From the timeline of Maxwell's prediction 1865 and Hertz's observation in 1887 that gave an understanding of the light wave as EM wave, How did the Black body radiation study understand the emitted radiation as what at that time since the timeline of Black body radiation was also in 1890s? Did they know that the radiation is EM wave? Or how did they understand the radiation that emitted?

  • $\begingroup$ See Wikipedia's history sections on black-body radiation and on optics. There were two competing theories of light since 17th century. Newton's corpuscular optics (light as rays of particles) dominated until early 19th century when it was displaced by Young's and Fresnel's wave optics (light as waves in luminiferous aether). Fresnel specifically discussed black-body radiation. $\endgroup$
    – Conifold
    Commented Feb 15 at 11:00
  • $\begingroup$ @Conifold Fresnel did not have the concept of blackbody. He just made some arguments against Newton's corpuscules in terms of conversion of light into the now deprecated idea of caloric. $\endgroup$
    – Mauricio
    Commented Feb 18 at 12:15

1 Answer 1


This is a new rewritten answer, after diving more into the contemporary references and finding some modern references. I deleted my old answer, which had information about black body radiation experiments, because this is essentially a new answer.

By the 1850s, the idea that heat radiation was the same as light was gaining increasing favor. It obeyed the same properties as light radiation (reflection, refraction, polarization, double refraction, interference) and sources of heat radiation were always closely connected with sources of light. Prior to these developments, heat radiation was understood phenomenologically, although its connection to light was suspected from the late 18th century. An excellent reference for the history up to the year 1800 is:

  • E.S. Cornell M.Sc. (1936) Early studies in radiant heat, Annals of Science, 1:2, 217-225, DOI: 10.1080/00033793600200171

From the conclusion to that paper:

Thus, before 1800, the existence of invisible heat rays had been generally accepted. It was known that these rays were propagated practically instantaneously and in straight lines, and were reflected according to the same law as that followed by light rays. It was suspected that they could also be refracted like light. Because of these similarities between the rays of heat and light, a close connection between the two was suspected, and [James] Hutton went so far as to maintain that the invisible heat rays were invisible light. The majority of scientists, however, bearing in mind the different absorption effects of glass on the rays of heat and light from the fire, now well known, preferred to reserve their judgment and used the terms "Obscure Heat" or "Radiant Heat" or "Radiant Caloric" for the invisible heat rays. There were two theories prevalent as to the nature of the invisible heat rays: -- (1) That they were a material emanation, and (2) that they were vibrations in an all-pervading heat fluid. Of these, the former was the more commonly held. Moreover, the reflection of cold had been clearly demonstrated, and satisfactorily explained by Prevost's theory. This was the state of the subject when Sir William Herschel made his discovery of the ifra-red heating rays in the solar spectrum in 1800. (p. 225, emphasis mine)

Herschel's discovery of radiant heat in the sun's spectrum led to the publication of Experiments on the solar, and on the terrestrial Rays that occasion Heat; with a comparative View of the Laws to which Light and Heat, or rather the Rays which occasion them, are subject, in order to determine whether they are the same, or different. (Phil. Trans. 90, 1800) (link to part 1 on HathiTrust). He says in the introduction,

I must also remark, that in using the word rays, I do not mean to oppose, much less to countenance, the opinions of those philosophers who still believe that light itself comes to us from the sun, not by rays, but by the supposed vibrations of an elastic ether, every where diffused throughout space; I only claim the same privilege for the rays that occasion heat, which they are willing to allow to those that illuminate objects. For, in what manner soever this radiance may be effected, it will be fully proved hereafter, that the evidence, either for rays, or for vibrations which occasion heat, stands on the same foundation on which the radiance of the illuminating principle, light, is built. (p. 294, emphasis mine)

This may seem like a decisive moment, but in fact the question was far from settled - discussions of Herschel's conclusion extended into the 1830s. The term "calorific rays" or "calorific radiation" was popular, if you'd like to dig up some sources from the period. See for example:

Powell, Baden (1825) An Experimental Enquiry into the Nature of the Radiant Heating Effects from Terrestrial Sources. Phil. Trans. 1825 p. 187. (Link)

It was the experiments between 1830 and 1850 performed by the likes of Ampere, Forbes, Melloni, and others, that helped the heat-rays-as-light theory gain favor. In the 1840s, Faraday was also proposing ideas that could be considered the beginnings of the electromagnetic theory of light. Simultaneously, the classical theory of thermodynamics was taking shape.

From a practical perspective, the question of what heat rays were (light, particles, other rays with their own aether, a combination of multiple effects) was less important than how the heat rays behaved thermodynamically, in particular with regards to thermodynamic equilibrium and the second law of thermodynamics. Now hopefully the information in my previous answer will make more sense.

Prevost (1791) postulated his theory of heat exchange, stating that every body radiates heat, and receives radiant heat from other bodies. From Pictet's experiments with heat radiation (1790), it was known that radiant heat could be reflected and focused, like light. Wollaston (1802), then later von Fraunhofer (1817), had discovered that the visible spectrum of the sun had dark lines (now called Fraunhofer lines), some of which aligned well with the spectral lines emitted by laboratory light sources. Sir John Herschel (1823) and William Hallows Miller (1833) experimented with passing the solar spectrum through gases and observing the resulting spectrum; additional dark lines which appeared were attributed to the absorption of radiation by the gases. Foucault (1849, 1860) demonstrated two important facts: (1) that an electric arc produces light of the same wavelength as what was known as the D line in the sun's spectrum, and (2) when light from a more uniform radiator was passed through this arc, a dark D line also appeared, indicating that the arc was both emitting and absorbing the same wavelength of radiation. Balfour Stewart (1858) performed experiments with rock salt and concluded that the radiating power of every kind of substance is equal to its absorbing power, for every kind of ray of radiant heat. [Whittaker, vol. 1, ch. XII].

Indeed, theories of black-body radiation using Maxwell's theory of electromagnetic light didn't come about until the late 19th century into the early 20th century, with e.g. the Rayleigh-Jeans law, but by this time Hertz's experiments had already demonstrated the existence of electromagnetic waves and classical electrodynamics was reaching maturity.

  • $\begingroup$ Thank you !!!!! $\endgroup$ Commented Mar 1 at 4:28

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