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Based on Planck’s law all matter can emit at all wavelengths at different intensities dependent of temperature. I was wondering if this holds true, does all matter absorb all wavelengths too, at different intensities ?

If the answer to this is yes, then how can we see colours? ( if all wavelengths are absorbed and emitted)

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The planck assumes a theoretical blackbody object that absorbs all incoming electromagnetic radiation and emits radiation at all wavelengths, depending on its temperature. The spectral distribution mathematical function by Planck describes the intensity of blackbody radiation at different wavelengths for a given temperature.

Hence, blackbody radiation described by Planck's law is a theoretical construct, that is often used as an idealized model, but it is not perfectly applicable in real life situations, as most physical body not perfectly behaves like a blackbody. Real objects emit and reflect light in much more complex ways, with the properties of the object influencing the specific wavelengths of light that are emitted or reflected.

Although, this law is fundamental of statistical mechanics and solid foundation of modern understanding of the physical nature of thermal radiation. The Planck's law does not provide any means to quantify the radiations from intrinsic atomic properties.

We have to extend this law to relate the atomic absorption and emission of radiation to microscopic properties of atoms through quantum mechanics of energy transition of different atom levels and mode cavities. Different atoms and molecules have different electronic and vibrational energy levels, which means that they can absorb and emit radiation at different specific wavelengths.

The temperature of an atom or molecule also affects the wavelengths it emits. At high temperatures, the atoms and molecules have more kinetic energy, and are more likely to make transitions to higher energy levels, resulting in the emission of shorter wavelength radiation. At lower temperatures, the atoms and molecules have less kinetic energy and are more likely to make transitions to lower energy levels, resulting in the emission of longer wavelength radiation.

For example, hydrogen gas emits light primarily at a wavelength of 656 nm (in the red region of the spectrum) when it makes transitions from the n=3 energy level to the n=2 energy level. However, at a high enough temperature, hydrogen atoms may be excited to higher energy levels and will emit light at a different set of wavelengths. Similarly, each element and molecule may have their own unique set of emission spectra based on its energy level transitions.

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  • $\begingroup$ I see. Suppose an objects with numerous atoms and molecules. Can this objects absorb all wavelengths but at different intensities? $\endgroup$ Jan 10, 2023 at 8:35
  • $\begingroup$ Yes, it is possible for an object composed of numerous atoms and molecules to absorb light at different wavelengths with different intensities. This is known as absorption spectroscopy and is commonly used to study the composition of a material. We dope different elements with each other to change energy levels and achieve different badwidths and light propogation/absorption/emission properties for different applications. For example in photodiode, Laser or amplifiers. $\endgroup$
    – Haris
    Jan 10, 2023 at 8:45
  • $\begingroup$ In practice, the absorption spectra of real-world samples are generally not as simple The absorption spectra of materials are also affected by temperature, pressure, concentration of the absorbing species, and so on. Also, the absorption coefficient, which relates the intensity of the light to the amount of absorbing material can vary depending on the specific material. $\endgroup$
    – Haris
    Jan 10, 2023 at 8:45
  • $\begingroup$ All in all, the absorption spectrum of a material gives a lot of information of the material itself and its surroundings, by using this information, we can gain insight into the chemical makeup of a material, how it is interacting with its environment, and how it is affected by various physical and chemical processes. $\endgroup$
    – Haris
    Jan 10, 2023 at 8:45
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    $\begingroup$ "Different atoms and molecules have different electronic and vibrational energy levels, which means that they can absorb and emit radiation at different specific wavelengths.", "and will emit light at a different set of wavelengths. ", "their own unique set of emission spectra based on its energy level transitions." So what is you answer to the question posed? $\endgroup$
    – ProfRob
    Jan 11, 2023 at 12:34
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Planck's law is not produced by some emission mechanism. It pre-supposes that there are physical processes that can both absorb and emit at all wavelengths. An object that emits a Planckian spectrum must be able to absorb all light incident upon it and there should be thermal equilibrium between the matter and the radiation. Any emission process will have an inverse absorption process but the two will only balance at thermal equilibrium.

To put it another way: an object that has no absorption at some wavelengths cannot emit a Planckian spectrum.

Most everyday objects do not absorb all radiation that is incident upon them; they are not in thermal equilibrium with their radiation environment; and they do not radiate a Planckian spectrum. They have wavelength-dependent reflectivity for example. This is what makes them coloured when they are illuminated by light with a broad spectrum.

Some objects have a pseudo-Planckian spectrum because they do provide effective absorption over a broad range of wavelengths. The mechanisms for this continuous absorption with wavelength are diverse and depend on the state of the matter (see for example answers to What are the various physical mechanisms for energy transfer to the photon during blackbody emission?), but everywhere emission is seen there must be the possibility for absorption.

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