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I liked it, though it bugs me a little when people equate infrared and heat. Infrared is light. Light can heat things, and hot things can glow, but "infrared is

I liked it, though it bugs me a little when people equate infrared and heat. Infrared is light. Light can heat things, and hot things can glow, but "infrared is heat" isn't exactly right.

Smoosh 3y ago
I agree with you, but I think that for the general public you have to relate to what they experience, and thus intuitively know, and that is that heat “seems” to be different from light.
amarant 3y ago
I know basically nothing about physics, so sorry if this is a dumb question. The existence of infrared LEDs seems to indicate to me that infrared light can exist without heat. The existence of infrared thermometers seems to imply that hot stuff radiates infrared light, at least usually. So my question is, is there any case where heat does not cause infrared radiation? What are those cases? Some special materials? Special colours(perhaps outside the visible spectrum)?
- elevatedastalt 3y ago
  That's a good question. All bodies above the temperature of absolute zero emit electromagnetic radiation across the whole spectrum (this is called Black-body radiation). Think of it as a mixture of different amounts of light of every possible wavelength. However, what the exact mixture is depends on the temperature of the body. As the body gets hotter, the 'peak' wavelength, i.e. the wavelength whose "amount" is highest in the mixture decreases. Objects at room temperature emit most of their energy outside of the visible spectrum, so they are not 'visible' in the dark. However, as you heat them up, the radiation mixture moves towards lower wavelengths, closer to infra-red. Heat it up further and things become red hot, yellow, blue hot and so on. Infra-red LEDs produce light of the the specific infrared wavelength through semi-conductors. They have nothing to do with the black-body radiation one associates with 'hot' objects.
- mensetmanusman 3y ago
  Metals don't generate much infrared light in certain wavelengths when are hot. This is why they are installed on window surfaces as thin layers to lower the windows emissivity.
- itishappy 3y ago
  The concept you're looking for is called Emissivity, and it describes how effectively an object can radiate (or absorb!) at a certain wavelength. https://en.wikipedia.org/wiki/Emissivity https://en.wikipedia.org/wiki/Emissivity > Real objects never behave as full-ideal black bodies, and instead the emitted radiation at a given frequency is a fraction of what the ideal emission would be. The emissivity of a material specifies how well a real body radiates energy as compared with a black body. This emissivity depends on factors such as temperature, emission angle, and wavelength. However, it is typical in engineering to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength so that the emissivity is a constant. This is known as the gray body assumption. https://en.wikipedia.org/wiki/Black-body_radiation https://en.wikipedia.org/wiki/Black-body_radiation In a previous life I worked for a lab designing a passive radiator that was reflective to visible light but absorptive/emissive in the atmospheric window (where the atmosphere is completely transparent). This material had the fun effect of absorbing energy via conduction/convection and then sending it out into space, maintaining a temperature slightly below ambient.

I liked it, though it bugs me a little when people equate infrared and heat. Infrared is light. Light can heat things, and hot things can glow, but "infrared is heat" isn't exactly right.

Smoosh 3y ago

I agree with you, but I think that for the general public you have to relate to what they experience, and thus intuitively know, and that is that heat “seems” to be different from light.

amarant 3y ago

I know basically nothing about physics, so sorry if this is a dumb question. The existence of infrared LEDs seems to indicate to me that infrared light can exist without heat. The existence of infrared thermometers seems to imply that hot stuff radiates infrared light, at least usually. So my question is, is there any case where heat does not cause infrared radiation? What are those cases? Some special materials? Special colours(perhaps outside the visible spectrum)?

elevatedastalt 3y ago
That's a good question. All bodies above the temperature of absolute zero emit electromagnetic radiation across the whole spectrum (this is called Black-body radiation). Think of it as a mixture of different amounts of light of every possible wavelength. However, what the exact mixture is depends on the temperature of the body. As the body gets hotter, the 'peak' wavelength, i.e. the wavelength whose "amount" is highest in the mixture decreases. Objects at room temperature emit most of their energy outside of the visible spectrum, so they are not 'visible' in the dark. However, as you heat them up, the radiation mixture moves towards lower wavelengths, closer to infra-red. Heat it up further and things become red hot, yellow, blue hot and so on. Infra-red LEDs produce light of the the specific infrared wavelength through semi-conductors. They have nothing to do with the black-body radiation one associates with 'hot' objects.
mensetmanusman 3y ago
Metals don't generate much infrared light in certain wavelengths when are hot. This is why they are installed on window surfaces as thin layers to lower the windows emissivity.
itishappy 3y ago
The concept you're looking for is called Emissivity, and it describes how effectively an object can radiate (or absorb!) at a certain wavelength. https://en.wikipedia.org/wiki/Emissivity https://en.wikipedia.org/wiki/Emissivity > Real objects never behave as full-ideal black bodies, and instead the emitted radiation at a given frequency is a fraction of what the ideal emission would be. The emissivity of a material specifies how well a real body radiates energy as compared with a black body. This emissivity depends on factors such as temperature, emission angle, and wavelength. However, it is typical in engineering to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength so that the emissivity is a constant. This is known as the gray body assumption. https://en.wikipedia.org/wiki/Black-body_radiation https://en.wikipedia.org/wiki/Black-body_radiation In a previous life I worked for a lab designing a passive radiator that was reflective to visible light but absorptive/emissive in the atmospheric window (where the atmosphere is completely transparent). This material had the fun effect of absorbing energy via conduction/convection and then sending it out into space, maintaining a temperature slightly below ambient.