Light and Sound

Have you noticed a beam of sunlight streaming through a gap in the curtains, or the straight beam of a torch in a dark room? Those beams show you something important: light travels in straight lines.

We draw the path of light as a ray — a straight line with an arrow showing the direction the light is moving.

Light comes from a light source. Some objects make their own light — we call them luminous. The Sun, a flame, a lamp and a glowing screen are all luminous.

Light travels extremely fast — so fast that when you switch on a lamp, the room looks lit up instantly. It is the fastest thing there is.

Because light goes in straight lines, it cannot bend around corners by itself. That single fact explains how we see things, and why shadows form — as you will see next.

Most things around you — a desk, a book, your hand — do not make their own light. They are non-luminous. So how do you see them?

Light from a source hits the object, bounces off it, and travels into your eye. This bouncing of light is called reflection. You see an object when reflected light from it reaches your eyes.

When light hits a smooth, shiny surface like a mirror, it reflects in a neat, predictable way.

A mirror reflects light so evenly that you see a clear image. A rough surface, like paper, scatters the light in all directions — that is why you do not see your reflection in a sheet of paper.

Because light travels in straight lines, it cannot bend around objects. So when something solid gets in the way, light is blocked — and a shadow forms.

A shadow needs three things: a light source, an object to block the light, and a screen or surface for the shadow to fall on.

The shadow is the same basic shape as the object, because the object blocks a straight slice of light.

You can change a shadow:

• Move the object closer to the light and the shadow gets bigger.
• Move the object further from the light and the shadow gets smaller.

Objects that block light completely are opaque. Objects that let light through, like clear glass, are transparent and do not make a strong shadow.

Touch your throat gently and hum. Feel that buzzing? That is a vibration — a fast back-and-forth movement. Every sound is made by something vibrating.

Here are some examples:

• A guitar string vibrates when you pluck it.
• A drum skin vibrates when you hit it.
• Your vocal cords vibrate when you speak.
• A loudspeaker has a cone that vibrates in and out.

When the vibrating gets stronger, the sound is louder. When the vibrating stops, the sound stops. You can test this: touch a ringing bell or a struck cymbal and the vibration — and the sound — stops the moment you hold it still.

So if you ever wonder what is making a sound, look for the part that is vibrating.

A vibration in one place would be useless if it could not reach you. Luckily, the vibration passes on to the material around it — usually the air — and travels outwards as a sound wave.

Sound can travel through solids, liquids and gases. It often travels even better through solids than through air — that is why you can hear footsteps through a floor.

But sound cannot travel through empty space (a vacuum). With no material to carry the vibration, there is nothing to pass it on. Space is silent.

When a sound wave reaches your ear, it makes your eardrum vibrate. Tiny parts inside your ear pass that vibration on, and your brain understands it as a sound.

This is a useful reminder: light can travel through empty space, but sound cannot. That is why we can see the Sun but never hear it.

Two things about a sound can change: how loud it is and how high or low it is.

Loudness (volume) depends on the size of the vibration, called the amplitude.

• A big vibration makes a loud sound.
• A small vibration makes a quiet sound.

Pitch is how high or low a sound is. It depends on how fast something vibrates, called the frequency.

• Fast vibrations make a high-pitched sound, like a whistle.
• Slow vibrations make a low-pitched sound, like a big drum.

A handy way to keep them straight: amplitude is the size of the wave, frequency is how often it repeats. Loudness goes with amplitude; pitch goes with frequency.

Understanding light and sound now sets you up well for what comes next:

• Energy — light and sound are both forms of energy that travel and transfer.
• Waves — you will study sound waves and light in much more detail.
• The eye and ear — biology builds on how we detect light and sound.
• Lenses and colour — later you will explore how light bends and how we see colour.
• Music and technology — instruments, speakers and microphones all use these ideas.

Whenever a topic about seeing or hearing feels tricky later, come back to two facts: light travels in straight lines and sound is a vibration that needs a material. Take your time — these simple ideas explain a surprising amount.