[Illustration:  FIG. 63.—­The surface of the paper, although smooth in appearance, is in reality rough, and scatters the light in every direction.]

Hot coals, red-hot stoves, gas flames, and candles shine by their own light, and are self-luminous.  Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light.  We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning.  When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions.  No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen.

CHAPTER XI

REFRACTION

107.  Bent Rays of Light.  A straw in a glass of lemonade seems to be broken at the surface of the liquid, the handle of a teaspoon in a cup of water appears broken, and objects seen through a glass of water may seem distorted and changed in size.  When light passes from air into water, or from any transparent substance into another of different density, its direction is changed, and it emerges along an entirely new path (Fig. 64).  We know that light rays pass through glass, because we can see through the window panes and through our spectacles; we know that light rays pass through water, because we can see through a glass of clear water; on the other hand, light rays cannot pass through wood, leather, metal, etc.

[Illustration:  FIG. 64.—­A straw or stick in water seems broken.]

Whenever light meets a transparent substance obliquely, some of it is reflected, undergoing a change in its direction; and some of it passes onward through the medium, but the latter portion passes onward along a new path.  The ray RO (Fig. 65) passes obliquely through the air to the surface of the water, but, on entering the water, it is bent or refracted and takes the new path OS.  The angle AOR is called the angle of incidence.  The angle POS is called the angle of refraction.

[Illustration:  FIG. 65.—­When the ray RO enters the water, its path changes to OS.]

The angle of refraction is the angle formed by the refracted ray and the perpendicular to the surface at the point where the light strikes it.

When light passes from air into water or glass, the refracted ray is bent toward the perpendicular, so that the angle of refraction is smaller than the angle of incidence.  When a ray of light passes from water or glass into air, the refracted ray is bent away from the perpendicular so that the angle of refraction is greater than the angle of incidence.

The bending or deviation of light in its passage from one substance to another is called refraction.