8.2 Light, Shadows & Sound
Key Takeaways
- Light travels in straight lines, so a shadow forms where an opaque object blocks the light
- Moving an object closer to the light source makes its shadow bigger and fuzzier; a larger light source makes shadows fuzzier too
- We see non-luminous objects only because they reflect light into our eyes
- Sound is produced by vibrations and needs a medium — it cannot travel through a vacuum
- Pitch is set by vibration frequency (faster = higher); volume is set by vibration size (bigger = louder)
8.2 Light, Shadows & Sound
Light and sound are the two kinds of energy you sense every second of the day, and ICAS tests both from the earliest papers onwards. Shadows appear on the Introductory paper, the production and uses of sound feature on Papers A and B (Years 3–4), and the senior papers stretch the same ideas into reflection, the electromagnetic spectrum and radio frequencies. This section builds the ideas in that order.
Light Travels in Straight Lines
Light moves in straight lines called rays. This single fact explains shadows, pinhole images and why you cannot see around corners. Sources that produce their own light — the Sun, a torch, a candle — are called luminous; everything else you see is non-luminous and visible only because it reflects light.
How Shadows Form — and How They Change
A shadow is the region where an opaque object blocks light from reaching a surface. Because light travels in straight lines, the shadow is a dark silhouette behind the object, on the side away from the light source.
- Distance: move the object closer to the light source and it blocks more of the spreading rays, so the shadow gets bigger; move it closer to the screen or wall and the shadow gets smaller and sharper.
- Sharpness: a small, point-like light source gives a sharp-edged shadow. A large light source sends rays from many points at once, so each point's shadow overlaps the others and the edge becomes fuzzy. That is why the shadow under a fluorescent ceiling light is softer than the shadow from a single small torch.
Reflection and How We See
When light hits a surface, some of it bounces off — this is reflection. A smooth surface like a mirror reflects rays in an even, predictable way, producing a clear image; a rough surface scatters rays in all directions. You see a book because light from the Sun or a lamp strikes the book and reflects into your eyes — the book itself produces no light. The Moon is the classic exam example: it shines only by reflecting sunlight.
Transparent, Translucent and Opaque
| Type | What light does | Examples |
|---|---|---|
| Transparent | Passes straight through — you can see clearly | Clear glass, clean water, cling wrap |
| Translucent | Partly passes through but is scattered — blurry view | Frosted glass, tissue paper, wax paper |
| Opaque | Blocked completely — casts a shadow | Wood, metal, cardboard, your hand |
Sound Is Produced by Vibrations
Every sound starts with something vibrating — shaking back and forth very fast. Guitar strings, drum skins, speaker cones and your own vocal cords all vibrate to make sound. You can feel this directly: rest your fingers on your throat while you hum, or touch a ringing tuning fork to the surface of water and watch it splash.
Unlike light, sound needs a medium — a solid, liquid or gas whose particles can pass the vibrations along. Sound usually travels fastest through solids, then liquids, then gases. In the vacuum of space there are no particles at all, so sound cannot travel: the tagline "in space, no one can hear you scream" is good physics.
Pitch and Volume
- Pitch — how high or low a note sounds — depends on the frequency of the vibration: how many vibrations happen each second. Faster vibrations (a tightened or shortened guitar string) give a higher pitch; slower vibrations give a lower pitch. Frequency is measured in hertz (Hz).
- Volume (loudness) depends on the size of the vibration — its amplitude. Pluck the string harder and it swings further, pushing more energy into the air, so the sound is louder. Plucking harder does not change the pitch.
Keep these two separate in your mind: tightening a string changes pitch, hitting it harder changes volume. ICAS loves asking which is which.
Light and Sound Compared
| Property | Light | Sound |
|---|---|---|
| What it is | Electromagnetic waves | Vibrations of particles |
| Needs a medium? | No — crosses empty space | Yes — cannot cross a vacuum |
| Speed | About 300,000 km/s | About 343 m/s in air |
| Everyday evidence of speed gap | Lightning seen instantly | Thunder heard seconds later |
The Electromagnetic Spectrum and Radio Waves
Light is just one small part of the electromagnetic (EM) spectrum — the family of waves that also includes, in order of increasing wavelength, gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. Radio waves have the longest wavelengths and lowest frequencies in the family. Radio stations each broadcast on their own frequency — for example a station at "101.1 FM" broadcasts radio waves at 101.1 megahertz (MHz), meaning 101.1 million vibrations per second — and your radio tunes in to one frequency at a time. Because the waves all travel at the speed of light, a longer wavelength always means a lower frequency. Paper D (Year 6) level questions keep this gentle: you mainly need the idea that radio waves are a kind of EM wave like light, and that different stations use different frequency ranges.
On a clear morning in Australia, the Sun is low in the eastern sky. A student notices the shadow of a tall flagpole on the playground. In which direction does the flagpole's shadow point, and why?
Which statement correctly explains how you are able to see the words printed in this study guide?
A guitarist tightens one of her strings and then plucks it harder than before. Compared with the loose, gently plucked string, how will the sound change?