The sunlight, fluorescent or some else light source (reflected light) etc. strike on the objects to create orderly predictable series of tones. Organised and control their relationship of light and dark gradations is the key for building three-dimensional image.
Review the last 2 exercises:
1.For my geometric solid I tried to catch up their graduations but to form the shadows it needs to have more gradations.
2.My 3 objects drawing with “X”and “+” symbols, the graduations are scrambled. I may shade them more tones.
For the exercise observing shadow using blocks of tone, for which I have drawn a teapot and a plate. I am quite happy with the tonal contrast of outcome.