All the colours of sky (continued)
- Black body – thermal processes:
A heated solid body or a dense gas emits electromagnetic waves
.The
intensity of the emission at various wavelengths at a fixed temperature is
represented by a relation called Planck's law or
black body
law.
When the temperature increases, the peak of the Planck's curve moves towards
shorter wavelengths. Planck's curves have peaks at wavelengths
corresponding to visible light for temperatures between 4000 and 8000
degrees.
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| Fig. 1: The diagram on the left shows black body curves at temperatures of 3000, 4000, 5000 and 6000 degrees; on the right the temperatures are 6000, 7500 and 9000 degrees. Depending on the temperature, different parts of the black body curve enter the optical window. |
In the optical window we basically see stars (considered as a hot black body).
The optical sky is a starry sky
At infrared wavelengths we detect the thermal emission of colder bodies (between 100 and 1000 degrees), such as dust clouds.
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