Question about Herschel experiment which discover Infra red light

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Discussion Overview

The discussion revolves around the Herschel experiment that led to the discovery of infrared light. Participants explore the relationship between light frequency, wavelength, and energy, particularly in the context of temperature measurements made during the experiment. The scope includes conceptual clarifications and technical explanations related to the experiment's findings.

Discussion Character

  • Exploratory, Conceptual clarification, Technical explanation

Main Points Raised

  • Charlotte questions the apparent contradiction in Herschel's findings, noting that while higher frequency light (shorter wavelength) is associated with higher energy, the highest temperature rise in the experiment occurred in the infrared region, which has a longer wavelength.
  • Warren points out that although individual blue photons have more energy than red photons, the sun emits a significantly larger number of red photons, which may contribute to the observed temperature effects.
  • A later reply suggests that the infrared thermometer's higher heating is due to the concentration of infrared light onto it, indicating that the spatial concentration of IR photons plays a role in the temperature measurement. It also notes that Herschel's prism cut off wavelengths longer than 2 microns, implying that the near-infrared range is primarily responsible for the heating observed.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between photon energy and temperature measurements, with no consensus reached regarding the interpretation of Herschel's findings and the implications of photon concentration.

Contextual Notes

The discussion highlights potential limitations in understanding the relationship between light energy and temperature, particularly regarding the definitions of energy, concentration of photons, and the specific wavelength ranges involved in the experiment.

Who May Find This Useful

Individuals interested in the historical context of light discovery, the physics of light and energy, and the implications of experimental findings in optics may find this discussion relevant.

b612_l_prince
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Hi there, I have a question about his experiment.

Herschel discovered IR by placing the blacken thermometer next to the red light (out from the prism), since he noticed the TREND OF INCREASING TEMPERATURE proceeded from BLUE (lowest temperature) TO RED (highest temperature). So he wanted to find out what the temperature would be just outside visible red light.

So my question is, we always say when frequency of a light is higher (wavelength is shorter), the energy that light carries is higher. However, in his experiment, Herschel found the highest temperature rise is indeed in Infra Red region... shortest wavelength?!

Is there anyone could give me some clues on this "contradiction"? What mistake have I made in having such a contradicting conclusion?

Thanks a lot.


Charlotte

P.S. The details of the experiment can be found at http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html
 
Science news on Phys.org
While each individual blue photon contains more energy than each individual red photon, the sun makes a whole lot more red photons than blue photons.

- Warren
 
thanks warren

thanks Warren for letting me know. I appreciate it. =)
 
A more correct answer

The Infrared thermometer is heated the most because the infrared light
is more concentrated onto it. The IR photons are spatially more concentrated by the prism.
Note - Herschel's prism cut-off longward of 2 microns so the the near-IR from 1-2 um is doing the heating.

See
http://home.znet.com/schester/calculations/herschel/index.html
 

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