Skin depression due to collision. Such wave-trains emanate from a center of vibration, the field persisting as long as the event persists. Wave-amplitude decreases with distance from the center. The wave train at any point in the field specifies the mechanical nature of the event. Organisms and organic substances normally emit volatile chemical compounds, specific to the organism, which travel outward by diffusion and air-currents until diluted in the air.
Propagation is slow, but emission is continuous.
These fields of energy are emitted by hot substances. Since propagation is rectilinear, the direction of the center of the field is exactly specified at any point in the field. Hence a photo-sensitive organism can orient itself to the source. Except for mirrors, all physical surfaces absorb a good part of the incident light and reflect the remainder in all directions. Different bits of the surface have different reflectances in accordance with the texture of the surface.