Nice explanation. You should cover in more detail how black bodies can only be composed of condensed matter. We have a popular fallacy spread by cosmologists that gas or plasma is a valid black body radiator. This is not the case as a gas can only emit discrete wavelengths. Only condensed matter can emit a continuous spectrum of wavelengths.
Thanks for your feedback. You're absolutely right, I could have mentioned that. The very fact that gases are generally transparent shows that radiation is obviously not (entirely) absorbed. Therefore, by definition, gases cannot be black bodies. In my opinion, the situation is different for stars, even though they are also made of gases. Although gases are usually transparent, this is not entirely true for the core of stars. For example, the Sun is mostly hot, dense plasma that exists at extremely high temperatures and pressures. In this state, gases can become ionized, causing them to absorb and emit radiation. The plasma in the Sun is therefore not transparent, but plays a crucial role in the generation and emission of electromagnetic radiation.
The thorough treatment of the black body prerequisites (absorption vs emission and thermal equilibrium) is much appreciated; this was extremely well done.
when using a soda can, you give the impression that any cavitty will do. what about perfect reflectors and resonant cavity like MRI? they are in equalibrium but are not black bodies. kirkhoff said that any material will do. "the radiation is independed of the walls" . that doesnt fit the real world.
Kirchhoff's law of thermal radiation states that for a body in thermal equilibrium, emissivity is equal to absorptivity. However, real-world materials, especially perfect reflectors or special cavities like those in MRI machines, deviate from the ideal blackbody behavior. A perfect reflector does not absorb or emit radiation, it reflects it and therefore does not behave like a blackbody. Resonant cavities in MRI machines are designed to maintain specific resonant frequencies and high quality factors, which means they interact with radiation differently than a blackbody. Kirchhoff's statement that "the radiation is independent of the walls" is an idealization. In practice, the material properties of the cavity walls affect the radiation characteristics. Ideal blackbodies are a theoretical construct, and real-world deviations exist. The soda can in the video is used as a simple illustrative example and does not capture these complexities.
Nice explanation. You should cover in more detail how black bodies can only be composed of condensed matter. We have a popular fallacy spread by cosmologists that gas or plasma is a valid black body radiator. This is not the case as a gas can only emit discrete wavelengths. Only condensed matter can emit a continuous spectrum of wavelengths.
Thanks for your feedback. You're absolutely right, I could have mentioned that. The very fact that gases are generally transparent shows that radiation is obviously not (entirely) absorbed. Therefore, by definition, gases cannot be black bodies.
In my opinion, the situation is different for stars, even though they are also made of gases. Although gases are usually transparent, this is not entirely true for the core of stars. For example, the Sun is mostly hot, dense plasma that exists at extremely high temperatures and pressures. In this state, gases can become ionized, causing them to absorb and emit radiation. The plasma in the Sun is therefore not transparent, but plays a crucial role in the generation and emission of electromagnetic radiation.
Thank you, sir. Wonderful video.
You're welcome
Thank you!!!
Thank you so much
you are welcome!
The thorough treatment of the black body prerequisites (absorption vs emission and thermal equilibrium) is much appreciated; this was extremely well done.
Thank you very much for your kind words!
when using a soda can, you give the impression that any cavitty will do. what about perfect reflectors and resonant cavity like MRI? they are in equalibrium but are not black bodies. kirkhoff said that any material will do. "the radiation is independed of the walls" . that doesnt fit the real world.
Kirchhoff's law of thermal radiation states that for a body in thermal equilibrium, emissivity is equal to absorptivity. However, real-world materials, especially perfect reflectors or special cavities like those in MRI machines, deviate from the ideal blackbody behavior.
A perfect reflector does not absorb or emit radiation, it reflects it and therefore does not behave like a blackbody. Resonant cavities in MRI machines are designed to maintain specific resonant frequencies and high quality factors, which means they interact with radiation differently than a blackbody.
Kirchhoff's statement that "the radiation is independent of the walls" is an idealization. In practice, the material properties of the cavity walls affect the radiation characteristics. Ideal blackbodies are a theoretical construct, and real-world deviations exist. The soda can in the video is used as a simple illustrative example and does not capture these complexities.
Is back body always black in color
no it is not, it depends on the temperature. See video @ 7:34. The sun is a very good blackbody, and I'm pretty sure it's not black ;-)