Why is sunlight white?

The strontium ions mentioned in the first post emit red light because of transitions in a single ion. Thermal radiation is a very different beast than the spectral line emission you see from the strontium ions or even the spectral band emission you see in the blue flame. Note well: the ions (and the flame) also emit thermal radiation. However, a 2000[sup]o[/sup]C flame temperature is too low to emit visible light. The thermal radiation a 2000[sup]o[/sup]C flame is well into the infrared range.

So what is thermal radiation? The tungsten atoms in the filament are bound to each other, forming a lattice. There is some separation between atoms in the lattice at which the potential energy reaches a minimum. At absolute zero, all of the atoms will be separated by this minimizing separation distance. At a nonzero temperature the atoms move around. The atoms can lose some of this energy by emitting photons. These thermal photons obey quite different rules than the spectral line photons you see in the flame from a bunsen burner.

In the case of spectral line emission, radiation is at a very specific frequency. In the case of thermal radiation, the radiation is continuous.

Planck showed that the energy is quantized. A photon can still have an arbitrary frequency.

 

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Thanks DH.
I think I get the idea now.

Energy absorbed is re-emitted as photons at a range of frequencies depending on the temperature of the heated body.

 

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