Question

Non luminous objects are those which don’t emit their own ………………. ?

Answer 1

Hint: We can see any object only if the light either gets reflected from it and falls on our eye or it may emit radiation on its own and then fall on our eye. By the word luminous we mean that it is visible to us. For an object to get visible there must be some radiation coming from it to our eye.

Complete step-by-step answer:
According to kelvin’s statement, every object emits their own radiation if it’s temperature is greater than 273K. But if it were true, then every object should be visible even in the dark! But we can’t see anything in the dark unless it’s a bulb, candle. So what’s wrong here? Is Kelvin’s statement incorrect? Well, it’s not so. Every object if not at a temperature of 0K, will emit radiation.
But it can only be seen if the wavelength of that radiation lies in the visible range of the electromagnetic spectrum.
For example, if we take a pen in a completely dark room, we can’t see it. On the other hand, if we take a red hot rod, it will be visible as red. Further, if we heat up the rod more, it will turn blue. Again further if we heat it, it will again get invisible, again because it’s radiation’s wavelength will cross the limit of the visible range’s wavelength.
Hence we can say, A non-luminous object is the one which either doesn’t emit their own radiation or whose radiation’s wavelength is not in the range of the visible region.

Note: Any object is visible to us only when the light reflected from it enters our eye or light emitted by it comes into our eye. Hence luminous objects are those which emit their radiation in the visible range’s limit. For example Sun, stars, torch, candle, red-hot rod, etc. Whereas non-luminous objects are those, which can’t reflect their own light, so reflect the one coming towards it from a luminous object. For example plastic, paper, rock, etc.