You hold your hand 3 feet above the ground and look at the shadiw cast by the sun. You repeat this inside using the light from a 2-foot by 4-foot fluorescent light boxin the ceiling. In which case will the penumbra be more pronounced?
In the case of the fluorescent light boxin the ceiling.
Light rays that are not parallel are the cause of a penumbra. The light rays from the sun are (nearly) parallel due to the long distance they have traveled . When your hand is hit by the rays from the light in the ceiling, light comes from all parts of the light to hit any single spot on your hand.
I hope this helps,