What Is Light?

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:
  • Reflection of light
  • Refraction of light

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.

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

Example 1: Luminous vs non-luminousCore

Classify each as luminous or non-luminous: a candle flame, the Moon, a book.

Show Answer

Candle flame: luminous.
Moon: non-luminous (reflects sunlight).
Book: non-luminous (reflects light).

Example 2: Seeing an objectCore

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

Show Answer

In complete darkness there is no light to reflect from the cat into your eyes, so no light enters your eyes from the cat and you cannot see it.

Example 3: Ray model statementCore

Write one sentence that states the ray model for light in a uniform medium.

Show Answer

In a uniform medium, light travels in straight lines and a ray shows the direction of travel.

Example 4: Speed of light (fact + unit)Core

State the speed of light in vacuum, and give its unit.

Show Answer

c = 3.0 × 10⁸ m s⁻¹

Example 5: Ray vs beamCore

State the difference between a ray and a beam.

Show Answer

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

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

G3 Physics / O-Level objective chains

K323 (2027) and 6091 have the same eleven Light outcomes, so learners keep this one indexed reflection → refraction → total internal reflection → converging-lens sequence. Current K323 objectives, activity versions, browser-storage namespaces and evidence gates remain separate from historical O-Level records; no mastery is projected between qualifications.

• Reflection and refraction practice check · static plan and re-test
• TIR and optical-fibre practice check · static plan and re-test
• Lens and ray-diagram practice check · static plan and re-test

Draw rays from a luminous source and from a lit non-luminous object into an observer’s eye. Continue to reflection of light.