Issues in calibration. Primary reference standards. A reference standard for a unit of measurement is an artifact that embodies the quantity of interest in a way that ties its value to the reference base. At the highest level, a primary reference standard is assigned a value by direct comparison with the reference base.
Refraction of Sound Waves. The speed of a wave depends on the elastic and inertia properties of the medium through which it travels. When a wave encounters different medium where the wave speed is different, the wave will change directions.
Figure 1. Pencil in glass of water. Refraction is the bending of light rays when they cross an interface between two materials with different refractive indices. For example, when light passes from air into the water at an angle, the light ray will bend toward the normal to the surface.