Light is an energy which can make visible an object. In a dark room we cannot see anything in the room even through keep our eyes open. But when a bulb or any source of light is switched on in the room, then all the objects are visible. As, light is an energy, it follows the conservation law of energy. i.e. light can neither be created nor be destroyed. We can only convert the light into another form of energy and vice versa. Sun is the main Light source in the world.
When light start from a point, then it is known as spotlight source. But if the light source has a geometrical shape, then it is known as area light source. Light travel in a straight line. If we place an opaque substance, then light cannot travel on it and create two types of blackish shadow just opposite side of the substance, first one is slightly darker, which is known as umbra and the shadow is called the penumbra.
We can see a lamp, because light came from this lamp to our eyes. Its came thought the air. Here air is the medium of light.
Optical Medium: The transparent medium is known as an optical medium by which light can travel from one location to another.
Homogeneous Medium: The optical medium is known as a homogeneous medium by which light can travel same speed in every direction. E.g. glass, air, water.
Heterogeneous Medium: The optical medium is known as a heterogeneous medium by which light can travel in different speed in different direction.
Opaque Medium: The optical medium is known as an opaque medium by which light cannot travel from one location to another. E.g. any metal body.
Semi-transparent medium: The optical medium is known as a semi-transparent medium by which light can travel partly from one location to another. E.g. scratch glass, oil paper, wax paper.