Figure 2-4. Light sources send waves in all directions.

TM 9-258

(3) Light rays. In considering visible light,

assume that it starts from a luminous point and travels

outwardly in all directions in waves through a medium of

constant density to form a sphere of which the luminous

point is the center (fig 2-4). The direction in which the

light is traveling is along the radii of the sphere of light

(fig 2-5) at right angles to the fronts of the waves. Light

traveling along these radii is termed light rays.

Figure 2-5. Light travels in direction of radii of waves.

(4) Light rays from distant source. The wave

front radiating from a light source is curved when it is

near the source (fig 2-6) and the radii of the waves

diverge or spread. The wave front becomes less and

less curved as the waves move outwardly, eventually

becoming almost straight (fig 2-7), and the radii of waves

from a distant source are virtually parallel.

Figure 2-4. Light sources send waves in all directions.

2-5