Origin of Radiation. Electromagnetic Waves

The origin of radiation is electromagnetic and is based on three phenomenological facts The Ampere law, the Faraday law, and the Lorentt force (Eg. 8.1) which are usually covered in a physics course. However, the usual form of Ampere s law, which gives 

The Lorentz force coupled with the Ampere law and the Faraday law leads to the balance of electromagnetic momentum2 which involves the (divergence of) electromagnetic (Maxwell) stress. The isotropic limit of this stress is the electromagnetic pressure 

1 A greenhouse is a glass-endosed structure used for cultivating plants with controlled temperature and humidity. 

2 The development of electromagnetic momentum is beyond the scope of this text. 

To an observer the hole appears to be black. The radiation escaping the cavity is also black for reasons to be clarified in Section 8.4. Since the electromagnetic radiation is altered by but not attached to an intervening medium, its energy u defined per unit volume leads, for total internal energy, to 

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