### Seeing things - Physics narrative

Reflection
Light, Sound and Waves

## Seeing things - Physics narrative

Physics Narrative for 5-11

A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.

It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).

### Light travelling in straight lines

Visible Light
Light, Sound and Waves

## Light travelling in straight lines

Physics Narrative for 5-11

## Linear paths

Not only does light travel, it travels in straight lines through a given medium.

Various first-hand sources of evidence point to light's linear path.

Further evidence is provided by shadows. If some light from an object (usually a luminous object) is blocked by something, then that something causes a shadow: a region where there's less light.

The shape of the something doing the blocking determines the shape of the shadow.

## Scattering

Some particles found in the atmosphere have the ability to scatter beams of light.

The incident beams of light are scattered in all directions. In some cases the scattering results from relatively large particles in the air (such as dust particles). In other cases the scattering is thought to be due to interactions between the incident light and molecules in the air.

## Absorption

Some particles found in the atmosphere have the ability to absorb beams of light.

The incident beams of light stop, or become dimmer and the particles move more.

### Reflection of light

Reflection
Light, Sound and Waves

## Reflection of light

Physics Narrative for 5-11

## Reflection from surfaces

Light beams are reflected from surfaces according to the law of reflection, which states that the angle of incidence is equal to the angle of reflection.

Here an explanation of the law builds up, step by step. (Remember that rays are figments of your imagination.)

The angles of incidence (i) and reflection (r) are measured between the incident (incoming) and reflected (outgoing) rays and the normal line. The normal line is a construction line drawn perpendicular to the reflecting surface at the point where the incident ray strikes.

## Predicting what will happen using ray diagrams

This ray diagram shows the formation of a shadow by an opaque thing. This might represent what happens when a narrow-beam torch forms the shadow of a book on a wall. This model represents the actual event in a number of ways:

• The light source is represented as a single point.
• Just two rays are shown, as straight lines leaving the source.
• The direction of travel of each ray is shown by an arrow.
• The book is represented as an opaque barrier.
• The position of the shadow on the wall is located between the points where the two rays meet the screen.

When thinking about ray diagrams, it is important that you remember that they are a model: they predict what will happen, but do not show a photo-realistic imitation of the phenomenon.

In the case of this shadow, you might be able to make the following predictions:

• What will happen to the size of the shadow if the torch is moved closer to the opaque barrier? (it gets bigger on the screen).
• What will happen to the size of the shadow if the screen is moved away from the opaque barrier? (it gets bigger on the screen).

### Not just mirrors

Reflection
Light, Sound and Waves

## Not just mirrors

Physics Narrative for 5-11

## Reflection from any surface

The law of reflection applies to the reflection of light at any surface, not just the shiny ones that you might usually associate with reflection.

For example, each point on a stone wall reflects light such that the angle of incidence equals the angle of reflection. Here you need to picture a tiny piece of the wall's surface, which acts as a flat reflector. Having to hand a piece of rock containing mica flakes, which are shiny, might help to make the link to the reflection of light from a duller rock.

Reflection from such a rough surface is sometimes referred to as diffuse reflection. It's what enables us to see many of the objects in our environment.

## Paint and reflections

Surfaces are made to be reflective in different ways to create different moods.

DIY shops sell different kinds of paint which are designed to provide contrasting effects not only in terms of colour but also in the ways in which light is reflected from the surface. The two extremes of finish are gloss and matt, with satin in between.

For gloss paint the particles at the surface of the paint are very small and when the paint dries they end up forming what is in effect a very flat, plane surface which acts just like a mirror. Any light shining onto this surface is reflected regularly and it may be possible to see the image of an object (your face!) as light is reflected.

At a microscopic level the surface of matt paint, when it dries, is very uneven. When magnified it resembles a pebbled beach. Here the light from a luminous source will hit each pebble and whilst each small part of each pebble will reflect light according to the laws of reflection, the overall effect is that the light is scattered in all directions. Our eyes gather light from parts of many different pebbles as diffuse reflection occurs, making the matt finish appear dull in comparison with the gloss surface.