How is the story of the upper world illuminated by the lower world in William Shakespeare's play Measure for Measure?
This question pertaining to Measure for Measure may depend on precisely what you mean by "upper world" and "lower world." In Greek and Roman mythology, the upper world is Earth and the abode of humankind, while the lower world is the habitation of the dead and is ruled by Hades and Persephone. On the other hand, in Christian terminology, the upper world is the spiritual world and the realm of angels and God, while the lower world is Earth and the abode of humankind.
If you are expressing your question in terms of mythology, then the lower world of death might be seen as illuminating the upper world of humankind by establishing a measuring stick for the valuation of life: escape from death is worth what cost to life? This is the question that the Duke's test and Angelo's villany forces Isabella to decide on. Her choice establishes her answer as stating that spiritual values exceed physical values (as a novice of the Church, chasity is a spiritual promise not a physical condition).
If you are speaking in terms of Christian theology, then the lower world of humankind might be seen as illuminating the upper world of spirituality and the abode of God by--well, actually, the same scenario with more applications: the physical world is secondary to spiritual truths. Again, Claudio's impending human death is less important than a spiritual truth of unity with God, or rather the promise of future unity with God. Another application is that selfish and carnal behavior, such as Angelo's (and Lucio's) crumbles when measured in the face of truth, true justice, and righteousness.