The simplest case is when the object you see is also the source of the light.
We are able to see objects when light from them enters the eye. In this case, light is given out by the object, which is referred to as a luminous source. The Sun is the most obvious example of a luminous source of light, along with car headlamps, torches, candle flames and so on.
Note that in the study of light we use familiar words in a specialist way. Thus the
thing that is being looked at is referred to generally as the object.
In this second case, the object itself does not give out light. Here light from a separate luminous source is reflected from the object and this reflected light is picked up by the detector. In this case the object is referred to as a non-luminous source. All of the objects (the table, chair, curtain, floor etc.) which we are able to see around us, and which are not themselves giving out light, are non-luminous sources.
These days it is not so easy to experience situations where there is no light whatsoever. Perhaps the most obvious place to achieve such a
light-free condition is underground. Of course, if you find yourself venturing into places where there isn't much light (this might be a cave underground or the cupboard under the stairs), you'd be well advised to take a portable, luminous source (a torch!) with you. Using the torch you will be able to detect objects.
It is quite tempting to think of this very familiar event in terms of the torch light
just lighting up the space, so that you can see. Indeed in day-to-day talk, people often refer to events in this way:
Light flooded the room when she switched the lamp on.
So does the torch
Just light up the space? In fact, when searching for the missing trainer in the cupboard under the stairs, you are directing light from the torch onto the various objects in the cupboard so that your eyes can detect the reflected rays:
Ah…There it is! The idea that we can see things if the space is lit up (with no reference to light entering the eye), is quite common and we discuss this further in teaching and learning challenges.