Light and Sound

In Grade 6 you saw light bounce off a mirror — that was reflection. But light can also bend when it travels into a new material, such as from air into water or glass. This bending is called refraction.

Have you ever noticed a straw in a glass of water looks bent or broken at the surface? Or that a swimming pool looks shallower than it really is? Those are refraction at work.

Light bends because it changes speed when it enters a new material. Light travels slower in glass and water than in air. As it slows down at an angle, its path bends.

There is a neat rule. The normal is an imaginary line at right angles to the surface. When light slows down entering a denser material, it bends towards the normal. When it speeds up leaving, it bends away from the normal.

Lenses in glasses, cameras and telescopes all work by refracting light in a controlled way.

White light looks plain and colourless — but it is actually a mixture of every colour. You can prove this by passing white light through a triangular block of glass called a prism.

The prism refracts the light, but here is the clever part: each colour bends by a slightly different amount. Red bends least, violet bends most. So the colours spread apart and you see them separately. This band of separated colours is called a spectrum.

The order of the spectrum is always the same: red, orange, yellow, green, blue, indigo, violet. A rainbow is a giant natural spectrum — tiny raindrops act like millions of little prisms, splitting sunlight into its colours.

If white light is a mix of all colours, why does a red apple look red and a leaf look green?

The answer is that objects absorb some colours and reflect others. The colour you see is the colour that bounces off the object into your eyes.

• A red apple reflects red light and absorbs all the other colours.
• A green leaf reflects green light and absorbs the rest.
• A white object reflects all the colours equally.
• A black object absorbs all the colours and reflects almost none — which is why black things look dark and heat up in sunlight.

This idea also explains a surprising trick. An object can only reflect a colour that is actually shining on it. If you shine pure red light on a green leaf, the leaf has no green light to reflect and no red light to absorb cleanly — so it looks dark, almost black. Take away the right colour of light and the object's usual colour disappears.

So colour is not really 'in' the object on its own — it depends on both the object and the light shining on it.

In Grade 6 you learned that sound is a vibration travelling through a material. Now think about how fast it travels.

Sound in air moves at about $340\ \text{m/s}$ — fast, but far slower than light. That difference is easy to notice in a thunderstorm: you see the lightning instantly, but the thunder arrives seconds later. The light reaches you almost at once; the sound takes time to travel.

Just as light reflects off a mirror, sound reflects off a hard surface. A reflected sound you hear again is an echo.

Because sound has a known speed, echoes are genuinely useful. Ships send a sound pulse down to the seabed and time the echo to measure the water's depth. Bats use echoes to find insects in the dark.

In Grade 6 you learned that frequency is how fast something vibrates, and it sets the pitch of a sound. Frequency is measured in hertz ($Hz$) — the number of vibrations each second.

Human ears cannot hear every frequency. We can only detect sounds roughly between 20 Hz and 20 000 Hz:

• below $20\ Hz$ the pitch is too low for us to hear — this is infrasound,
• above $20\,000\ Hz$ the pitch is too high for us — this is ultrasound.

Other animals are different. Dogs hear higher frequencies than we can, which is why a 'silent' dog whistle works — it makes ultrasound. Bats and dolphins also use ultrasound. Our hearing range also tends to shrink as we get older, especially at the high-frequency end.

The Stage 8 ideas in this topic set you up well for what comes next:

• Lenses and optical instruments — refraction explains how glasses, cameras and microscopes form images.
• The electromagnetic spectrum — visible colour is just one part of a much wider family of waves.
• Waves in detail — you will study how all waves reflect, refract and carry energy.
• The eye and ear — biology builds on how we detect colour and the range of sounds.
• Ultrasound technology — medical scans and sonar all use sound beyond human hearing.

Whenever a light question feels tricky later, ask two things: is the light bouncing (reflection) or bending (refraction)? And for sound, remember it is far slower than light. Take your time — these ideas reach into a surprising amount of physics ahead.