Rate of Heat Absorption from Visible vs Infrared Light

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Discussion Overview

The discussion centers on the rate of heat absorption from visible versus infrared light, specifically in the context of an experiment using a Crookes's Radiometer. Participants explore the relationship between light intensity, spectrum, and material composition in terms of energy absorption.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant observed that a Crookes's Radiometer spun faster under a bulb with higher lumens, suggesting a potential relationship between light absorption rates and the spectrum of light.
  • Another participant noted that energy absorption depends on both intensity and the spectrum of radiation.
  • There is a question regarding whether "higher the spectrum" refers to intensity or frequency, indicating a need for clarification on terminology.
  • It was proposed that the relationship between absorption rate and frequency is not straightforward and may depend on the material's composition.
  • A participant explained that different parts of the electromagnetic spectrum interact differently with materials, with specific examples of how colors reflect and absorb light.

Areas of Agreement / Disagreement

Participants generally agree that the absorption of energy is influenced by both the intensity and spectrum of light, but there remains uncertainty regarding the specific relationship between frequency and absorption rates, as well as the terminology used to describe these concepts.

Contextual Notes

There are unresolved questions about the empirical nature of the relationship between absorption rates and frequency, as well as the complexity introduced by different material compositions.

sphoenixee
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I recently did an experiment with the Crookes's Radiometer.

The radiometer was first placed under a 100W 1600 lm bulb and then a 100 W 2360 lm bulb. Though they both emit the same energy, the radiometer spun about 30% faster under the 2360 lm.

Why is this? I was thinking maybe this was because visible light is absorbed faster than the infrared, but don't know for sure.

Thanks,

sphoenixee
 
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You're right. In general, the amount of energy absorbed by a surface depends on both the intensity and spectrum of the radiation hitting it.
 
Just to clarify, it's the higher the spectrum, the higher the absorption rate?
 
Just to clarify, it's the higher the spectrum, the higher the absorption rate?

What do you mean by "higher the spectrum" - more intensity or higher frequency?
 
What do you mean by "higher the spectrum" - higher frequency or higher intensity?

In the case of frequency, there is no simple relationship - it depends specifically on the composition of the material.
 
Sorry about all the repeated messages. I was experiencing internet problems so I couldn't tell what went through.
 
My bad, but yes, I meant the higher the frequency. So the relationship between absorption rate and frequency is empirical? Or is there just a complicated relationship? Does anyone know of any good articles about this?

Thanks,

sphoenixee
 
It is not quite empirical. There is a lot of theory there, but it is quite complicated, depending on the composition of the surface as well as the frequency of the radiation. the simplest example would be the fact that a red surface reflects red and absorbs other colors, while a blue surface reflects blue and absorbs others (this is overly simplified). Different parts of the spectrum (ranging from radio waves to gamma rays) alll act differently. Low frequency such as microwaves interact with molecules, while higher frequencies such as gammma rays interact with electrons and nuclei.
 
Last edited:
Thanks for clearing that up mathman!
 

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