The instructions :
Frame three small (2-3 inch) rectangles on exercise canvas. 1. In the first rectangle, paint an orange from memory. Include its form and cast shadows, a highlight and a background. (No example of this)
2. In the second rectangle (top left) paint another orange painting the orange with opaque paints and the background in transparent paints.
3. In the third rectangle (bottom left) , paint the shadows (both form and cast) shadows with traent paint. Paint the lit part of the orange with opaque paint. In the background, directly beside opaque paint use transparent paint. Directly beside transparent paint in the shadow areas, paint opaque paint and then vary the remaining areas of the background with a variety of paints, small color or value changes.
What should be seen from this exercise is that the opaque paint physically advances and the transparent paint recedes. The progression from your first rectangle to the third rectangle should make this apparent.