incident ray

Corner reflectors retroreflect light, producing reflected rays that travel back in the direction from which the incident rays came.

A white body is one with a "rough surface [that] reflects all incident rays completely and uniformly in all directions."

A major challenge that must be addressed in any solar concentrating technology is the changing intensity of the incident rays (the rays of sunlight striking the mirrors) as the sun progresses throughout the day.

Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, to avoid the vignetting and color crosstalk that occur in color filter array-based digital image sensors with oblique incident rays.

The refractive index need not be twice the ambient but can be anything exceeding 1.5 times as high; due to spherical aberration, there exists a radius from the centerline at which incident rays are focused at the center of the rear surface of the sphere.