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What Is Light?

Key idea: O Level light basics: the ray model, luminous vs non-luminous objects, and key ideas before reflection and refraction.

  • SEC G3 Physics 2027
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Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

1. Definition

A. Light (O Level model)

For O Level optics, light is treated as an electromagnetic wave and described using the ray model:

  • A ray shows the direction light travels.
  • In a uniform medium, light travels in straight lines.

2. Key Ideas

  • Light is an electromagnetic wave (it can travel through vacuum).
  • You see an object only if light enters your eyes.
  • Luminous objects emit their own light (e.g. Sun, lamp).
  • Non-luminous objects are seen because they reflect light from a luminous source (e.g. book, Moon).
  • In vacuum, light travels at c = 3.0 × 10⁸ m s⁻¹.
  • Reflection and refraction are covered in:
Where this lesson leads

Light study continues through reflection, refraction, total internal reflection and thin converging lenses. This lesson supplies the ray model and basic terms used throughout.

3. Detailed Explanations

A. How we see objects (luminous vs non-luminous)

  • A luminous object produces light that travels directly to your eyes.
  • A non-luminous object does not produce light; it reflects light from a luminous source into your eyes.
Luminous and non-luminous light pathsPanel A shows light from a luminous source entering the eye. Panel B shows light from a source reflecting off a non-luminous object into the eye.A: Luminous objectSource emits lightB: Non-luminous objectReflected light reaches eye
Luminous objects are seen directly; non-luminous objects are seen by reflected light.

B. Rays and beams

  • A ray is drawn as a straight line with an arrow showing the direction of travel.
  • A beam is a group of rays (it can be parallel, converging or diverging).

Ray diagrams help you predict where light will go after reflection or refraction.

C. Evidence that light travels very fast

In a thunderstorm, you see lightning before you hear thunder. This is because light travels much faster than sound.

D. Why the ray model works for O Level optics

When the medium is uniform (same material throughout), light travels in straight lines. When light reaches a boundary between materials (e.g. air and glass), it can be:

  • reflected (bounce off)
  • refracted (change direction because speed changes)

4. Common Mistakes

  • Saying you can see in complete darkness (you need light entering your eyes).
  • Mixing up luminous and non-luminous objects.
  • Drawing rays without arrowheads (direction matters in ray diagrams).

5. Exam Tips

  • Use the phrase “light travels in straight lines in a uniform medium”.
  • If asked “how do we see?”, start with: “light from a luminous source enters the eye”.
  • In ray diagrams, always draw:
    • a straight ray,
    • an arrow for direction,
    • a clear point of incidence on the surface.

6. Worked Examples

Modelled example 1

Luminous vs non-luminous

Core

Problem

Classify a candle flame, the Moon and a book as luminous or non-luminous, and give the deciding test.
Study the worked solution

  1. Apply the definition

    Method

    Ask whether each object produces its own visible light.

    Reason

    Luminous objects emit light; non-luminous objects are seen by reflected light.

    Working

    Candle flame: luminous.

  2. Classify reflected-light objects

    Method

    Place the Moon and book in the non-luminous group.

    Reason

    The Moon reflects sunlight and the book reflects incident light.

    Working

    Moon and book: non-luminous.

Guided practice 2

Seeing an object

About 4 min

Problem

Explain why you cannot see a black cat in a completely dark room.

Try this before viewing the solution

Hints

Hint 1: start with what must enter the eye
A non-luminous object needs incident light before it can send reflected light towards an observer.
View solution step by step

  1. Check for incident light

    Method

    State that complete darkness provides no light to illuminate the cat.

    Reason

    The cat is non-luminous.

    Working

    No incident light reaches the cat.

  2. Complete the path

    Method

    Therefore no reflected light from the cat enters the eyes.

    Reason

    Vision requires light from the object to reach the observer.

    Working

    The cat cannot be seen.

Common misconception 3

Ray model statement

Find and correct the mistake

Learner statement

A learner writes, “Light always travels in straight lines.” Locate the overclaim and write the bounded ray-model statement.

Try this before viewing the solution

Missing qualifier

View solution step by step

  1. Bound the claim

    Method

    Add “in a uniform medium”.

    Reason

    Light can change direction at a boundary between media.

    Working

    In a uniform medium, light travels in straight lines.

  2. State the representation

    Method

    A ray is a straight line with an arrow showing travel direction.

    Reason

    The ray is a model of the light path, not a physical filament.

    Working

    Ray arrow → direction of travel.

Examiner practice 4

Speed of light (fact + unit)

2 marks

Examination question

State the speed of light in vacuum and its SI unit. [2 marks]

Try this before viewing the solution

View solution step by step

  1. State the value

    1 mark

    Method

    Give 3.0 × 10⁸.

    Reason

    This is the accepted vacuum speed at this course level.

    Working

    c = 3.0 × 10⁸.

  2. Attach the unit

    1 mark

    Method

    Use metres per second.

    Reason

    Speed is distance divided by time.

    Working

    c = 3.0 × 10⁸ m s⁻¹

Challenge 5

Ray vs beam

Minimal support

Representation transfer

State the difference between a ray and a beam, then describe what several parallel arrowed lines represent.

Try this before viewing the solution

Hints

Hint 1: one path versus a collection
A ray represents one travel direction; a beam describes a group of rays.
View solution step by step

  1. Distinguish the models

    Method

    A ray is one arrowed line; a beam is a group of rays.

    Reason

    The terms refer to different scales of representation.

    Working

    Ray: one direction; beam: collection.

  2. Interpret the diagram

    Method

    Several parallel arrowed lines represent a parallel beam.

    Reason

    The rays share a direction and do not converge or diverge.

    Working

    Parallel rays → parallel beam.

7. Mind Stretchers

Mind stretcher 1: Why the Moon is visibleExtension

The Moon has no light of its own. Why is it bright at night?

Show Answer

It reflects sunlight. The reflected light travels to our eyes, so we can see the Moon.

Mind stretcher 2: Why shadows have sharp edgesExtension

Why can a small point-like light source produce sharp shadows?

Show Answer

Because light travels in straight lines. Rays from a point source either reach a region or are blocked, so the boundary between light and shadow can be sharp.

8. Practice and next step

Draw rays from a luminous source and from a lit non-luminous object into an observer’s eye. Then continue to reflection of light, where the direction of those rays is analysed at a surface.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027