Radiative properties of magnetic elements - I. Why are G-band bright points bright?

Photospheric magnetic elements are most conspicuously visible in high-resol ution G-band filtergrams. We show that their enhanced contrast in the G-ban d is due to a reduction of the CH abundance by dissociation in the deep pho tospheric layers of the flux tube, where it is hotter than in the surroundi ng atmosphere. As a consequence, the CH-lines weaken, allowing more of the continuum to "shine" through the forest of G-band CH-lines. We suggest that other molecular bands or atomic lines may exhibit a similar behaviour.