← Ray Model of Light

Lesson 2 of 8 · Textbook 5.1–5.2

How we see things

You see something only when light from it enters your eyes. Everything in this lesson comes from taking that sentence literally.

Objects that make their own light

Definition Objects that are able to emit light on their own are called luminous objects, or sources of light.

Luminous objects emit light in all directions. The light then travels in straight lines from the object to our eyes, which is how we see it.

Natural sourcesMan-made sources
The sun · the northern lights · the Milky Way · fireflies A lamp · a candle · a mobile phone screen · a light bulb
Where the sun's light comes from The core of the sun reaches about 15 million degrees Celsius. It produces its enormous heat and light through nuclear fusion, in which hydrogen nuclei combine to form helium nuclei.

Objects that do not

Most objects around you emit no light of their own — they are non-luminous. You can see them only in the presence of a light source, because light from that source bounces off them and enters your eyes.

Definition The bouncing of light off a surface is called reflection. Light travels in straight lines even when it bounces off a surface.

So seeing a flower takes two straight lines: one from the sun to the flower, and a second from the flower to your eye. Miss either and you see nothing.

A totally dark room

candle white ball black ball eye
The candle—
The white ball—
The black ball—

Blow the candle out and both balls disappear. This is the Learning Point on page 11, and “you can see the white one” is the wrong answer.

The moon is not a light source

The moon is a non-luminous object. We see it because its surface reflects light from the sun into our eyes. The moon completes one orbit of Earth in about 27 days, and during each orbit it is lit from different angles by the sun — which is why the intensity of moonlight varies.

Two straight lines, every time For a luminous object: light travels from the object → your eye. One line.
For a non-luminous object: light travels from a source → the object → your eye. Two lines.

If a question asks you to explain how someone sees a flower, a book or the moon, draw both lines and label the source. Drawing only the second one loses the mark.

Check yourself