Lesson focus: Light transfers energy through space and reaches our eyes either directly from a source or after interacting with matter.
Use rayA straight line with an arrow, drawn to show the path light takes. models to explain vision and distinguish transmission, reflection, absorptionWhen a material takes in light or sound energy instead of letting it bounce off or pass through. and scattering.
Build on earlier learning
Investigating the speed of soundSound required a material medium and travelled comparatively slowly. Light provides a transverse wave that can cross a vacuum at far greater speed.
You will be successful when you can…
- Distinguish luminous and non-luminous objects.
- Draw ray arrows in the direction light travels.
- Classify transparent, translucent and opaque materials.
- Compare light and sound propagation through a vacuum.
Words that carry the science
- luminous
- Emitting light.
- ray
- A straight line with an arrow used to model the direction of light travel.
- transparent
- Transmitting light with little scattering so objects can be seen clearly through it.
- translucent
- Transmitting some light but scattering it so clear images are not formed.
- opaque
- Transmitting essentially no light; incident light is reflected, absorbed or both.
- absorption
- Transfer of light energy to a material, often increasing internal energy.
Track light from source to eye
01Sources emit; objects become visible by reflection
A luminousGives out its own light, like the Sun or a torch. object emits light. A non-luminous object becomes visible when light from a source reflects from it and then enters an eye.
Ray diagrams use straight lines in a uniform medium and arrows showing the direction of travel. rayA straight line with an arrow, drawn to show the path light takes. must point source → object → eye, not outward from the eye.
02Materials divide incident light between outcomes
A transparentLets light straight through so you can see clearly through it, like clear glass. material transmits light with little scattering. A translucentLets some light through but you cannot see clearly through it, like frosted glass. material transmits but scatters light. An opaqueDoes not let any light through, like wood or metal. material transmits essentially none.
Real surfaces can reflect and absorb different fractions at the same time. “Opaque” does not mean all light is absorbed; a mirror is opaque and reflects strongly.
03Light can cross empty space
Light is an electromagnetic transverse wave and does not require a material medium. In a vacuum it travels at about 3.0 × 10⁸ m/s.
Sunlight takes roughly 8 minutes 20 seconds to reach Earth. We can see astronomical events through space, while sound from them cannot cross the vacuum directly.
Light travels in straight lines: shadows
Light travels in straight lines. An opaque object blocks it and makes a shadow. Translucent materials let some light through; transparent ones let almost all through. Move the object closer to the lamp and the shadow grows.
Controls
Shadow height = object height × (lamp to screen) ÷ (lamp to object), from similar triangles. Play to see the light moving.
Draw how a person sees a book
A lamp illuminates a book. Describe the minimum ray path that explains vision.
- Draw a ray from the luminousGives out its own light, like the Sun or a torch. lamp to the book.
- Add an arrow pointing from lamp to book.
- Draw a reflected ray from book to eye.
- Add an arrow toward the eye and label the book non-luminous.
How to make the reasoning visible: Light emitted by the lamp reflects from the book and enters the eye.
Classify interactions with materials
A material can show more than one interaction. Classify by what happens to the transmitted light.
| Material | Transmission evidence | Other interaction | Classification |
|---|---|---|---|
| Clear window | Clear image visible | Some reflection | Transparent |
| Frosted glass | Light passes; image blurred | Strong scattering | Translucent |
| Black card | No light detected behind | Mostly absorption | Opaque |
| Mirror | No light transmitted | Strong reflection | Opaque |
Why are both black card and a mirror opaque although they look different?
Explain why a clear window can still produce a reflection.
Check your understanding
Answer from memory, then check against the suggested answer.
What is a luminous object?
What do arrows on rays show?
Can light travel through a vacuum?
Explain why the Moon is visible but is not normally a luminous object.
A material transmits 60% of incident light, reflects 25% and absorbs 15%. Explain why one-word labels are useful but incomplete.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student sees a red book under a white lamp.
Explain how the student sees the book.
(3)Explain why the book cannot be seen in a perfectly dark room.
(1)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., write the light path in order and keep ray directions explicit.