See how the penumbra changes. When the light is large, the penumbra is also large. As the light shrinks in size, the shadow becomes primarily an umbra. If the effects are puzzling, try using the step and 20 steps buttons to see the individual rays and determine where the different light factors are coming from. Remember to press the complete button to re-enable the workspace after using step or 20 steps.
Shadows affect both diffuse and specular light, but a penumbra is an effect of diffuse light.
A penumbra exists wherever only part of a lightsource is blocked. The raytracing algorithm considers each lightsource as a single point. Wherever a ray comes in contact with a surface, the algorithm checks every light to see if the center is visible to that spot. In other words, the light is either completely visible or completely hidden. The radiosity algorithm however turns all lightsources into shapes that will radiate light.