Temat 16: Perception of colors

The human eye
The human eye is a so-called camera eye. Like a photographic camera, and in contrast to insect eyes and other compound eyes, the vertebrate camera eye works by producing an image of the outer world on a surface consisting of light sensors, the retina. The retina covers more than half of the inside of the eye ball, whose typical diameter in an adult is about 16.7mm. The pupil has a diameter between 2mm – below which one gets problems with diffraction – and 7mm – for which lens aberrations are just acceptable. The image on the retina has low image distortion, low chromatic aberrations (about 1 dioptre between red and blue) and low coma; the eye achieves this performance by using an deformable aspheric gradient-index lens and a cornea whose shape is always near the ideal shape within 30 μm – an extremely good value for a deformable body.The eye, together with the brain, also has a powerful autofocus – still not fully understood – and an excellent motion compensation and image stabilization system built in. A section of this amazing device is shown in Figure.

The retina is an outgrowth of the brain. It contains 120 million rods, or black and white pixels, and 6 million cones, or colour pixels. Each pixel can detect around 300 to 500 intensity levels (9 bit). The eye works over an intensity range of 8 to 10 orders of magnitude; the involved mechanism is incredibly complex, takes place already inside the receptors, involves calcium ions, and is fully known only since a few years.The region of highest resolution, the fovea, has an angular size of about 1°. The resolution of the eye is about 1'.The integration time of the retina is about 100ms – despite this, nothing is seen during the saccades.The retina itself is 200μm thick and is transparent: this means that all cables leading to the receptors are transparent as well.
The retina has very low energy consumption and uses a different type of neurons that usual nerves: instead of using spikes, the neurons in it use electrotonic potentials, not the action potentials or spikes used in most other nerves,which would generate interferences that would make seeing impossible. In the fovea, every pixel has a connection to the brain.
At the borders of the retina, around 10 000 pixels are combined to one signal channel. (If all pixels were connected 1 to 1 to the brain, the brain would need to be as large as a typical classroom.) As a result, the signals of the fovea, whose area is only about 0.3% of the retina, use about 50% of the processing in the brain’s cortex.
To avoid chromatic aberrations, the fovea has no blue receptors.The retina is also a graphic preprocessor: it contains three neuronal layers that end up as 1.3 million channels to the cortex, where they feed 5 million axons that in turn connect to 500 million neurons.
The compression methods between the 125 million pixel in the retina and the 1.3 million channels to the cortex is still subject of research. It is known that the signals do not transport pixel data, but data streams processed in about a dozen different ways. The streams do not carry brightness values, but only contrasts, and they do not transmit RGB values, but colour differences.The streams carry motion signals in a compressed way and the spatial frequency data is simplified.
Explorations have shown how the ganglions in the retina provide a navigational horizon, how they detect objects moving against the background of the visual field, and how they subtract the motion of the head. The coming years and decades will provide many additional results; several data channels between the eye and the brain are still unknown.
Apart from rods and cones, human eyes also contain a third type of receptor.This receptor type, the photosensitive ganglion cell or intrinsically photosensitive retinal ganglion cell, has only been discovered in the early 1990s, sparking Ref. 136 a whole new research field.
Photosensitive ganglion cells are sensitive mainly to blue light, use melanopsin as photopigment and are extremely slow.They are connected to the suprachiasmatic nucleus in the brain, a small structure of the size of a grain of rice that controls our circadian hormone cycle. For this reason you should walk a lot outside, where a lot of blue light is available, in order to reset the body’s clock and get rid of jet-lag. Photosensitive ganglion cells also produce the signals that control the diameter of the pupil.

Temat 15: What is light?

The nature of light has fascinated explorers of nature since at least the time of he ancient Greeks. The answer appeared in 1848, when Gustav Kirchhoff noted hat the experimental values on both sides of the equation

agreed within measurement errors. This suggested the answer to the question asked two thousand years earlier:
⊳ Light is an electromagnetic wave.
Ten years later, in 1858, Bernhard Riemann* proved mathematically that any electromagnetic wave must propagate with a speed c given by the above equation. Note that the right-hand side contains electric and magnetic quantities, and the left-hand side is an optical quantity.The expression of Kirchhoff and Riemann thus unifies electromagnetism and optics. The modern value for the speed of electromagnetic waves, usually called c from Latin celeritas, is
c = 299 792 458 m/s .
The value for c is an integer number, because the meter is nowadays defined in such a way as to exactly achieve this number.
In 1865, Maxwell summarized all data on electricity and magnetism collected in the 2500 years in his equations. Almost nobody read his papers, because he wrote them using quaternions. The equations were then simplified independently by Heinrich Hertz and Oliver Heaviside.They deduced the original result of Riemann: in the case of empty space, the equations of the electromagnetic potentials can be written as

Experiments in empty space confirm that the phase velocity c is independent of the frequency of the wave.This phase velocity thus characterizes electromagnetic waves, and distinguishes them from all other types of waves in everyday life.


What are electromagnetic waves? To get a clearer idea of electromagnetic waves, we explore their properties. The wave equation for the electromagnetic field is linear in the field; this means that the sum of two allowed situations is itself an allowed situation. Mathematically speaking, any superposition of two solutions is also a solution. We therefore know that electromagnetic waves must show interference, as all linear waves do.

FIGURE 50 The general structure of a plane, monochromatic and linearly polarized electromagnetic wave at a specific instant of time.

Linearity also implies that two waves can cross each other without disturbing each other, and that electromagnetic waves can travel undisturbed across static electromagnetic fields.
Linearity also means that every electromagnetic wave can be described as a superposition of harmonic, or pure sine waves, each of which is described by expression. The simplest possible electromagnetic wave, the harmonic plane wave with linear polarization, is illustrated in Figure 50. Note that for this simplest type of waves, the electric and the magnetic field are in phase. (Can you prove this experimentally and by calculation?) The surfaces formed by all points of maximal field intensity are parallel planes, spaced by (half the) wavelength; these planes move along the direction of the propagation with the phase velocity.
After Riemann and Maxwell predicted the existence of electromagnetic waves, in the years between 1885 and 1889, Heinrich Hertz* discovered and studied them. He fabricated a very simple transmitter and receiver for 2GHz waves, shown in Figure 53. Such waves are still used today: cordless telephones and the last generation of mobile phones work at this frequency – though the transmitters and the receivers look somewhat differently nowadays. Such waves are now also called radio waves, since physicists tend to call all moving force fields radiation, recycling somewhat incorrectly a Greek term that originally meant ‘light emission.’

Today Hertz’s experiment can be repeated in a much simpler way. As shown in Figure 54, a budget of a few euro is sufficient to remotely switch on a light emitting diode with a gas lighter. (After each activation, the coherer has to be gently tapped, in order to get ready for the next activation.) Attaching longer wires as antennas and ground allows this set-up to achieve transmission distances up to 30 m.
Hertz also measured the speed of the waves he produced. In fact, you can also measure the speed at home, with a chocolate bar and a (older) kitchen microwave oven. A microwave oven emits radio waves at 2.5GHz – not far from Hertz’s value. Inside the oven, these waves form standing waves. Just put the chocolate bar (or a piece of cheese) in the oven and switch the power off as soon as melting begins. You will notice that the bar melts at regularly spaced spots. These spots are half a wavelength apart. From the measured wavelength value and the frequency, the speed of light and radio waves simply follows as the product of the two.
If you are not convinced, you can measure the speed directly, by telephoning a friend on another continent, if you can make sure of using a satellite line (choose a low cost provider). There is about half a second additional delay between the end of a sentence and the answer of the friend, compared with normal conversation. In this half second, the signal goes up to the geostationary satellite, down again and returns the same way.
This half second gives a speed of c ≈ 4 ⋅ 36 000 km/0.5 s ≈ 3 ⋅ 105 km/s, which is close to the precise value. Radio amateurs who reflect their signals from the Moon can perform a similar experiment and achieve higher precision.
In summary, electromagnetic waves exist and move with the speed of light.

Temat 14: What is electricity?

The answer to this question is: Electricity is more the name for a field of inquiry, and less the name for any specific observation or effect. Electricity is not a specific term; the term is used to refer to the effects of electric charges, of their motion and their fields. In fact the vocabulary issue hides a deeper question: what is the nature of electric charge? In order to solve this issue, we start with the following question.

Can we detect the inertia of electricity?
If electric charge really is something flowing throughmetals, we should be able to observe the effects shown in Figure 12: electric charge should fall, should have inertia and should be separable from matter. And indeed, each of these effects has been observed.

FIGURE 12 Consequences of the flow of electricity.

For example, when a long metal rod is kept vertically, we can measure an electrical potential difference, a voltage, between the top and the bottom. In other words, we can measure the weight of electricity in this way. Similarly, we can measure the potential difference between the ends of an accelerated rod. Alternatively, we can measure the potential difference between the centre and the rim of a rotating metal disc.The last experiment was, in fact, the way in which the ratio q/m for currents in metals was first measured with precision.
The result is:
q/m ≈ −1.8(2) ⋅ 10¹¹ C/kg (7)
for all metals, with small variations in the second digit. The minus sign is due to the definition of charge. In short, electrical charge in metals has mass, though a very small one.
If electric charge hasmass, whenever we switch on an electrical current, we get a recoil. This simple effect can easily be measured and confirms themass to charge ratio just given. Also, the emission of current into air or into vacuum is observed; in fact, every cathode picture. The emission works best for metal objects with sharp, pointed tips. The rays created this way – we could say that they are ‘free’ electricity – are called cathode rays. Within a few per cent, they show the same mass to charge ratio as expression (7). This correspondence thus shows that charges move almost as freely in metals as in air; this is the reason that metals are such good conductors of electric current.
If electric charge falls inside vertical metal rods, we can make the astonishing deduction that cathode rays should not be able to fall through a vertical metal tube. As we will see later, cathode rays consist of free electrons. The name ‘electron’ is due to George Stoney. Electrons are the smallest and lightest charges moving in metals; they are, usually – but not always – the ‘atoms’ of electricity. In particular, electrons conduct electric current inmetals. The charge of an electron is small, 0.16 aC, so that flows of charge typical of everyday life consist of huge numbers of electrons; as a result, electrical charge effectively behaves like a continuous fluid. The particle itself was discovered and presented in 1897 by Johann Emil Wiechert (b. 1861 Tilsit, d. 1928 Göttingen) and, independently, three months later, by Joseph John Thomson (b. 1856 Cheetham Hill, d. 1940 Cambridge).
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The speed of electricity is too slow for many people. Computer chips could be faster if it were higher. And computers that are connected to stock exchanges are located as near as possible to the stock exchange, because the time advantage the short communication distance (including the delay inside switching chips) provides is essential for getting a good financial performance in certain trading markets.
In summary, experiments show that all charges have mass. And like all massive bodies, charges move slower than light. Charge is a property of matter; images and light have no charge.

Temat 13: Albert Einstein

Albert Einstein (b. 1879 Ulm, d. 1955 Princeton) was one of the greatest physicists ever. (The ‘s’ in his name is pronounced ‘sh’.)
In 1905, he published three important papers: one about Brownian motion, one about special relativity and one about the idea of light quanta.
The first paper showed definitely that matter is made of molecules and atoms;
the second showed the invariance of the speed of light;
and the third paper was one of the starting points of quantum theory.
Each paper was worth a Nobel Prize, but he was awarded the prize only for the last one.
Also in 1905, he proved the famous formula E₀ = mc² (published in early 1906), after a few others also had proposed it.
Although Einstein was one of the founders of quantumtheory, he later turned against it. His famous discussions with his friend Niels Bohr nevertheless helped to clarify quantum theory in its most counter-intuitive aspects.
Later, he explained the Einstein–de Haas effect which proves that magnetism is due to motion inside materials. After many other discoveries, in 1915 and 1916 Einstein published his highest achievement: the general theory of relativity, one of the most beautiful and remarkable works of science. In the remaining forty years of his life, he searched for the unified theory of motion, without success.
Being Jewish and famous, Einstein was a favourite target of attacks and discrimination by theNational Socialist movement; therefore, in 1933 he emigrated from Germany to the USA; since that time, he stopped contact with Germans, except for a few friends, among them Max Planck. Until his death, Einstein kept his Swiss passport in his bedroom. He was not only a great physicist, but also a great thinker; his collection of thoughts about topics outside physics are well worth reading. But his family life was disastrous, and he made each of his family members unhappy.
Anyone interested in emulating Einstein should know first of all that he published many papers.* He was both ambitious and hard-working.
Moreover, many of his papers werewrong; he would then correct themin subsequent papers, and then do so again.
This happened so frequently that hemade fun of himself about it. Einstein indeed realized the well-known definition of a genius as a person who makes the largest possible number of mistakes in the shortest possible time.

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* All his papers and letters are now freely available online, at einsteinpapers.press.princeton.edu .


Check your knowledge: Special relativity
1. Which of the following has not been observed to change due to motion?
- Time
- Length
- Speed of light in a vacuum
2. An astronaut takes a long space journey at close to light speed. The astronaut's twin sister remained on Earth. On a return to Earth, what is the astronaut's age compared to her earthbound twin?
- Older
- The same
- Younger
3. As it approaches the speed of light, what will happen to the length of a spacecraft as observed by a passenger inside? - It will increase
- It will remain constant
- It will decrease
4. A spacecraft of length 780 meters is travelling at a constant speed of 0,95 c . The spacecraft travels at this speed for 10 years, as measured by a clock on the Earth.
Calculate the time elapsed, in years, as measured by a clock in the spacecraft.
- 2.23 years
- 3.12 years
- 32.0 years
5. A spacecraft of length 780 meters is travelling at a constant speed of 0,95 c. The spacecraft travels at this speed for 10 years, as measured by a clock on the Earth. If the spacecraft passes Earth at this speed what length would an observer on Earth measure for the spacecraft?
- 174 m
- 244 m
- 2500 m
6. Imagine a train travelling at 100 m/s with a passenger who throws a ball ahead of the train at 5 m/s . What is the resultant velocity of the ball?
- 5 m/s
- 100 m/s
- 105 m/s
7. Imagine a train is travelling at 2,9 x 108 m/s . A passenger shines a torch from the front of the train. What speed does the passenger see the light from the torch move at?
- 0,1 x 108 m/s
- 3 x 108 m/s
- 5,9 x 108 m/s
8. Imagine a train is travelling at 2,9 x 108 m/s . A passenger shines a torch from the front of the train. A stationary observer on the train platform measures the speed of light from the torch. How fast does the light appear to move according to the stationary observer?
- 0,1 x 108 m/s
- 3 x 108 m/s
- 5,9 x 108 m/s

Temat 12: The physics of blood and breathing

Fluid motion is of vital importance. There are at least four fluid circulation systems inside the human body.
First, blood flows through the blood system by the heart.
Second, air is circulated inside the lungs by the diaphragm and other chestmuscles.
Third, lymph flows through the lymphatic vessels, moved passively by body muscles.
Fourth, the cerebrospinal fluid circulates around the brain and the spine, moved by motions of the head.
For this reason, medical doctors like the simple statement: every illness is ultimately due to bad circulation.
Why do living beings have circulation systems?
Circulation is necessary because diffusion is too slow. Can you detail the argument?
We now explore the two main circulation systems in the human body.
Blood keeps us alive: it transports most chemicals required for our metabolism to and from the various parts of our body.
The flow of blood is almost always laminar; turbulence only exists in the venae cavae. The heart pumps around 80 ml of blood per heartbeat, about 5 l/min. At rest, a heartbeat consumes about 1.2 J.
The consumption is sizeable, because the dynamic viscosity of blood ranges between 3.5 ⋅ 10 ⁻³ Pa s (3.5 times higher than water) and 10⁻² Pa s, depending on the diameter of the blood vessel; it is highest in the tiny capillaries.
The speed of the blood is highest in the aorta, where it flows with 0.5 m/s, and lowest in the capillaries, where is as low as 0.3 mm/s. As a result, a substance injected in the arm arrives in the feet between 20 and 60 s after the injection. In fact, all animals have similar blood circulation speeds, usually between 0.2 m/s and 0.4 m/s. Why?
To achieve blood circulation, the heart produces a (systolic) pressure of about 16 kPa, corresponding to a height of about 1.6 m of blood. This value is needed by the heart to pump blood through the brain. When the heart relaxes, the elasticity of the arteries keeps the (diastolic) pressure at around 10 kPa.
These values are measured at the height of the heart. The values vary greatly with the position and body orientation at which they are measured: the systolic pressure at the feet of a standing adult reaches 30 kPa, whereas it is 16 kPa in the feet of a lying person.
For a standing human, the pressure in the veins in the foot is 18 kPa, larger than the systolic pressure in the heart. The high pressure values in the feet and legs is one of the reasons that leads to varicose veins. Nature uses many tricks to avoid problems with blood circulation in the legs.
Humans leg veins have valves to avoid that the blood flows downwards; giraffes have extremely thin legs with strong and tight skin in the legs for the same reason. The same happens for other large animals.
At the end of the capillaries, the pressure is only around 2 kPa. The lowest blood pressure is found in veins that lead back from the head to the heart, where the pressure can even be slightly negative. Because of blood pressure, when a patient receives a (intravenous) infusion, the bag must have a minimum height above the infusion point where the needle enters the body; values of about 0.8 to 1m cause no trouble. (Is the height difference also needed for person-to-person transfusions of blood?)
Since arteries have higher blood pressure, for the more rare arterial infusions, hospitals usually use arterial pumps, to avoid the need for unpractical heights of 2m or more.

The physics of breathing is equally interesting. A human cannot breathe at any depth under water, even if he has a tube going to the surface, as shown in Figure 242 (right):

At a few metres of depth, trying to do so is inevitably fatal!
Even at a depth of 50 cm only, the human body can only breathe in this way for a few minutes, and can get badly hurt for life.Why?
Inside the lungs, the gas exchange with the blood occurs in around 300 millions of little spheres, the alveoli, with a diameter between 0.2 and 0.6 mm. To avoid that the large one grow and the small ones collapse,  the alveoli are covered with a phospholipid surfactant that reduces their surface tension. In newborns, the small radius of the alveoli and the low level of surfactant is the reason that the first breaths, and sometimes also the subsequent ones, require a large effort.
We need around 2% of our energy for breathing alone.The speed of air in the throat is 10 km/h for normal breathing; when coughing, it can be as high as 160 km/h. The flow of air in the bronchi is turbulent; the noise can be heard in a quiet invironment. In normal breathing, the breathingmuscles, in the thorax and in the belly, exchange 0.5 l of air; in a deep breath, the volume can reach 4 l.
Breathing is especially tricky in unusual situations. After scuba diving* at larger
depths than a few meters for more than a few minutes, it is important to rise slowly, to avoid a potentially fatal embolism.Why?
The same can happen to participants in high altitude flights with balloons or aeroplanes, to high altitude parachutists and to cosmonauts

*The blood pressure values measured on the two upper arms also differ; for right handed people, the pressure in the right arm is higher

How blood pressure works


Understanding Blood Pressure
Human Anatomy and Physiology video 3D animation

This is a biology/anatomy video for Grade 10-11 students about Blood Pressure, its causes and effects. The pressure with which blood flows in the blood vessels is called Blood Pressure or BP. BP is measured using a special device called Sphygmomanometer

Temat 11: Fluids and their MOTION

Fluids can be liquids or gases, including plasmas. And the motion of fluids can be exceedingly intricate, as Figure 233 shows.
In fact, fluid motion is so common – think about breathing, blood circulation or the weather – that exploring is worthwhile.

F IGURE 233 Examples of fluid motion: a vertical water jet striking a horizontal impactor, two jets of a glycerol–water mixture colliding at an oblique angle, a water jet impinging on a reservoir (all © John Bush, MIT) and a dripping water tap (© Andrew Davidhazy).

The state of a fluid
To describe motion means to describe the state of a system. For most fluids, the state at every point in space is described by composition, velocity, temperature and pressure. We will explore temperature below. We thus have one new observable: The pressure at a point in a fluid is the force per area that a body of negligible size feels at that point. Pressure is measured with the help of barometers or similar instruments. The unit of pressure is the pascal: 1 Pa is 1N/m2.
Pressure is not a simple property. Can you explain the observations of Figure 235?

F IGURE 235 The hydrostatic and the hydrodynamic paradox (© IFE).

If the hydrostatic paradox – an effect of the so-called communicating vases – would not be valid, it would be easy to make perpetuum mobiles. Can you think about an example?
Another puzzle about pressure is given in Figure 236.

FIGURE 236 A puzzle: Challenge 545 s Which method of emptying a container is fastest? Does the method at the right hand side work at all?

Air has a considerable pressure, of the order of 100 kPa. As a result, it is not easy to make a vacuum; indeed, everyday forces are often too weak to overcome air pressure. This is known since several centuries, as Figure 237 shows. Your favorite physics laboratory should posess a vacuum pump and a pair of (smaller) Magdeburg hemispheres; enjoy performing the experiment yourself.

F IGURE 237 The pressure of air leads to surprisingly large forces, especially for large objects that enclose a vacuum. This was regularly demonstrated in the years from 1654 onwards by Otto von Guericke with the help of his so-called Magdeburg hemispheres and, above all, the various vacuum pumps that he invented (© Deutsche Post, Otto-von-Guericke-Gesellschaft, Deutsche Fotothek).

Laminar and turbulent flow
Like all motion, fluid motion obeys energy conservation. In the case that no energy is transformed into heat, the conservation of energy is particularly simple. Motion that does not generate heat is motion without vortices; such fluid motion is called laminar. If, in addition, the speed of the fluid does not depend on time at all positions, it is called stationary.
For motion that is both laminar and stationary, energy conservation can be expressed with the help of speed v and pressure p:
½ pv² + p + qgh = const
where h is the height above ground. This is called Bernoulli’s equation. *

* Daniel Bernoulli (b. 1700 Bale, d. 1782 Bale), important mathematician and physicist. His father Johann and his uncle Jakob were famous mathematicians, as were his brothers and some of his nephews. Daniel Bernoulli published many mathematical and physical results. In physics, he studied the separation of compound motion into translation and rotation. In 1738 he published the Hydrodynamique, in which he deduced all results from a single principle, namely the conservation of energy.The so-called Bernoulli equation states that (and how) the pressure of a fluid decreases when its speed increases. He studied the tides and many complex mechanical problems, and explained the Boyle–Mariotte gas ‘law’. For his publications he won the prestigious prize of the French Academy of Sciences – a forerunner of the Nobel Prize – ten times.

F IGURE 234 Daniel Bernoulli (1700–1782)

In this equation, the last term is only important if the fluid rises against ground. The first term is the kinetic energy (per volume) of the fluid, and the other two terms are potential energies (per volume). Indeed, the second term is the potential energy (per volume) resulting from the compression of the fluid.This is due to a second way to define pressure:
Pressure is potential energy per volume
Energy conservation implies that the lower the pressure is, the larger the speed of a fluid becomes.
We can use this relation to measure the speed of a stationary water flow in a tube. We just have to narrow the tube somewhat at one location along the tube, and measure the pressure difference before and at the tube restriction.The speedv far from the constriction is then given as

(What is the constant k?)
A device using this method is called a Venturi gauge. If the geometry of a system is kept fixed and the fluid speed is increased – or the relative speed of a body in fluid – at a certain speed we observe a transition: the liquid loses its clarity, the flow is not laminar anymore. We can observe the transition whenever we open a water tap: at a certain speed, the flow changes from laminar to turbulent. At this point, Bernoulli’s equation is not valid any more.
The description of turbulence might be the toughest of all problems in physics. When the young Werner Heisenberg was asked to continue research on turbulence, he refused – rightly so – saying it was too difficult; he turned to something easier and he discovered and developed quantum mechanics instead. Turbulence is such a vast topic, with many of its concepts still not settled, that despite the number and importance of its applications, only now, at the beginning of the twenty-first century, are its secrets beginning to be unravelled.
It is thought that the equations of motion describing fluids, the so called Navier–Stokes equations, are sufficient to understand turbulence.** But the mathematics behind them is mind-boggling. There is even a prize of one million dollars offered by the Clay Mathematics Institute for the completion of certain steps on the way to solving the equations.
** They are named after Claude Navier (b. 1785 Dijon, d. 1836 Paris), important engineer and bridge builder, and Georges Gabriel Stokes (b. 1819 Skreen, d. 1903 Cambridge), important physicist and mathematician.

FIGURE 238 Left: non-stationary and stationary laminar flows; right: an example of turbulent flow (© Martin Thum, Steve Butler).

Temat 10: Forces and Motion

Forces and Motion - REVISION PODCAST

This revision podcast is for Edexcel IGCSE physics (4PH0 or 4SC0), and covers all of topic 1 - forces and motion. It is also suitable for other GCSEs.
There's no hard and fast rule about what to plot on each axis of a graph - I have plotted Force on the y-axis and Extension on the x-axis so that the gradient of the graph is k - the spring constant. (Consider the equation F=kx and the general equation for a straight line, y=mx).
I know that generally, students at GCSE are taught that the independent variable (what you change) goes on the x-axis, and the dependent variable (what you measure) goes on the y-axis, but sometimes it dies make sense to swap the axes, if the gradient is more meaningful.