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Illustration Example of Radiant Exitance of the Sun

Radiant exitance of the sun
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The sun emits light of different wavelengths: UV light, red light, green light, blue light and other wavelengths. If we measure the intensity of sunlight of all these different wavelengths and plot it on a graph, we get a curve as shown in the illustration. From this you can see that the blue light, which has wavelength \( \class{blue}{ \lambda_{\text{max}} \), has the highest intensity. This maximum of the curve can be used to calculate the solar temperature using the Wien's radiation law.

More precisely, not the intensity is plotted on the vertical axis, but the so-called Radiant exitance \( M_{\lambda} \), which is the derivative of the radiant power \(M\) with respect to the wavelength: \( M_{\lambda} = \frac{\text{d} M}{ \text{d} \lambda } \).