Light & Colour
Discover how light travels, reflects, and refracts — and understand how white light splits into the colours of the rainbow through prisms and water droplets over the Australian sky.
Properties of Light
Light is a form of electromagnetic radiation that travels as waves. It is the fastest thing in the universe — light travels at approximately 300,000 km per second (3 × 10⁸ m/s) in a vacuum. Light from the sun takes about 8 minutes to reach Earth.
Light travels in straight lines
Light rays travel in straight lines (called rectilinear propagation). This is why shadows are formed and why you cannot see around corners without a mirror.
Luminous vs Non-luminous
Luminous objects produce their own light (sun, stars, fire, torches). Non-luminous objects are seen by reflected light (the moon, this page, a kangaroo).
Australian Curriculum Connection
This lesson aligns with AC9S7U04: "Light and sound are forms of energy; light can be reflected, refracted and absorbed; white light can be split into its component colours."
Reflection of Light
Reflection occurs when light bounces off a surface. The Law of Reflection states that the angle of incidence (incoming ray) always equals the angle of reflection (outgoing ray), both measured from the normal (a line perpendicular to the surface).
Law of Reflection
Specular (Regular) Reflection
Occurs on smooth, flat surfaces (mirrors, calm water). Creates a clear, sharp image. Like seeing your reflection in a still lake in the Australian outback.
Diffuse (Scattered) Reflection
Occurs on rough surfaces. Light reflects in many directions — no clear image. This is how you see most objects around you (paper, fabric, rocks).
Refraction and the Visible Spectrum
Refraction is the bending of light as it passes from one medium to another (e.g. air to glass or water). This occurs because light changes speed in different media. The speed of light in glass or water is slower than in air, causing the ray to bend.
White Light Dispersed by a Prism
Remember the colours with: Roy G Biv (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
Dispersion
When white light enters a prism or water droplet, different colours (wavelengths) refract by slightly different amounts. Violet light bends the most; red light bends the least. This separates white light into its spectrum.
Rainbows in Australia
Rainbows form when sunlight enters water droplets (rain), is refracted, internally reflected, and refracted again — separating into colours. The sun must be behind you and rain in front. Common after tropical storms in Queensland.
Key Vocabulary
Reflection
The bouncing of light off a surface. The angle of incidence equals the angle of reflection (Law of Reflection).
Refraction
The bending of light as it passes from one medium to another due to a change in speed.
Dispersion
The separation of white light into its component colours (spectrum) when it passes through a prism or water droplet.
Visible Spectrum
The range of colours that human eyes can detect: Red, Orange, Yellow, Green, Blue, Indigo, Violet (Roy G Biv).
Worked Examples
A light ray strikes a mirror at an angle of 35° to the normal. What is the angle of reflection?
Law of Reflection: angle of incidence = angle of reflection.
Angle of incidence = 35° (measured from the normal).
Answer: The angle of reflection = 35°.
Why does a straw in a glass of water appear bent?
Reason: Light travels at different speeds in air and water.
When light passes from water to air (at the water's surface), it changes speed and refracts (bends).
Result: Your eye receives the refracted rays and traces them back in a straight line, making the underwater part of the straw appear to be in a different position — the straw looks bent or broken at the water's surface. This is refraction.
Why is a red T-shirt red?
White light from the sun (or lights) contains all colours of the visible spectrum.
The dye in the T-shirt absorbs all colours except red.
Red light is reflected back to your eyes.
Answer: The T-shirt appears red because it reflects red light and absorbs all other colours of the spectrum. An object's colour is determined by which wavelengths of light it reflects.
Knowledge Check
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Key Concepts Summary
- ✓ Light travels at 300,000 km/s in a vacuum and travels in straight lines.
- ✓ Law of Reflection: angle of incidence = angle of reflection (both measured from the normal).
- ✓ Refraction: light bends when it changes speed moving from one medium to another (e.g. air to water).
- ✓ White light is made up of all colours of the visible spectrum (Roy G Biv); a prism disperses it into its colours.
- ✓ The colour of an object is determined by which wavelengths of light it reflects to our eyes.