Year 10 Science Unit 3 · Waves and Motion Lesson 6 of 20 45 min SC5-WAM-01

Light Waves and Properties

Light is all around us — it lets us see the world, warms the Earth and carries information across the universe. But what exactly is light? In this lesson you will discover that light is a transverse electromagnetic wave, and you will explore how light behaves when it meets different materials: reflecting off mirrors, bending through water, being absorbed by dark surfaces and scattering through the atmosphere.

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Think First

Before You Begin

Think about these everyday observations involving light.

Write down your answers before reading on:

  • Why does a pencil look bent when you place it in a glass of water?
  • Why can you see your face in a mirror but not in a sheet of paper?
  • Why is the sky blue during the day but red at sunset?
Write your thinking in your book before reading on.

Work mode: Digital — answers typed below

Learning Intentions

Know

  • That light is a transverse electromagnetic wave
  • The four main properties of light: absorption, reflection, refraction and scattering
  • How to draw simple ray diagrams for reflection and refraction

Understand

  • Why light bends when it passes from one medium to another
  • How different surfaces interact with light in different ways
  • That the speed of light changes depending on the medium it travels through

Can Do

  • Draw simple ray diagrams showing reflection and refraction
  • Predict how light will behave at a boundary between two materials
  • Design a practical investigation to observe light properties
Key Terms
Transverse wave A wave in which the oscillations are perpendicular to the direction of energy transfer.
Electromagnetic wave A wave made of oscillating electric and magnetic fields that can travel through a vacuum.
Reflection When light bounces off a surface, such as a mirror, without passing through it.
Refraction The bending of light as it passes from one medium to another, caused by a change in speed.
Absorption When light energy is taken in by a material and converted to other forms, such as heat.
Scattering When light is deflected in many directions by particles in a medium, such as air molecules.
Ray diagram A simple diagram using straight lines with arrows to show the path that light takes.
Core Content
1

Light as a Transverse Electromagnetic Wave

Energy carried by electric and magnetic fields

Wave Properties

Wave Properties

Light is a transverse electromagnetic wave. This means two important things. First, it is transverse: the oscillations of the electric and magnetic fields are perpendicular to the direction the wave travels. Second, it is electromagnetic: it does not need a medium and can travel through the vacuum of space at approximately 300 000 km/s.

Unlike sound, which is a longitudinal mechanical wave, light can travel from the Sun to Earth across 150 million kilometres of empty space. When light travels through transparent materials such as glass or water, it slows down slightly, but it still behaves as a transverse wave.

Remember Light is a transverse electromagnetic wave. It can travel through a vacuum and its oscillations are perpendicular to its direction of travel.

Comparing light and sound waves

PropertyLight waveSound wave
TypeElectromagneticMechanical
Transverse or longitudinalTransverseLongitudinal
Need a mediumNoYes
Speed in air~300 000 km/s~340 m/s
Can travel through glassYesOnly if very thin
2

Properties of Light

Absorption, reflection, refraction and scattering

When light encounters a material, it can behave in four main ways:

  1. Absorption: The material takes in the light energy and converts it to heat. Dark-coloured surfaces absorb more light than light-coloured surfaces. This is why black asphalt feels hot on a sunny day — it absorbs most of the sunlight that hits it.
  2. Reflection: The light bounces off the surface. Smooth, shiny surfaces such as mirrors reflect light in a predictable direction. Rough surfaces also reflect light, but in many directions — this is called diffuse reflection, and it is why you can see objects that are not shiny.
  3. Refraction: Light bends as it passes from one medium to another, such as from air into water or from air into glass. This happens because light changes speed when it enters a different material. The bending makes objects appear distorted or displaced — like the "bent" pencil in a glass of water.
  4. Scattering: Light is deflected in many directions by small particles in a medium. In Earth's atmosphere, nitrogen and oxygen molecules scatter shorter-wavelength blue light more than longer-wavelength red light. This is why the sky appears blue during the day and why sunsets are red (the light travels through more atmosphere, scattering most of the blue light away).
Important Refraction occurs because light changes speed when it moves from one medium to another. It slows down in denser materials like water and glass, causing the light ray to bend toward the normal.
Mirror surface Normal Incident ray Reflected ray REFLECTION Water / Glass Normal Air Bends toward normal REFRACTION
Simple ray diagrams showing reflection (left) and refraction (right). In reflection, the angle of incidence equals the angle of reflection. In refraction, light bends toward the normal when entering a denser medium.
3

Practical Investigation

Observing light behaviour

You can investigate the properties of light using simple equipment in the classroom or at home:

Investigating reflection: Place a plane mirror on a sheet of paper. Shine a ray of light from a ray box or torch at the mirror. Trace the incident ray and the reflected ray. Measure the angle between each ray and the normal (an imaginary line perpendicular to the mirror surface). You should find that the angle of incidence equals the angle of reflection.

Investigating refraction: Place a rectangular glass block on a sheet of paper. Shine a ray of light through the block. Trace the path of the ray entering and leaving the block. You will see the ray bend toward the normal as it enters the glass, and bend away from the normal as it leaves.

Investigating absorption: Place pieces of different coloured paper (black, white, red, blue) in sunlight for five minutes. Use a thermometer or simply feel the surfaces. Dark surfaces become warmer because they absorb more light energy. White surfaces stay cooler because they reflect most of the light.

Safety Never look directly into a laser or bright light source. When using a ray box, avoid shining light into anyone's eyes. Wear safety glasses if required by your teacher.

Common Misconceptions

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"Light always travels in straight lines, so it cannot bend." No — light travels in straight lines in a uniform medium, but it bends (refracts) when it passes from one medium to another because its speed changes. This is why a swimming pool looks shallower than it really is.

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"Reflection only happens in mirrors." No — all objects reflect light. If they did not, you would not be able to see them. Mirrors produce clear images because their surfaces are smooth, causing regular reflection. Rough surfaces produce diffuse reflection, which is why most objects do not produce clear images.

✗trong>"The colour of an object is painted onto it." No — the colour we see depends on which wavelengths of light the object reflects and which it absorbs. A red shirt looks red because it reflects red light and absorbs most other colours.

Australian Context

Light in Australia

The Great Barrier Reef: Light penetration in seawater is critical for coral survival. As sunlight enters the ocean, water absorbs red and orange light first, leaving mainly blue-green light at depth. This is why corals that live deeper in the reef tend to be less colourful — there is less red light available to reflect back.

Bushfire smoke and scattering: During severe bushfire seasons, such as the 2019–20 Black Summer, smoke particles in the atmosphere cause increased scattering of light. This can turn the sky an eerie orange or red colour because the smoke scatters shorter blue wavelengths and allows longer red wavelengths to pass through.

Indigenous astronomical knowledge: Aboriginal and Torres Strait Islander Peoples have sophisticated knowledge of light and observation, using starlight for navigation, seasonal calendars and ceremony. Understanding that light travels from distant sources and can be observed and interpreted is foundational to both traditional and modern astronomy.

✍ Copy Into Your Books

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Light as a Transverse EM Wave

  • Light is a transverse electromagnetic wave
  • It can travel through a vacuum at ~300 000 km/s
  • Oscillations are perpendicular to direction of travel

Four Properties of Light

  • Absorption: light energy taken in and converted to heat
  • Reflection: light bounces off a surface
  • Refraction: light bends when changing medium
  • Scattering: light deflected in many directions by particles

Reflection and Refraction Rules

  • Angle of incidence = angle of reflection
  • Light bends toward the normal when entering a denser medium
  • Light bends away from the normal when leaving a denser medium
Activities
Activity 1

Light Property Detective

For each scenario, identify which property of light is being demonstrated: absorption, reflection, refraction or scattering.

1 A straw in a glass of water appears to bend at the water's surface.
Answer in your book.
2 During a bushfire, the sky over Sydney turns an orange-red colour.
Answer in your book.
3 A black car parked in the sun becomes much hotter than a white car.
Answer in your book.
Activity 2

Ray Diagram Practice

Describe what happens to the light ray in each situation. Use the terms normal, angle of incidence, angle of reflection and bend toward/away from the normal.

1 A light ray strikes a plane mirror at 30 degrees to the normal. Describe the path of the reflected ray.
Answer in your book.
2 A light ray travels from air into a glass block. Explain why the ray bends and in which direction.
Answer in your book.
3 A light ray inside a glass block exits into air. Describe how the ray's path changes and explain why.
Answer in your book.
Multiple Choice
Q

Test Your Understanding

UnderstandBand 3

1. Which statement best describes light?

AA longitudinal mechanical wave that requires a medium
BA transverse electromagnetic wave that can travel through a vacuum
CA transverse mechanical wave that needs air to travel
DA longitudinal electromagnetic wave that only travels through solids
UnderstandBand 3

2. What happens to light when it passes from air into water?

AIt speeds up and bends away from the normal
BIt slows down and bends away from the normal
CIt slows down and bends toward the normal
DIt maintains the same speed and does not bend
ApplyBand 4

3. A student shines a torch at a mirror. The light ray hits the mirror at 40 degrees to the normal. At what angle to the normal does the reflected ray leave the mirror?

A40 degrees
B50 degrees
C90 degrees
D20 degrees
ApplyBand 4

4. Why does the sky appear blue during the day?

AThe ocean reflects blue light into the sky
BThe Sun only emits blue light during the day
CClouds absorb all other colours and reflect blue
DAir molecules scatter shorter blue wavelengths more than longer red wavelengths
AnalyseBand 5

5. A student claims that when light passes from air into glass, it bends because the glass "pulls" the light toward it. Which statement BEST evaluates this claim?

AThe claim is correct because glass has a magnetic attraction to light
BThe claim is incorrect because bending is caused by light slowing down in the denser medium, not by a pulling force
CThe claim is correct because glass is denser than air and therefore attracts all waves
DThe claim is incorrect because light does not bend at all when entering glass
Short Answer
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Short Answer Questions

UnderstandBand 3

1. Explain why a pencil appears to bend when placed in a glass of water. In your answer, include the terms refraction, medium and speed. 4 MARKS

Answer in your book.
ApplyBand 4

2. Compare absorption and reflection. Include in your answer: (i) what happens to the light energy in each case, and (ii) one everyday example of each. 4 MARKS

Answer in your book with reasoning.
AnalyseBand 5

3. During severe bushfires in Australia, the sky can appear orange or red. Using your knowledge of scattering, explain why this happens and why the effect is stronger during bushfire seasons. 4 MARKS

Answer in your book.

Revisit Your Thinking

Go back to your Think First answer. Has your understanding changed?

  • Can you now explain why the pencil looks bent using the term refraction?
  • Can you explain mirror reflection and sky colour using the properties of light?
Update your thinking in your book.

Answers

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MCQ 1

B — Light is a transverse electromagnetic wave. This means its oscillations are perpendicular to the direction of travel, and it does not need a medium — it can travel through a vacuum.

MCQ 2

C — When light passes from air into water, it slows down because water is denser. This causes the light ray to bend toward the normal.

MCQ 3

A — According to the law of reflection, the angle of incidence equals the angle of reflection. Both angles are measured relative to the normal, so the reflected ray leaves at 40 degrees to the normal.

MCQ 4

D — Air molecules in the atmosphere scatter shorter-wavelength blue light more effectively than longer-wavelength red light. This scattered blue light reaches our eyes from all directions, making the sky appear blue.

MCQ 5

B — The claim is incorrect because it uses an incorrect mechanism. Light bends (refracts) because its speed changes when entering a different medium, not because of a "pulling" force. Glass is denser than air, so light slows down and changes direction.

Short Answer 1

Model answer: The pencil appears to bend because of refraction. When light travels from the pencil through the water and into the air, it passes from one medium (water) to another (air). Light travels more slowly in water than in air, causing the light rays to bend at the boundary. Our brain assumes light travels in straight lines, so it interprets the bent rays as coming from a different position, making the pencil look bent at the water's surface.

Short Answer 2

Model answer: During absorption, the material takes in light energy and converts it to heat. For example, a black road surface absorbs sunlight and becomes hot. During reflection, light bounces off a surface without being absorbed. For example, a mirror reflects light in a predictable direction, allowing us to see a clear image. White surfaces reflect most light, while dark surfaces absorb most light.

Short Answer 3

Model answer: During bushfires, smoke particles enter the atmosphere and cause increased scattering of light. Normally, air molecules scatter shorter blue wavelengths more than red wavelengths, making the sky blue. During bushfires, the larger smoke particles scatter and absorb more of the shorter blue and green wavelengths, allowing the longer red and orange wavelengths to pass through and dominate what we see. The effect is stronger during bushfire seasons because there are far more particles in the air than usual, increasing the scattering and absorption of shorter wavelengths.

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Boss Battle

Test your knowledge in a rapid-fire quiz battle. Defeat the boss by answering questions correctly!

Mark lesson as complete

Tick when you have finished all activities and checked your answers.

Unit 3 overview · Science subject page