
Herschel performed further experiments on what he called "calorific rays" (derived from the Latin word for 'heat') beyond the red portion of the spectrum. He found that they were reflected, refracted, absorbed and transmitted just like visible light. What Sir William had discovered was a form of light (or radiation) beyond red light. These "calorific rays" were later renamed infrared rays or infrared radiation (the prefix infra means `below'). Herschel's experiment was important not only because it led to the discovery of infrared light, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes.
The video below was produced by FLIR Systems, makers of infrared thermal imaging systems. It begins by describing Herschel's experiment and infrared light in general, before describing the use of thermal imaging systems in the modern world.
Over the next century or so the rest of the electromagnetic spectrum was discovered.
The diagram below shows where light and infra red fit into the the electromagnetic spectrum. Infra red has a wavelength between 0.7 and 300 micrometres. A micrometre (micrometer for Americans. The symbol for a micrometre is µm.
Joseph Fourier and the Greenhouse Effect
Writers and researchers in the second half of the 19th century credited Fourier with being the first to allude to the greenhouse effect.
In an article in 1924 (and reprinted in English in 1927) he wrote:
the temperature (of the Earth) can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non-luminous heat.
Source: Fourier J (1824). "Remarques Générales Sur Les Températures Du Globe Terrestre Et Des Espaces Planétaires". Annales de Chimie et de Physique 27: 136-67, quoted at this link.
The quotation sounds remarkably like the modern undestanding of the greenhouse effect, but fourier's understanding of the phenomon is often overstated. He seems to mean by the quote something like the operation of greenhouses, which do not work the same way as the modern effect named (erroneously) after them. Fourrier also thought that the atmosphere was warmed more effectively by other factors including the internal heat of the planet and heat from the stars.
For a detailed discussion of Fourier and the greenhouse effect see this link, from pages 55 to 64. A brief analysis can be found on pages 2 and 3 of this document.
The Discovery Greenhouse Gasses by John Tyndall
John Tyndall (1820 - 1893) was a very active and prominent 19th physisist who made many original discoveries but is best remembered for his work on greenhouse gasses. His main experimental work in this field was in 1959. He discovered that the main constituents of the atmosphere, nitrogen and oxygen are transparent to infra red radiation, but that a number of trace gasses in the atmospere were effective in absorbing infra red. The main ones were water vapour and carbon dioxide (CO2).
Tyndall's experimental apparatus is shown in the drawing below. To see a larger version and a description of the apparatus click on the drawing.
Tyndall was aware of the implications of his finding when he noted:
To the eye, the gas within the tube might be as invisible as the air itself, while to the radiant heat it behaved like a cloud which it was almost impossible to penetrate. Thus, the bold and beautiful speculation has been made an experimental fact. The radiant heat of the sun does certainly pass through the atmosphere to the Earth with greater facility than the radiant heat of the Earth can escape into space.
A source for the quotation can be found here.
(It is interesting to note that in the chair of Tyndall's demonstration of his discovery was Albert the prince consort. It is sad reflection on the current political classes that a significant fraction of today's poiticians do not take the active interest in science that Prince Albert did. Many of the current crop of politicians and journalists reject and distort the science on political and ideological grounds.)
Like many scientists of his time, Tyndall was particularly interested in explaining changes of climate in the past, as it was becomming increasingly clear that Europe had suffered ice ages in the past.
Tyndall was aware that the growth of industry was putting CO2 into the atmosphere but it was almost four decades before a scientists seriously investigated the effect of increasing CO2 in the atmosphere.
For more information on Tyndall see this link.
Svante Arrhenius's Greenhouse Calculations
Arrhenius was a Swedish physicist who published a study in 1896 into the effect on climate of changing the amount of CO2 in the atmosphere. Tyndall had shown that increasing CO2 would warm the climate and decreasing it would have a cooling effect. The interesting question was by how much would temperature be effected? Arrhenius performed long and tedius pen and paper calculations to provide an answer to this question. He realised that the effect of CO2 is complicated by feedbacks, what he called "the mutual reaction of the physical conditions". A major feedback process involves water vapour, which is a much more effective greenhouse gas than CO2. The increase in termperature caused by an increase in CO2 would result in more water vapour in the atmosphere which would amplfy the warming produced by increasing CO2.
Arrhenius calculated that doubling the amount of CO2 in the atmosphere would result in a 50C to 60C increase in global temperature. This wasn't a bad result as the current accepted value is about 30C.
A few years after Arrhenius published his result Ångström published a strong criticism of it. Ångström's basic argument was that the infrared is already saturated and that increasing the CO2 in the atmosphere would not effect the absorbtion of the infrared radiation. This argument is still used by climate science deniers today. For an explanation of why this argument by Ångström was incorrect see the posts here and here.
For biographies of Arrhenius see posts here and here.
Guy S. Callendar (February 1898 - October 1964)
After Ångström's criticism of Arrhenius two decades passed before serious analysis of the role of CO2. From 1938 until his death in 1964 Callendar argued that CO2 had an important role in climate. For more than ten years he played a lone hand in making these argumants.
As this site notes:
Callendar took his own weather observations at his home in Sussex and compiled a massive amount of temperature data from around the world. Noting an upward trend in temperatures for the first four decades of the twentieth century, he combined these results with studies of the retreat of glaciers, measurements of rising concentrations of atmospheric carbon dioxide since pre-industrial times, and information newly available concerning the infrared absorption bands of atmospheric constituents. He concluded that the trend toward higher temperatures was significant, especially north of the forty-fifth parallel; that increased use of fossil fuels had caused a rise of the concentration of CO2 in the atmosphere of about ten percent from nineteenth century levels; and that increased sky radiation from the extra CO2 was linked to the rising temperature trend. Although he was an amateur meteorologist, Callendar worked on a truly global scale, compiling a reliable world data set of surface temperatures from earliest times and insisting- long before it became fashionable to do so-that climatology must deal with physics and atmospheric dynamics.
Most scientists during the 1930s and 1940s rejected Callendar's climate view for two reasons:
- the saturation argument, a la Ångström, and,
- the view that most if not all of the CO2 released into the atmosphere from human sources, would be absorbed by the oceans