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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).
Using your eyes
Imagine the scene. You are sitting in your living room at home concentrating on the latest episode of your favourite TV soap. Out of the corner of your eye you are aware that the cat has just come into the room from the kitchen. The dull glow from the fire tells you that more coal is needed. Just then, the security light bursts into life outside and the shadow of a figure is thrown up against the curtains. At last! you think, the pizza has arrived!
In day-to-day living we pick up huge amounts of information through our eyes: most of the time the sheer quantity is too much and we selectively attend to some things while ignoring others.
So, how are we able to use our eyes to detect all of these things? You probably have some pretty clear ideas about this question, but let's start by considering the act of seeing as one end of a chain linking a source of illumination to a detector (your eyes).
Seeing with light
Seeing luminous things
The simplest case is when the object you see is also the source of the light.
We are able to see objects when light from them enters the eye. In this case, light is given out by the object, which is referred to as a luminous source. The Sun is the most obvious example of a luminous source of light, along with car headlamps, torches, candle flames and so on.
Note that in the study of light we use familiar words in a specialist way. Thus the
thing that is being looked at is referred to generally as the object.
Seeing non-luminous things
In this second case, the object itself does not give out light. Here light from a separate luminous source is reflected from the object and this reflected light is picked up by the detector. In this case the object is referred to as a non-luminous source. All of the objects (the table, chair, curtain, floor etc.) which we are able to see around us, and which are not themselves giving out light, are non-luminous sources.
Experiencing no light
These days it is not so easy to experience situations where there is no light whatsoever. Perhaps the most obvious place to achieve such a
light-free condition is underground. Of course, if you find yourself venturing into places where there isn't much light (this might be a cave underground or the cupboard under the stairs), you'd be well advised to take a portable, luminous source (a torch!) with you. Using the torch you will be able to detect objects.
It is quite tempting to think of this very familiar event in terms of the torch light
just lighting up the space, so that you can see. Indeed in day-to-day talk, people often refer to events in this way:
Light flooded the room when she switched the lamp on.
So does the torch
just light up the space? In fact, when searching for the missing trainer in the cupboard under the stairs, you are directing light from the torch onto the various objects in the cupboard so that your eyes can detect the reflected rays:
Ah… There it is! The idea that we can see things if the space is lit up (with no reference to light entering the eye), is quite common and we discuss this further in teaching and learning challenges.
The idea that light travels is not uncommon in the 21st century. Pupils will often refer to things moving
at the speed of light when they are talking about things moving very quickly. In the simple models set out in the previous section, the light travels from a luminous source to the eye, the light travels from the torch to the object and then to the eye.
Teacher: Just how fast does light travel?
The answer is 300 million metres in each second or 3 × 108 metre / second .
To be precise, what we usually call the speed of light is really the speed of light in a vacuum. In reality, the speed of light depends on the material (often called a medium) that it moves through. Light moves more slowly in water and glass than in air, and in all cases the speed is less than in a vacuum.
Here are a few values of the speed of light in different media for your interest: vacuum, 299 792 000 metre second-1 ; air, 299 703 000 metre second-1 ; water, 225 408 000 metre second-1 ; glass, 199 862 000 metre second-1.
We'll return to this slowing down of light as it passes into different substances (such as water or glass), when we consider the refraction of light in episode 03.
The fact that light travels so quickly means that all of those day-to-day events involving light (such as light being reflected from the face of your watch and travelling to your eyes) appear to happen instantaneously, and of course to all intents and purposes they do. These experiences can undermine the essential idea that light necessarily involves movement.
Light travelling in straight lines
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.
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.
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.
How far can light travel?
The fact that we can see the Sun and stars shows that light can travel over enormous distances (150 million kilometres from the Sun). In fact there is no known limit to how far light can travel. However, as you will be aware from observing torch beams or car headlights, there is a limit to the distance over which these are effective sources of illumination.
There are two reasons for this.
First, as light spreads from a bulb the level of illumination it provides is reduced (this is why a given source of light appears dimmer as you get further away from it).
Second, some of the light from the bulb will be scattered and absorbed by particles in the air and this further reduces the level of illumination it provides at any given distance.
Light meeting obstacles
What happens when light travels through the air and meets the surface of an object, such as a table, mirror, window, or curtain? Various outcomes are possible.
Notice that all of the energy of the incident beam can only be shared out amongst the three possibilities: energy is conserved.
If the object is:
- Opaque: no light is transmitted (the light is reflected and absorbed to varying degrees).
- Translucent: some light is transmitted (the rest of the light is reflected and absorbed to varying degrees).
- Transparent: ideally all light is transmitted (although in practice some light will be reflected and absorbed).
Notice that all the energy must be accounted for – energy is conserved.
Reflection of light
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 the 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 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 wiill 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.
Diffuse reflection of light
A smooth reflection
Imagine the scene. You are standing at the edge of a large lake, gazing out across the water at a range of mountains which rears up into the sky. You can see the mountain peaks directly and you are also aware that there is a perfect reflection of the mountain from the water. So you imagine two rays from a pair of selected points – one point near the top of the mountain, and one near the bottom.
To model the direct view of the mountain you imagine rays drawn straight from those points to your eye. To model the reflection of the mountain, you imagine rays drawn down to the lake surface from the mountain and being reflected from the flat surface to your eye.
How do these pairs show that the top and bottom will be reversed in the reflection?
The same rays, regrouped by the view to which they contribute
Here we've redrawn the scene, and hope that helps. Now the rays are grouped differently, with the rays coloured to show their origin.
To get further you'll have to model the eye as a pinhole camera (more on that in episode 02), to remember that the eye inverts all images that appear on the retina. You might like to draw out a ray diagram to see if you can figure out what is going on, before moving on.
Tracing rays inside the eye
Here we've simplified the eye (right down to a pinhole eye), taking the rays from the previous diagram. By the time it gets to be as simple as the pinhole camera, so just a box, you can very clearly see the predictions of the ray model of the original view of the mountains.
The green rays, predicting what the light beams from the top of the mountain will do, swap positions with the blue rays (that predict what the light beams from the bottom of the mountain will do) as you move from the reflected to the direct view, and back again.
As they swap position, top for bottom, and bottom for top, so the eye will see the mountain up the right way and upside down. Now all you need to do is find a mountain and a lake on a clear still day and see if the predictions are correct.
A rippled surface
Just then, a sudden squall of wind blows up across the lake, disturbing the water's surface. The reflected image of the mountain disappears as the rays of light are now reflected in all directions. You can use the models you've just developed to make sense of this.
For each ray the angle of incidence is equal to the angle of reflection, but the incident rays strike different regions of water, which are inclined at different angles to each other. The outgoing rays are reflected at many different angles and so the image is jumbled up.