Light and Sound

In Grade 6 you saw that light reflects — it bounces off a surface. There is a precise rule for this: the angle the light arrives at equals the angle it leaves at, both measured from the normal (an imaginary line at right angles to the surface).

But light does something else too. When light passes from one transparent material into another — say from air into water or glass — it bends. This bending is called refraction.

Light bends because it changes speed. It travels fastest in air, and more slowly in water or glass:

• entering a denser material (air to glass), light slows down and bends towards the normal,
• leaving a denser material (glass to air), light speeds up and bends away from the normal.

Refraction is why a straw in a glass of water looks bent, and why a swimming pool looks shallower than it really is.

A lens is a shaped piece of glass or plastic that uses refraction to bend light in a useful way.

A convex lens (also called a converging lens) is thicker in the middle and thinner at the edges. As parallel rays of light pass through it, they all bend inwards and meet at a single point called the focal point.

Because a convex lens gathers light to a point, it can form an image — a picture of an object made of focused light. The lens in a camera, a magnifying glass and your own eye all work this way.

A concave lens does the opposite: it is thinner in the middle and spreads light rays apart.

The most important lens of all is the one inside you. The eye uses a convex lens to focus light, as you will see next.

Your eye is a remarkable natural camera. Light passes through several parts before you can see.

Light travels through the eye like this:

• the cornea — the clear front layer — does most of the bending of the light,
• the pupil — the black hole in the middle — lets light in; it gets smaller in bright light,
• the lens — a flexible convex lens — fine-tunes the focus,
• the retina — the light-sensitive layer at the back — is where the image forms.

The retina turns the image into electrical signals, which travel along the optic nerve to your brain. The eye's lens can change shape to focus on near or far objects — a clever trick called accommodation.

In Grade 6 you learned sound is made by vibrations. Now describe it properly: sound travels as a wave.

A wave carries energy from place to place without carrying the material along with it. When a loudspeaker vibrates, it pushes the air particles back and forth. They bump into their neighbours, passing the vibration along — but each particle just wobbles in place.

Every wave has three key properties:

• Frequency — how many complete vibrations happen each second, measured in hertz (Hz). Frequency sets the pitch: high frequency means a high-pitched sound.
• Amplitude — the size of the vibration, shown as the height of the wave. Amplitude sets the loudness: large amplitude means a loud sound.
• Wavelength — the distance from one point on the wave to the same point on the next.

So pitch and loudness, which you met in Grade 6, are really just the frequency and amplitude of a wave.

Your ear is built to capture sound waves and turn them into something your brain can read.

Here is the journey of a sound:

• the outer ear acts like a funnel, collecting sound waves and directing them inwards,
• the waves travel down the ear canal to the eardrum — a thin, tight skin,
• the sound waves make the eardrum vibrate,
• tiny bones in the middle ear pass the vibration on and make it stronger,
• the vibration reaches the cochlea — a coiled, fluid-filled part — which turns it into electrical signals,
• the signals travel along the auditory nerve to the brain, which understands them as sound.

Every step is a kind of energy transfer: sound energy in the wave becomes movement energy in the eardrum and bones, then electrical energy in the nerve.

This is why very loud sounds can be harmful. A very large amplitude makes the eardrum and the delicate parts of the cochlea vibrate violently, which can damage them. Protecting your hearing means keeping loud sounds short and not too close.

Your ears cannot detect every sound. Each animal has a range of hearing — the lowest and highest frequencies it can pick up.

A healthy young human can usually hear frequencies from about 20 Hz (a very low rumble) up to about 20 000 Hz (a very high whine). Sounds outside this range are still real — your ears just cannot detect them.

• Sound with a frequency above 20 000 Hz is called ultrasound.
• Sound with a frequency below 20 Hz is called infrasound.

Many animals hear differently from us. Dogs can hear higher frequencies, which is why a 'silent' dog whistle works — it makes ultrasound that the dog hears but you cannot. Bats and dolphins use ultrasound to find their way and hunt, sending out high-frequency sounds and listening for the echoes.

Humans use ultrasound too. Ultrasound scans send high-frequency sound into the body; the echoes are used to build an image, for example of a baby before it is born. Ultrasound is also used to check for cracks deep inside metal parts.

As people get older, the top of their hearing range usually drops, so they can no longer hear the very highest frequencies.

Treating light and sound as waves prepares you for a lot of later science:

• Waves in detail — you will study wave speed, the wave equation and the electromagnetic spectrum.
• Optics — lenses lead into telescopes, microscopes, cameras and correcting eyesight.
• Sound technology — microphones, speakers and ultrasound scanners all build on these ideas.
• The senses — biology builds on how the eye and ear detect light and sound.
• Communication — radio, light signals and fibre optics all rely on waves.

Whenever a topic about seeing or hearing feels tricky later, return to two ideas: light bends when it changes speed and sound is a wave with frequency and amplitude. Take your time — these wave ideas explain a huge amount of physics.