Properties of Light and Sound
What Is This?
Light and sound are both forms of energy that travel as waves, but they behave very differently: light travels as electromagnetic waves and can move through a vacuum; sound travels as mechanical waves and requires a medium (matter) to move through. Understanding their properties explains everyday experiences — why mirrors reflect, why water makes objects look bent, why we hear echoes, and why sound cannot travel in space.
Why Does It Matter?
The properties of light and sound underpin an enormous range of technologies and natural phenomena — from eyeglasses and cameras to musical instruments and medical ultrasound. Understanding these properties builds foundational knowledge for physics, engineering, and the natural sciences, and helps explain many familiar everyday experiences through scientific principles.
Light
How Light Travels
Light travels as electromagnetic waves — it does not need a medium and can travel through the vacuum of space. In a vacuum, light travels at approximately 300,000 kilometres per second — the fastest anything in the universe can travel.
Light typically travels in straight lines (called rays) until it encounters a surface or passes from one medium to another.
Transparent, Translucent, and Opaque Materials
Materials interact with light in different ways:
- Transparent: Light passes through clearly; objects on the other side can be seen distinctly (glass, clean water)
- Translucent: Light passes through partially; objects on the other side appear blurry (frosted glass, wax paper)
- Opaque: Light cannot pass through; a shadow forms on the other side (wood, metal, most rocks)
Reflection
Reflection occurs when light bounces off a surface. The angle at which light hits a surface (the angle of incidence) equals the angle at which it reflects (the angle of reflection) — this is the Law of Reflection.
- Smooth, shiny surfaces (like mirrors) produce regular reflection — clear, distinct images
- Rough surfaces (like a brick wall) produce diffuse reflection — light scatters in many directions; no clear image forms
Refraction
Refraction occurs when light passes from one medium into another (e.g., from air into water or glass) and bends due to the change in speed. Light slows down when it enters a denser medium and bends toward the normal (an imaginary line perpendicular to the surface); it speeds up when entering a less dense medium and bends away.
Everyday examples of refraction:
- A straw appearing bent when placed in a glass of water
- A swimming pool appearing shallower than it really is
- Lenses in glasses, cameras, and microscopes bending light to focus images
Absorption
When light strikes a surface and is neither reflected nor transmitted, it is absorbed — its energy is converted to thermal energy (heat). Dark-colored and matte surfaces absorb more light; light-colored and shiny surfaces reflect more.
The Color of Light
White light (like sunlight) is a mixture of all visible colors. When white light passes through a prism, refraction separates it into the visible spectrum: red, orange, yellow, green, blue, indigo, violet (ROYGBIV).
Objects appear a certain color because they reflect that color of light and absorb all others. A red apple absorbs most colors of light but reflects red wavelengths back to our eyes.
Sound
What Is Sound?
Sound is a form of energy created by vibrations — back-and-forth movements that disturb surrounding particles and create a wave of pressure that travels outward from the source.
Example: When a guitar string is plucked, it vibrates, pushing and pulling air particles around it, creating a wave of compressions (high pressure) and rarefactions (low pressure) that travels through the air to your ears.
Sound Requires a Medium
Unlike light, sound cannot travel through a vacuum — it needs particles to transmit its vibrations. Sound travels through:
- Gases (air) — slowest
- Liquids (water) — faster than gases
- Solids (wood, metal) — fastest
Speed of sound in air: approximately 343 m/s at room temperature — far slower than light (which is why we see lightning before hearing thunder).
Properties of Sound Waves
Frequency and Pitch:
- Frequency is how many vibrations occur per second, measured in Hertz (Hz)
- High-frequency vibrations → high-pitched sounds (a whistle)
- Low-frequency vibrations → low-pitched sounds (a bass drum)
Amplitude and Volume:
- Amplitude is the size of the vibration (how much the particles are displaced)
- Large amplitude → loud sounds
- Small amplitude → quiet sounds
- Sound volume is measured in decibels (dB)
Reflection of Sound: Echoes
When sound waves strike a hard surface, they can reflect back — creating an echo. Echoes are used by animals like bats and dolphins (echolocation) to navigate and locate prey in the dark.
Absorption of Sound
Soft, porous materials (like foam, carpet, and curtains) absorb sound rather than reflecting it, reducing echoes. This is why recording studios use acoustic foam panels.
Comparing Light and Sound
| Property | Light | Sound |
|---|---|---|
| Type of wave | Electromagnetic | Mechanical (pressure wave) |
| Needs a medium? | No — travels through vacuum | Yes — needs matter |
| Speed (approximate) | 300,000 km/s | 343 m/s in air |
| Can be reflected? | Yes | Yes (echoes) |
| Can be refracted? | Yes | Yes (bends in different media) |
Common Mistakes
Mistake 1: Thinking sound travels faster than light
Light travels almost a million times faster than sound. This is why we see lightning before hearing thunder — both happen simultaneously, but the light reaches us almost instantly while the sound takes several seconds.
Mistake 2: Thinking sound can travel through space
Sound requires a medium — in the vacuum of space, there are no particles to carry vibrations. Explosions in space would be completely silent.
Mistake 3: Confusing pitch and volume
Pitch is determined by frequency (how fast something vibrates); volume is determined by amplitude (how big the vibration is). A loud, low-pitched sound has high amplitude and low frequency.
Key Takeaways
- Light travels as electromagnetic waves at 300,000 km/s and needs no medium; sound travels as pressure waves and requires a medium
- Light interactions: transmission (transparent/translucent), reflection, refraction, absorption
- Law of Reflection: angle of incidence equals angle of reflection
- Refraction bends light when it passes between media of different density
- White light separates into ROYGBIV through a prism
- Sound is created by vibrations; higher frequency = higher pitch; greater amplitude = louder sound
- Sound travels fastest through solids, slower through liquids, slowest through gases
- Sound cannot travel through a vacuum (space)
Practice and Resources
Ready to practise? Try our Grade 3-5 Properties of Light and Sound Worksheet with property identification, comparison activities, and real-world reasoning. Test yourself with the Grade 3-5 Science Practice Test.