We see objects by means of what we call light. Light comes from the sun by means of vibrations and produces an effect on the eye. These vibrations may also come from illuminated objects, but such objects give off only waves of light that fall on them from some other source. Bodies which give out light waves directly from themselves are called luminous; those that do not are called non-luminous. Light travels to our eyes very rapidly, and always in straight lines. A line of light is called a ray. A number of rays are called a beam of light.
Light passes through some objects, such as a piece of glass, very readily. Such objects are spoken of as being transparent. If light passes through a body with difficulty, the body is said to be translucent. When light fails to pass through a body at all, the body is said to be opaque. In this latter case, the light passes by the extremities or outline of the object, and a shadow is erected.
Objects may also be seen by means of reflected light. When rays of light fall on a smooth, opaque body, which is polished, they are reflected at the same angle at which they strike the surface (Fig. 62). These reflected rays form an image. When the image is quite distinct, the surface is called a mirror. When the surface is rough the rays are not reflected regularly, but at different angles (Fig. 63). This action is called diffused reflection. Diffused reflection throws the rays of light in all directions and assists, therefore, in illumination.
Fig. 62. - Regular Reflection of Light.
Fig. 63. - Irregular Reflection of Light.