PHYSICS · Stage 8 · Chapter 7: Light

Physics 7.1 How do we see things? — Luminous and Non-Luminous Objects and Light Rays

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Physics 7.1 How do we see things?

Part 1 of 6What light is

Light a way of transferring energy.
Luminous objectsLight sources give out their own light, such as the Sun, other stars, a candle flame and a light bulb.
No medium neededUnlike sound, light can travel through empty space, so light from the Sun and stars reaches us.
Infrared radiationSometimes called heat or thermal radiation, it is similar to light and also carries energy from the Sun.

Luminous objects — We see them because the light they emit enters our eyes.
Non-luminous objects — They do not give out light and are seen because light from a source is reflected from them into our eyes.
- Not only mirrors reflect light; all objects reflect at least some light.
Sunlight and moonlight — Sunlight is emitted by the Sun, whereas moonlight is sunlight reflected from the non-luminous Moon.

Demonstration — Cut holes in three cards and line them up between a candle and your eye.
- The flame is only visible when all the holes are exactly in line.
- Shadows and sharp-edged beams also show straight-line travel.
Rays — Light travelling from a source to a detector is drawn as straight rays with arrows.

Transmitted — Light passes through the object.
Reflected — Light is turned back from the surface.
Absorbed — Light stays in the object, neither transmitted nor reflected.
- Energy cannot disappear, so the light energy is transferred to thermal energy.
- The object heats up a little.

Type	What happens	Example
Transparent	Light passes through and you can see clearly	Clear glass
Translucent	Some light passes through, but you cannot see clearly	Frosted glass, white cloth
Opaque	No light passes through; light is absorbed or reflected	Bricks, wood, metal

Spreading — Light spreads out as it moves away from the source, so less light hits a given area further away.
- A source looks dimmer with distance because the light is spread out, not because light runs out.
- Stars look dim because their light has spread over enormous distances.
Looking at the Moon — Light travels from the Sun, reflects off the Moon and enters the eye.
Looking in a mirror — Light from a lamp reflects off you, reflects again from the mirror and then enters the eye, one extra reflection.
Shadows — An opaque object blocks light and forms a shadow because light travels in straight lines.

Last updated: 13 October 2026