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Seeing with light - Physics narrative
- Imagine the scene
- Seeing with light
- Travelling light
- A central role for the speed of light
- Light travelling in straight lines
- How far can light travel?
- Light travelling not so far: the scattering and absorption of light
- There's more to seeing than meets the eye
- The electromagnetic spectrum
- Seeing and lighting
The Electromagnetic Spectrum
Light, Sound and Waves
Seeing with light - Physics narrative
Physics Narrative for 11-14
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).
Core ideas of the Light topic:
- Seeing is all about the source-medium-detector model.
- Rays as predictive tools.
- Reflections - diffuse and specular.
- Refraction and its use.
- Characterising light: brightness and intensity, frequency and spectral colour.
- Seeing colour: perceptual colour, selective absorption and reflection.
- Trip times, ranging and speed.
- Speed, frequency and wavelength.
The ideas outlined within this subtopic include:
- Seeing with light
- Source-medium-detector
- Light travels in straight lines
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 light to a detector (your eyes).
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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.
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Travelling light
Light travels
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.
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A central role for the speed of light
Light in physics
The speed of light plays a central role in astronomy and in physics.
According to Einstein's theory of relativity, no signal can travel faster than light. Furthermore, contrary to normal intuition, the theory of relativity tells us that light always travels at the same speed relative to an observer, no matter how that observer moves relative to the source of the light.
Thus, light emitted from a moving aeroplane does not travel with the speed of light plus the speed of the aeroplane, as recorded by an observer on Earth. It travels with the speed of light, no matter what the speed of the aeroplane. In a vacuum, light always travels at a speed of 3 × 108 metre/second, no matter how its speed is measured. Although this seems strange, it has been confirmed in many experiments. These experiments show that it is our common sense that is wrong in this case.
While these ideas are way beyond the scope of work appropriate for most 11–14 pupils, they are often very interested to hear about them, with the promise that they will find out more when they study advanced level physics.
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Light travelling in straight lines
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.
Shadows
Further evidence is provided by shadows. If some light from an object is blocked by something, then that something causes a shadow: a region where there's less light.
The shape of the something obstructing the light 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.
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How far can light travel?
Reasons for not seeing glowing objects
Wrong Track: The light runs out before it gets to us.
Wrong Track: Light's only got quite a long range: then it stops.
Right Lines: Light keeps on going until it gets absorbed. But there might not be enough light entering your eye for you to see something – although your dog might still be able to see it.
Light doesn't run out or get used up
Thinking about the learning
On the one hand pupils are quite prepared to accept that light can travel 150 million kilometres from the Sun to the Earth and yet at the same time believe that light from their torch beam gets used up
in a matter of a few metres.
The light from the torch becomes progressively more spread out and may also be scattered and absorbed by particles in the air (more in the physics narrative). The light is not lost
, it just becomes more spread out, scattered and absorbed.
This applies to the very bright sunlight, or to the somewhat dimmer torch. Light just does not get lost
.
Thinking about the teaching
Although the spreading and scattering of light are not referred to in all curricula, they are certainly necessary to offer any kind of plausible explanation for the relatively short range of a torch beam. One teaching colleague regales his pupils with the mantra:
Teacher: Light doesn't run out, it spreads out!
Furthermore, the only reason that you can actually see the torch beam cutting through the night air is because light is being scattered to your eye. These are ideas that pupils are ready to accept and we would advise that you use them. See the teaching activity that depends upon the scattering of the intense laser beam by chalk particles in the air.
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Light travelling not so far: the scattering and absorption of light
Light, Sound and Waves
Light travelling not so far: the scattering and absorption of light
Physics Narrative for 11-14
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.
With light absorption, the incident light interacts with matter in the air and energy is transferred from the light to the matter, resulting in an increase in molecular motion.
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There's more to seeing than meets the eye
Sight
You will probably be familiar with the mechanism of seeing. Light enters the eye through the pupil and the cornea and lens act together to create an image on the retina. When light lands on the retina, the light-sensitive cells in the surface produce tiny electrical signals which travel to the brain via the optic nerve.
What then happens within our brain is amazing. The signals from the retinal cells in each eye move rapidly around the brain as we seek to make sense of them. Different parts of the brain process these signals by comparing the combinations of electrical data with existing learned patterns. Different parts of the brain have different functions. For example, some parts focus on recognition of faces, others on trying to interpret written words, others focus on colour or shape recognition.
A simple ray diagram (more details about rays diagrams follow in the next episode) demonstrates a rather surprising feature of the image formed on the retina.
Seeing upside down
The image is upside down! Light from the top of the object passes through the pupil and then appears low down on the retina. Light from the bottom of the object appears high up on the retina. This happens because the rays from the top and the bottom of the object must cross over as they pass through the pupil.
Of course this crossing-over also occurs for rays from the left and right-hand sides of the object, which end up on opposite sides of the image. In this respect the image formed on the retina is not only flipped vertically but also horizontally.
Does this mean we see the world upside down and the wrong way round? Thankfully no! The brain intervenes and processes the signals so that we perceive the object as being the correct way up and the right way around. In this respect it is fair to say that there is more to seeing than meets the eye!
The brain is central to the process of seeing.
The reversal of images also occurs in cameras. In episode 02, we consider in detail how a ray diagram can be constructed to demonstrate this for a pinhole camera.
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The electromagnetic spectrum
What is light?
This question might well be posed by an interested pupil. The approach taken in this topic starts on familiar ground with light being given out by the Sun, travelling in straight lines, and allowing us to see. The idea that light is some kind of wave motion is quite common, but these wave ideas add little (if anything) to the physics narrative at this level. Nevertheless if you are interested in going deeper, read on for more about the electromagnetic spectrum. The SPT: Radiations and radiating topic is much more comprehensive.
When we refer to the visible light that is given out by the Sun and that allows us to see the world around us, we are referring to one part of the electromagnetic spectrum. The spectrum of visible colours that you will be familiar with is just one small part of a huge family of radiations, ranging from radio waves to gamma rays.
Teacher Tip: You'll also meet spectra in the SPT: Sound topic
Properties of electromagnetic radiation
These various kinds of radiation have quite different properties, so what is it that they all have in common which allows us to say that they are part of the same family?
The physical nature of the radiation is the same throughout the spectrum. We refer to electromagnetic radiation because all parts of the family consist of linked electric and magnetic fields that oscillate perpendicular to each other. The radiation moves in a direction perpendicular to both of the fields.
As a result all of these radiations have common behaviours, so they're referred to as a family. One common behaviour is that they all travel at the same speed in a vacuum. All of these electromagnetic waves (whether radio, visible or gamma) travel at the same speed. They all travel at the 'speed of light' through a vacuum, that is at 3 × 108 m s-1.
The significant difference from one part of the spectrum to another is in frequency.
Electromagnetic radiation consists of linked, oscillating electric and magnetic fields.
Radiation as waves
Electromagnetic radiation, such as visible light, is wave-like in nature because it consists of these oscillating electric and magnetic fields.
While water waves are made up of particles, moving up and down at right angles to the direction of travel of the wave, electromagnetic waves consist of oscillating fields. If you think that this is a difficult idea to get hold of, you'd be right! It is one matter to think of water moving up and down to create a wave, but something quite different to imagine oscillating fields (since a field is itself an abstract mathematical idea). (If you ask any watersports enthusiast, they'll tell you that the particles in water waves don't only move up and down, but any additional movements are small compared to the up and down movement. The approximation is helpful here.)
Since radiations from all parts of the spectrum travel at the same speed, and the frequencies vary, the wavelengths of the different parts of the electromagnetic spectrum must also be different.
The differences in wavelength can be huge. For example, radio waves might have a wavelength of 1500 metre (the 'metric mile'), while X-rays might have a wavelength of 1 × 10-10 m (close to the separation between molecules in solids).
Frequency, speed and wavelength
Here are the quantities, and how they're measured: frequency is measured by counting the number of cycles per second in hertz; wavelength is the shortest distance between points of the wave in step in metres; speed is the distance the wave travels each second in metres / second.
For any wave (whether water, light, sound), there is a relationship between the wave speed, frequency and wavelength.
Here's a precise way of writing it out, so that every term is just a number:
Wavespeedmetre/second = frequencyhertz × wavelengthmetre
You can also write (making notes to yourself about the units):
wavespeed measured in metre/second = frequency measured in hertz × wavelength measured in metre
Or even express it rather concisely as:
wavespeed = frequency × wavelength
Since all of the radiations of the electromagnetic spectrum share the same speed, it follows that radiations with a relatively high frequency (such as light) must have a relatively short wavelength and vice versa. Here are some selected values: radio waves have a frequency of 2 × 105 Hz and a wavelength of 1.5 × 103 m; visible light has a frequency of 3 × 1015 Hz and a wavelength of 1 × 10-7 m; X-rays have a frequency of 3 × 1018 Hz and a wavelength of 1 × 10-10 m .
For now, we suggest that you concentrate on the fundamentals of frequency and amplitude. Wavelength can come later.
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Seeing and lighting
Starting out in thinking about seeing and light
Light travels from source to detector. Sources are luminous objects. Our eye is a detector that can see things only when light travels from the source to the detector. We can also see non-luminous objects when light reflects from their surfaces and arrives at the eye.
Not all the light emitted from the source reaches the detector:
- some will head off in other directions
- some will be absorbed
- some will be scattered
But all of the light continues to travel until it is absorbed, somewhere.