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Physics Lesson 19.1.2 - Total and Spectral Emissivity
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Welcome to our Physics lesson on Total and Spectral Emissivity, this is the second 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.
Total and Spectral Emissivity
The energy radiated by the unit area of an object in every second from all possible wavelengths is known as the total emissivity E of an object. It is measured in [J/(s·m2)] or [W/m2]. All wavelengths of thermal radiation contribute in the total emissivity. There is another quantity which represents the contribution of a particular wavelength in thermal emissivity. It is known as spectral emissivity e(λ) and is a function of wavelength. Spectral emissivity depends from the type of object and its temperature. The graph showing the dependence of spectral emissivity from wavelength of light is given below.
The graph has the shape of a bell, where the highest point represents the maximum value of thermal emissivity reached for λ = λm. Other wavelengths close to λm also give a considerable contribution in thermal emissivity and transportation of energy of radiation. Wavelength that are 10 times greater or smaller than λm give little or no contribution in this regard.
Objects in normal temperature are not visible because they are incandescent (glowing) but because they reflect part of radiation falling on them. An object absorbs all the other wavelengths of solar radiation falling on it, except the wavelength representing the colour of object. Only this wavelength is reflected from the object's surface and eventually comes to our eyes. This is how we see the objects around us. In the next paragraph we will see how emissivity and absorptivity of a material relate with each other.
You have reached the end of Physics lesson 19.1.2 Total and Spectral Emissivity. 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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