Surfaces contain content. They provide clear focal points within the user's field of view, which can contain content or actions.
Depth creates a sense of comfort and hierarchy
Depth on glasses is the combination of placement in z-space and shadows to establish a sense of hierarchy through varying levels of emphasis.
Use surface color variants to emphasize depth
Use surface color variants like Surface Container Low to create a more contrasting effect for depth. Shadows need to be fully opaque black to achieve effect.
Surfaces should always be transparent to maximize contrast in most environments.
Do
Make sure to set blend mode to screen to preview how UI surfaces will display on transparent displays.
Don't
Use normal blend mode while designing as it will not give a realistic perception.