Light: reflection and shadows

Learning objectives

By the end of this topic you should be able to:

• describe how light travels and explain how we see objects
• explain how shadows form and classify materials
• apply the law of reflection and describe mirror images
• explain everyday uses of reflection and scattering
• analyse shadow and reflection data and plan fair tests
• discuss road safety and building design

Before you start

You will use these skills. If any feel shaky, review them first.

• energy transfers (see Energy stores and transfers)
• measuring angles with a protractor
• reading tables and patterns

Key vocabulary

Luminous

Giving out its own light, like the Sun or a lamp.

Transparent / translucent / opaque

Lets light through clearly / partly and scattered / not at all.

Shadow

A dark area where light is blocked by an opaque object.

Normal

A line at 90° to a surface where a ray hits it.

Angle of incidence / reflection

The angle between the incoming / reflected ray and the normal.

Scattering (diffuse reflection)

Light reflecting in many directions from a rough surface.

Understanding the ideas

1. Light travels in straight lines

   Light comes from luminous sources such as the Sun and lamps. It travels in straight lines, very fast (about 300 000 km every second). We see non-luminous objects because light reflects off them into our eyes.

2. Shadows

   Opaque objects block light, forming shadows with shapes set by the straight paths of light. Transparent materials let light through clearly; translucent ones let some light through but scatter it.

3. Reflection

   When light hits a smooth mirror, it reflects so that the angle of incidence equals the angle of reflection. Mirror images appear as far behind the mirror as the object is in front and are reversed left to right. Rough surfaces scatter light, so they do not form images.

4. Why does it matter?

   Reflection is used in periscopes, road mirrors, reflective safety vests and car headlights, but strong reflections from glass buildings can dazzle drivers and harm birds.

5. What does it connect to?

   This topic links to energy, colour, sound and waves in physics, and to the eye and vision in biology.

Reflection rules

• Angles are measured from the normal, not from the mirror.
• Angle of incidence = angle of reflection.
• A plane mirror image is the same size, the same distance behind the mirror as the object is in front, and reversed left to right.
• Smooth surfaces give regular reflection (images); rough surfaces scatter light (no images).

Light in the real world

Mirrors at sharp bends on mountain roads help drivers see oncoming traffic. Reflective strips on vehicles and vests make people visible at night. Periscopes and fibre-optic cables use reflection to guide light.

Worked examples

Assessment tips

Light questions often use ray diagrams and simple data. Expect to:

• Draw and read ray diagrams, measuring angles from the normal.
• Explain shadows, seeing and reflection using straight-line light.
• Process data and spot anomalies.
• Plan fair tests with light sources and sensors.

Common mistakes: measuring angles from the mirror instead of the normal; thinking light comes out of our eyes; saying the Moon is luminous; and saying rough surfaces do not reflect light.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.