There is no sharply drawn line between transparent and opaque bodies. Very thin sheets of gold transmit a greenish light, and experiments have shown that substances as transparent as clear water absorb enough light so that at considerable depths in an ocean or lake little or no light is ever found. All light whether from luminous bodies or reflected from non-luminous objects shows certain properties which will now be considered. =354. The Rectilinear Propagation of Light.=--If a beam of light passes through a hole in a window shade into a darkened room, it is seen to follow a perfectly straight course. If a person while coughing holds a book before the face, the sound passes around the book and is heard at any point in the room while the face is hidden by the book. In other words, light ordinarily does not pass around corners as sound does, but travels in _straight lines_. This fact is made use of when one aims a gun or merely looks at an object. So well established in our minds is the idea that an _object_ is in the direction from which we see the light coming to us from it, that we are sometimes deceived as to the real position of an object, when the course of the light from it has been changed by a mirror or some other reflecting surface. Many _illusions_ are produced in this way, of which the _mirage_ of the desert is one example. (See Art. 381.) [Illustration: FIG. 346.--Shadow from a small source of light.] [Illustration: FIG. 347.--Shadow when source of light is large.] =355. Shadows.=--_A shadow is the space from which light is cut off by an opaque body._ Thus if a book (see Fig. 346) is held between a screen, _N_, and a _small_ source of light, _L_, a shadow is produced which extends from the book to the screen. Notice that the shadow is a _space_ and not an _area_. If a _large_ gas flame (see Fig. 347) is used as the source of light, the shadow of the book is no longer clear cut at the edges as before, but has a darker central part with a lighter fringe of partial shadow at the edges. The dark portion within the shadow has all the light excluded from it and is called the _umbra_. The lighter portion of the shadow at the edges has only a part of the light from the flame cut off. This portion is called the _penumbra_. when one stands in sunlight his shadow extends from his body to the ground or object on which the shadow falls. At night we are in the earth's shadow, which extends out into space beyond the earth. [Illustration: FIG. 348.--Character of the earth's shadow.] =356. Eclipses.=--Since the sun is a very large object the shadow cast by the earth contains both umbra and penumbra. (See Fig. 348.) When the moon passes into the shadow of the earth, there is said to be an eclipse of the moon, while if the moon's shadow falls upon the earth, the portion of the earth cut off from the sun's light has an eclipse of the sun.