10.4 Energy: Light & Sound
Key Takeaways
Luminous objects such as the Sun, a candle flame and a firefly give off their own light; non-luminous objects such as the Moon and a mirror are seen only because they reflect light.
Light travels in straight lines, so an opaque object blocks it and forms a shadow; transparent materials let most light through and translucent materials let some through.
Reflection is light bouncing off a surface, while refraction is light bending as it passes from one transparent material into another, which makes a pencil look bent in water.
Sound is made by vibrations and needs a medium to travel; it moves through solids, liquids and gases but not through empty space.
Pitch depends on how fast an object vibrates and loudness depends on the size of the vibrations; prolonged loud noise can damage hearing.
10.4 Energy: Light & Sound
Light and Sound is a Grade 6 Science unit in the National Standards Curriculum (Term 1, Unit 2, theme Energy, Forces and Matter). Students are expected to investigate the properties of light and sound, explain everyday effects such as reflection and refraction, and discuss why loud sounds can be harmful. Light and sound are both forms of energy that travel from a source to our eyes and ears.
Luminous and Non-Luminous Objects
| Type | Meaning | Examples |
|---|---|---|
| Luminous | Gives off its own light | The Sun and other stars, a candle flame, a light bulb that is switched on, a firefly (peenie wallie) |
| Non-luminous | Does not make light; we see it only when light bounces off it into our eyes | The Moon, a mirror, a book, a bicycle reflector, people |
The Moon looks bright at night only because it reflects sunlight. In a completely dark room you cannot see a non-luminous object at all, because no light is reaching it to be reflected.
How Light Travels
- Light travels in straight lines, which scientists draw as rays with arrows.
- Light travels extremely fast, about 300,000 kilometres every second, and it can travel through empty space; that is how sunlight reaches Earth.
- Evidence for straight-line travel: sunbeams through a gap in a zinc fence are straight, and you cannot see a torch beam around a corner.
Transparent, Translucent and Opaque Materials
| Material type | How much light passes through | What you see through it | Examples |
|---|---|---|---|
| Transparent | Almost all | Clear images | Clean window glass, clear plastic, clean water |
| Translucent | Some | Blurry shapes and light | Frosted glass, wax paper, tracing paper, thin cotton |
| Opaque | None | Nothing | Wood, metal, cardboard, brick, your hand |
Shiny and dull surfaces: smooth, shiny surfaces such as mirrors and aluminium foil reflect most of the light that hits them in a regular way, so they can form images. Dull, rough surfaces such as cloth or unpainted wood scatter light in many directions or absorb more of it.
Shadows
A shadow forms when an opaque object blocks light travelling in straight lines, leaving a dark area behind it.
- A shadow has the same outline as the object but no colour or detail.
- Moving an object closer to the light source makes its shadow bigger; moving it away makes the shadow smaller.
- Outdoors, shadows are long in the early morning and late afternoon, when the Sun is low, and short around midday, when the Sun is high.
Reflection and Mirrors
Reflection happens when light bounces off a surface. On a smooth mirror, light bounces off at the same angle at which it arrived (the angle of incidence equals the angle of reflection). The image in a flat (plane) mirror is:
- the same size as the object;
- upright;
- the same distance behind the mirror as the object is in front;
- laterally inverted, meaning left and right are swapped. That is why the word AMBULANCE is printed back-to-front on the front of an ambulance: drivers read it correctly in their rear-view mirrors.
Everyday uses of reflection include rear-view and side mirrors on vehicles, periscopes, bicycle reflectors and the reflective strips on school-crossing vests.
Refraction and Lenses
Refraction is the bending of light as it passes from one transparent material into another, such as from air into water, because light changes speed.
- A pencil standing in a glass of water looks bent at the water's surface.
- A swimming pool or river looks shallower than it really is, which is why it is dangerous to dive into unfamiliar water.
- Lenses use refraction. A convex lens (thicker in the middle) brings light together and can magnify; it is used in magnifying glasses, cameras, microscopes and some spectacles. A concave lens (thinner in the middle) spreads light out and is used in spectacles for short-sighted people.
- A rainbow forms when sunlight is refracted and split into its colours by raindrops: red, orange, yellow, green, blue, indigo and violet.
Sound Is Made by Vibrations
Every sound comes from something vibrating, or moving quickly back and forth: a guitar string, a drum skin, the vocal cords in your throat, a speaker cone. The vibrations pass from particle to particle through a medium.
- Sound travels through solids, liquids and gases. It travels fastest through solids, which is why you can hear a distant train by putting your ear to the rail (never try this on a real track).
- Sound cannot travel through a vacuum (empty space), because there are no particles to carry the vibrations.
- Sound is much slower than light: about 340 metres per second in air. During a storm you see the lightning before you hear the thunder.
Pitch and Loudness
| Property | What it means | What changes it |
|---|---|---|
| Pitch | How high or low a sound is | How fast the object vibrates. Shorter, tighter or thinner strings vibrate faster and give a higher pitch; longer, looser or thicker strings give a lower pitch |
| Loudness | How loud or soft a sound is | The size (strength) of the vibrations. Hitting a drum harder makes bigger vibrations and a louder sound. Loudness is measured in decibels (dB) |
Pleasant Sounds, Unpleasant Sounds and Noise Pollution
Whether a sound seems pleasant or unpleasant depends on its loudness, pitch, regularity and the situation: soft music can be relaxing, while a sudden screech or a sound system at full volume at 2 a.m. is unpleasant. Noise pollution is unwanted or harmful sound in the environment.
- Common sources in Jamaica: loud sound systems at night, traffic and horns, construction machinery, aircraft near airports, and factory equipment.
- Why loud sound is harmful: long exposure to very loud sound can permanently damage the inner ear and cause hearing loss. Noise also disturbs sleep, raises stress, and makes it hard for students to concentrate.
- Reducing noise: lower the volume, keep distance from loud sources, wear ear protection around machinery, use sound-absorbing materials such as curtains and foam, plant trees as sound barriers, and respect laws such as Jamaica's Noise Abatement Act.
Investigating Light and Sound Fairly
In a fair test, change only one variable and keep everything else the same. To test which material blocks light best, use the same torch, the same distance and the same size of each material, and change only the material. To test whether string length affects pitch, pluck strings of different lengths that are equally tight and equally thick. Write a hypothesis first ("A shorter string will give a higher pitch"), record results in a table, and use the data, not guesses, to reach a conclusion.
Which of these objects is non-luminous?
A burning candle
A firefly
The Moon
The Sun
A straight pencil standing in a glass of water looks bent at the water's surface. Which property of light explains this?
Reflection of light off the curved glass surface
Refraction as light passes between water and air
Absorption of light by the wooden pencil
A shadow forming behind the glass of water
A student tightens a guitar string and plucks it with the same force as before. What happens to the sound?
It becomes louder because the string vibrates more slowly
It stops because tight strings cannot vibrate
It becomes lower in pitch because the string is longer
It becomes higher in pitch because the string vibrates faster
Sections you finish are checked off in the contents.