When starting a new construction Archimedes Geo3D uses a so-called ’Headlight’. The direction of the light is the same as that of a lamp mounted to the viewers head, thus the name.
You can define your own lights with Extras - Light. The headlight will be turned off in this case.
Those lights behave - besides being lights - like free points. You can position them freely in the scene and attach them to points, lines, segments, circles, planes and spheres. You get nice effects if you define an animation with a light!
Please notice that such a light is just a simple simulation that computes the color of a surface from the angle between light, surface and viewer. You cannot generate really realistic lights as in raytracing-software. Consider that raytracing-software is roughly a thousand times slower than realtime-software like Archimedes. With a bit of skill and patience you will get beautifully lit scenes, though. See the example ’lightshow.geosave’.