Objects that emit light on their own are called luminous objects. The light emitted by luminous objects enables us to see things around us. Examples of luminous objects are a tubelight, the sun, a lit candle, a glowing bulb, a bonfire and a lit torch.
Objects that do not emit light on their own are called non-luminous objects. The light emitted by luminous objects falls on non-luminous objects, and then bounces back to our eyes, which enables us to see non-luminous objects. Examples of non-luminous objects are the moon, a book, a pen, a wooden box, a cupboard and a chair.
Light travels in a straight line, and its rays represent the path of light. The material that light passes through is called a medium.
Objects through which we cannot see anything are called opaque objects. A medium that does not allow light to pass through it is called an opaque medium. Examples of opaque objects are a pencil box, a wooden screen, a book, a towel, a ceramic plate and chart paper. Most objects in our surroundings, like buildings and trees, are opaque objects.
Objects through which we can see clearly are called transparent objects. A medium that allows all the light incident on it to pass through it is called a transparent medium. Examples of transparent objects are a plain glass plate, a clear plastic scale, windowpanes, a soap bubble, and pure water.
Objects through which we cannot see the objects on the other side clearly but can see some light are called translucent objects. A medium that allows only a part of the light incident on it to pass through it is called a translucent medium. Examples of translucent objects are ground glass, frosted glass, smoked glass, sun glasses and butter paper.