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Welcome to our Physics lesson on What is Thermal Radiation?, this is the first lesson of our suite of physics lessons covering the topic of Thermal Radiation. Photon as the Quantum of Light, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
In a given temperature above the absolute zero, a thermal motion produces various types of motion in atoms and molecules of matter such as electronic, vibrational and rotational (when spinning around themselves). When particles move to lower energetic levels, they produce EM radiation, otherwise known as thermal radiation.
The EM spectrum is a continuous spectrum, which theoretically includes wavelengths from 0 to infinity. Different wavelengths give different contributes in the transportation of energy produced by thermal radiation. However, there is a characteristic wavelength λm which gives the main contribution in this process for a given temperature. This characteristic wavelength decreases with the increase in temperature of material. Thus, in normal temperatures (around 300 K or 27° C), this wavelength is in the range of infrared part of EM spectrum. This is the reason why we cannot see objects radiate at normal temperature (IR radiation is invisible for humans). Objects start to glow at 800 K roughly. For example, the resistor of a heater does not glow immediately after turning on the switch; it needs a few seconds for the metal conductor to reach the above temperature and start glowing. However, the conductor has been radiating EM waves long before turning the switch on; this radiation was simply invisible for our sight.
The Sun is the main contributor of thermal (EM) radiation coming to the Earth. The surface of Sun is about 5800 K and for this temperature, the main contribution in thermal radiation is given by the wavelength λm ≈ 500 nm, which corresponds to the cyan colour - "the colour of the sky" in our perception. Obviously, without the presence of the Sun everything would dive into darkness.
You have reached the end of Physics lesson 19.1.1 What is Thermal Radiation?. There are 6 lessons in this physics tutorial covering Thermal Radiation. Photon as the Quantum of Light, you can access all the lessons from this tutorial below.
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