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, 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 eye, 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.
Objects through which we cannot see are called opaque objects. A medium that does not allow light to pass through it is called an opaque medium. Examples of opaque medium are a pencil box, a wooden screen, a book, a towel, a ceramic plate and chart paper.
If we are able to see anything clearly through an object, then such an object is said to be a transparent object. A medium that allows all the light incident on it to pass through it is called a transparent medium. Examples of transparent objects are plain glass, a reading glass, a plastic scale, window panes, a soap bubble and pure water.
Objects that allow only a part of the light incident on them to pass through it are called translucent media. Examples of translucent media are a shower stall, smoked glass, sun glasses and butter paper. Light travels in a straight line, and its rays represent the path of light.
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