Concrete is the most common artificial composite material of all and typically consists of loose stones (aggregate) held with a matrix of cement. Concrete is a very robust material, much more robust than cement, and will not compress or shatter even under quite a large compressive force.
2) Y-axis contains absolute brightness Why do astronomers use the H-R Diagram? Astronomers use H-R Diagrams to classify stars and to understand how stars change over time.
The axis is the curvature of the eye surface in a specific meridian (like degrees or clock hours). I80 is right on the horizontal meridian, and 90 is right on the vertical meridian. The axis is always a number between 1 and 180. On your eyeglass prescription, the axis refers to the first number (the sphere power).