Light
Light is a form of energy that travels in straight lines at incredible speed, about 300 000 km every second. Light from the Sun takes about 8 minutes to reach Earth.
Sources of light
A light source makes its own light: the Sun, stars, light bulbs, fire and glow sticks. Objects like the Moon, mirrors and your shirt aren’t light sources. We see them because light bounces off them into our eyes.
How light interacts with materials
| Type | What light does | Examples |
|---|---|---|
| Transparent | Passes through almost completely | Clear glass, air, water |
| Translucent | Passes through partly and scatters | Wax paper, frosted glass, tissue |
| Opaque | Is blocked completely | Wood, metal, cardboard |
Opaque objects make shadows.
Reflection
Reflection happens when light bounces off a surface. Smooth, shiny surfaces like mirrors reflect light neatly, so you can see a clear image. Rough surfaces scatter light in many directions.
The angle at which light hits a mirror equals the angle at which it bounces off. That’s why you can use a mirror to see around a corner.
Refraction
Refraction is the bending of light when it passes from one material into another, like from air into water. This is why a straw in a glass of water looks bent or broken.
Lenses use refraction to bend light. Magnifying glasses, eyeglasses, cameras and microscopes all use lenses.
Colour
White light is a mixture of all colours. A prism or raindrops can split white light into a rainbow: red, orange, yellow, green, blue, indigo and violet.
We see colours because objects reflect some colours and absorb others. A red apple reflects red light and absorbs the rest.
Sound
Sound is energy made by vibrations. When something vibrates, it pushes on the particles of air around it, making them vibrate too. The vibrations travel as a wave until they reach your ear.
Sound needs something to travel through
Sound can travel through gases (air), liquids (water) and solids (walls, the ground). It travels:
- fastest through solids
- slower through liquids
- slowest through gases
Sound cannot travel through empty space, because there are no particles to vibrate. Space is silent!
Sound travels about 343 metres per second in air, much slower than light. That’s why you see lightning before you hear thunder.
Pitch
Pitch is how high or low a sound is.
| High pitch | Low pitch |
|---|---|
| Fast vibrations | Slow vibrations |
| Short, thin or tight strings | Long, thick or loose strings |
| Small instruments (flute, violin) | Large instruments (tuba, cello) |
| A whistle, a bird | A foghorn, a big drum |
Volume
Volume (loudness) depends on how much energy the vibrations have. Bigger vibrations make louder sounds. Loudness is measured in decibels (dB). Sounds above about 85 dB, heard for a long time, can damage your hearing.
| Sound | About |
|---|---|
| Whisper | 30 dB |
| Normal talking | 60 dB |
| Lawn mower | 90 dB |
| Rock concert | 110 dB |
Echoes
An echo is a sound that reflects off a hard surface and comes back. Bats and dolphins use echoes to find food, and ships use sonar to measure the depth of the ocean.
Common mistakes
- Thinking we see because light comes from our eyes. Light enters our eyes.
- Thinking sound travels through space.
- Confusing pitch and volume.
Try it yourself
- Put a pencil in a glass of water. Draw what you see from the side. Explain it.
- Fill glass bottles with different amounts of water. Blow across the tops. Which makes the highest pitch?
- Hold a ruler on a desk with part hanging off. Twang it, then change the length. What happens to the pitch?
Quick recap
- Light travels in straight lines and very fast.
- Materials can be transparent, translucent or opaque.
- Reflection bounces light; refraction bends it.
- Sound is made by vibrations and needs a material to travel through.
- Pitch depends on how fast something vibrates; volume depends on energy.