Sources of LightEvery object is in one way or the other a source of light.
The substance through which light passes is called an optical medium.
Depending upon their behaviour towards light, a medium can be classified as transparent, translucent or opaque.
Ray and Beam of LightThe path along which the light travels is shown by drawing a straight line outward from the source with an arrow mark. This line is called a light ray. The direction of the arrow represents the direction of light travel.
It is impossible to obtain a single ray of light from a source. A group of adjacent light rays obtained from a source is called a beam of light.
Virtual ImageThe image formed is virtual if the rays do not actually meet, but appear to meet when produced backwards. Since the rays do not actually meet, we cannot obtain the image on the screen. Virtual image is always erect.