Sources of Light
Simple Explanation
A luminous object produces its own light (like the Sun, a candle flame, or a light bulb). A non-luminous object does not produce light of its own — it is only visible because it reflects light from a luminous source into our eyes. We see any object at all only because light from it (either its own, or reflected) enters our eyes.
Why Do We Need It?
Understanding that most of what we see is REFLECTED light (not self-produced light) is the foundation for everything else in this chapter — reflection, mirrors, and images all depend on this basic idea.
Worked Example
Classify objects as luminous or non-luminous
Classify each as luminous or non-luminous: (a) the Moon, (b) a lit torch, (c) this page of text.
Why Does This Work?
The luminous/non-luminous distinction is really about the ORIGIN of the light reaching your eye — whether it was generated right there at the object, or generated somewhere else and simply bounced (reflected) off the object on its way to you.
Real-Life Example
Why the Moon appears to "shine"
The Moon appears bright in the night sky, leading many people to assume it produces its own light.
In reality, the Moon is non-luminous — every bit of moonlight is actually reflected sunlight, which is why the Moon has different visible "phases" as the fraction of sunlit surface facing Earth changes.
Practice
Which of these is a luminous object?
EasyCommon mistake
Assuming anything bright-looking must be luminous — the Moon and planets look bright but are entirely non-luminous, only reflecting light from the Sun.
Quick Review
- Luminous: produces its own light (Sun, flames, light bulbs).
- Non-luminous: only visible by reflecting light from a luminous source.
- We see anything only because light from it enters our eyes.