How Light Travels
We never expect to see around a corner, and if we wish to see through pinholes in three separate pieces of cardboard, we place the cardboards so that the three holes are in a straight line. When sunlight enters a dark room through a small opening, the dust particles dancing in the sun show a straight ray. If a hole is made in a card, and the card is held in front of a light, the card casts a shadow, in the center of which is a bright spot. The light, the hole, and the bright spot are all in the same straight line. These simple observations lead us to think that light travels in a straight line.
We can always tell the direction from which light comes, either by the shadow cast or by the bright spot formed when an opening occurs in the opaque object casting the shadow. If the shadow of a tree falls towards the west, we know the sun must be in the cast; if a bright spot is on the floor, we can easily locate the light whose rays stream through an opening and form the bright spot. We know that light travels in a straight line, and following the path of the beam which comes to our eyes, we are sure to locate the light.
FIG. - The candle cannot be seen unless the three pinholes are in a strait line.