What is light?
Light is a form of energy that lets us see. It travels from a source, bounces off objects, and enters our eyes. Without light, there would be no sight, no colour, and no photosynthesis to grow food.
Sources of light
| Luminous (makes its own light) | Non-luminous (reflects light) |
|---|---|
| The Sun and other stars | The Moon |
| Light bulbs, candles, fire | Planets |
| Fireflies, some jellyfish | Mirrors, books, people |
| Lightning, TV screens | Almost everything around you |
How light is produced
| Type | How | Example |
|---|---|---|
| Incandescence | Something gets so hot it glows | The Sun, old light bulbs, fire, hot metal |
| Fluorescence | A material absorbs invisible UV and glows | Fluorescent tubes, highlighter ink |
| Phosphorescence | Glows after the light source is removed | Glow-in-the-dark stickers |
| Bioluminescence | Living things make light through chemical reactions | Fireflies, anglerfish |
| Electroluminescence | Electricity passes through a material | LEDs |
| Chemiluminescence | A chemical reaction gives off light | Glow sticks |
Light travels in straight lines
Light travels in straight lines, which we draw as rays. That’s why:
- you can’t see around corners
- shadows have sharp edges when the light source is small
- sunbeams through clouds look like straight bars
Shadows
A shadow forms when an opaque object blocks light.
- Shadows are bigger when the object is closer to the light.
- Outdoor shadows are long in the morning and evening (Sun low) and short at noon (Sun high).
- During a solar eclipse, the Moon’s shadow falls on Earth. During a lunar eclipse, Earth’s shadow falls on the Moon.
How materials affect light
| Type | Light | Examples |
|---|---|---|
| Transparent | Passes straight through; you see clearly | Clear glass, clean water, air |
| Translucent | Some passes through but scatters; blurry view | Frosted glass, wax paper, tissue paper |
| Opaque | Blocked completely | Wood, metal, cardboard, your hand |
When light hits an object, it can be:
- reflected (bounced back)
- absorbed (taken in, often becoming heat)
- transmitted (passed through)
- refracted (bent as it passes into a new material)
The speed of light
Light is the fastest thing in the universe: about 300 000 km per second in a vacuum.
| Light travelling from… | Time to reach Earth |
|---|---|
| The Moon | About 1.3 seconds |
| The Sun | About 8 minutes 20 seconds |
| Proxima Centauri (nearest star) | About 4.2 years |
Light could circle Earth about 7.5 times in one second.
How we see
We see objects when light from them enters our eyes.
- Light comes from a source, like the Sun or a lamp.
- It reflects off an object.
- The reflected light enters your eye.
- Your eye focuses it onto the retina.
- The retina sends signals to the brain, which forms the image.
Ancient Greek thinkers believed eyes sent out rays to "touch" objects. In about 1000 CE, the Arab scientist Ibn al-Haytham showed that light travels into the eye, not out.
White light and colour
White light is a mix of all the colours of the rainbow: red, orange, yellow, green, blue, indigo and violet. A prism splits white light into these colours.
We see an object’s colour because it reflects that colour and absorbs the others. A red apple reflects red light. A black object absorbs nearly all colours. A white object reflects them all.
Light safety
- Never look directly at the Sun, even during an eclipse, without certified eclipse glasses.
- Never shine laser pointers at people’s eyes or at aircraft.
- Wear sunglasses that block UV light.
Common mistakes
- Thinking the Moon gives off its own light.
- Thinking we see because light leaves our eyes.
- Thinking shadows are made of darkness. They are areas where light is blocked.
Try it yourself
- Test objects with a flashlight. Sort them into transparent, translucent and opaque.
- Make a shadow puppet show. How can you make shadows bigger or smaller?
- Use a prism or a glass of water in sunlight to make a rainbow.
Quick recap
- Light is energy that lets us see.
- Luminous objects make light; non-luminous objects reflect it.
- Light travels in straight lines at about 300 000 km/s.
- Materials are transparent, translucent or opaque.
- We see objects when reflected light enters our eyes.