Alternatively, they also use the term 'black body' radiation — a black body is a theoretical object that completely absorbs all of the light that it receives and reflects none. A black body also is a perfect emitter of light over all wavelengths, but there is one wavelength at which its emission of radiation has its maximum intensity.
Hotter objects emit more radiation than colder objects over all wavelengths. Furthermore, the hotter an object, the shorter the wavelength of its peak emission see figure 1. Recalling that short wavelengths correspond to higher energies, this means that hotter objects give off more high-energy radiation than colder objects—as intuition might suggest.
Figure 1: Graph showing black body radiation as a function of wavelength and temperature. Astronomers consider stars to be approximate black bodies that are capable of absorbing light at every wavelength without any reflection.
For example, the hottest among stars, with surface temperatures of 10, K or more, will emit most of their radiation at ultraviolet UV wavelengths. In contrast, stars like the Sun, which are approximately K at the surface, emit most of their radiation at visible wavelengths, and even cooler stars emit most in the infrared IR range. This relation is particularly useful to astronomers as it allows them to infer the temperature of stars and other cosmic objects, which lie at enormous distances from us, by simply measuring the 'colour' of their light —more precisely, by measuring the wavelength at which their emission is most intense.
Estimating the temperature of stars, interstellar gas and other astronomical sources is crucial to a better understanding of the physical processes that take place therein.
In fact, all objects and astronomical sources are only approximate black bodies. Astronomers have discovered that an almost perfect black body is represented by the Cosmic Microwave Background CMB , the relic radiation from the Big Bang which pervades the entire Universe. This radiation is the first light ever emitted in cosmic history. When it was released, just , years after the Big Bang, its energy was enormous, but as the Universe expanded, it cooled and its photons have stretched to increasingly longer wavelengths.
Yes, all objects, including human bodies, emit electromagnetic radiation. The wavelength of radiation emitted depends on the temperature of the objects.
Such radiation is sometimes called thermal radiation. Most of the radiation emitted by human body is in the infrared region, mainly at the wavelength of 12 micron. The wavelength of infrared radiation is between 0. This wavelength is longer than that of red visible light so this explains the name 'infrared', meaning 'beyond the red'. Stars behave approximately like blackbodies, and this concept explains why there are different colors of stars.
Red stars are cooler, and they emit the most radiation in the red wavelengths. We don't see any stars as green because stars with peak wavelengths in the green also emit lots of radiation in the red and blue part of the spectrum. Our eyes combine all of these colors and we see white in this case. Even hotter stars and other objects emit the most radiation in the blue, ultraviolet or even x-ray and gamma ray part of the spectrum.
Objects like these appear blue to our eyes. Much cooler objects like planets and humans emit the most radiation in the infrared.
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