Diffraction

Diffraction in the process of waves spreding out after having passed a barrier. The extent to which waves diffract relies upon the width of the gap in comparison to the wavelength of the wave. Diffraction is a useful property to understand when studying the comparative properties of short-wave and long-wave radio communication.
Physics - Waves - Reflection, Refraction and Diffraction


This article is part 4 of the series 'Reflection, Refraction and Diffraction'. Prior knowledge may be required. If you have difficulty with this article, read its precursor here: http://exploringgcses.blogspot.com/2012/01/total-internal-reflection.html

In order to understand this fully, consider a wave that starts from one central point and radiates out. It will radiate in all directions. The waves in this article should be considered as many of these placed directly next to each other, moving forwards. Each one will radiate to the right and left, but this will be counteracted by the opposite movement of the areas right next to them, so the wave moves in a straight line forward.

When the wave passes the barrier, the very edge of it has no wave to the side of it. This means that nothing is preventing it from radiating to the side as well as forwards. This leads to the wave's edges spreading out in a circular way from the edge of the wave's main body. This diagram should illustrate the principle:

It is not neccessary for there to be two barriers and a gap: diffraction would also happen if a wave were to pass and be partially cut off by one barrier.


When the width of the aperture is equal to the wavelength, there are no pieces of the wave 'sandwiched', so all of the waves have been diffracted and there are no straight parts. Diffraction is a property shown by all waves.


This is the last of 4 articles in the series 'Reflection, Refraction and Diffraction' under the topic 'Waves' in Physics

Total Internal Reflection

Total internal reflection is the reflection of light incident at the boundary of a medium back into the medium whence it came. A varying proportion of the incident light may be totally internally reflected depending on the angle of incidence of the light and the critical angle of the medium.
Physics - Waves - Reflection, Refraction and Diffraction 


This article is part 3 of the series 'Reflection, Refraction and Diffraction'. Prior knowledge may be required. If you have difficulty with this article, read its precursor here: http://exploringgcses.blogspot.com/2012/01/refraction-of-light.html

Total internal reflection occurs alongside refraction, and only in the passage of light from a more optically dense material to a less optically dense material. When the light is incident at the boundary of the medium in this case, most of it is refracted, but a small proportion is reflected back into the medium.

The proportion of light reflected back remains small until the light is incident at an angle greater than the critical angle. Once this happens, all of the light is totally internally reflected. When a light ray incident at exactly the critical angle, the refracted ray runs perpendicular (at 90 degrees) to the normal.

If the angle of incidence increases such that it is greater than c, all of the light will be totally internally reflected.

The value of c depends not only on the optical density of the first medium, but also on the optical density of the medium that the light would be refracted to. In other words, it relies on the density of the materials either side of the boundary. This is important to remember.

The equation sin(c) = 1/n relates a medium's refractive index to its critical angle in air.


Total internal reflection has several uses. It can be used to carry optical signals along bundles of optical fibres because the light inside just reflects against the side continuously as it moves forwards. It is used in periscope prisms as well: because the critical angle for glass is 42 degrees, it will be totally internally reflected in a periscope arrangement because it will be incident to the boundary at 45 degrees, which is greater than c.


Read part 4 of this series, 'Diffraction', http://exploringgcses.blogspot.com/2012/01/diffraction.html

Refraction of light

Refraction describes the change in direction of a ray of light as it travels from one medium to another medium with a different optical density. The change in direction happens as a result of the change in velocity of the light as it travels into this other medium.
Physics - Waves - Reflection, refraction and diffraction


This article is part 2 of the series 'Reflection, Refraction and Diffraction'. Prior knowledge may be required. If you have difficulty with this article, read its precursor here: http://exploringgcses.blogspot.com/2011/12/reflection-of-waves.html

The optical density (also called absorbance)  of a medium (a material through which light can travel) describes the speed at which light travels in it. For example, a vacuum (a space with no matter of any kind in it) has an optical density of 0. This is because there is absolutely nothing to slow it down, the vacuum is completely empty. The 'speed of light' (300 million m/s) actually means the speed of light in a vacuum because that's where it travels fastest.

The refractive index of a medium is derived from the optical density of that medium. The optical density defines how much the light's speed changes, and that in turn defines how much it is refracted. How much it is refracted is the refractive index; it is easy to see that these two terms are closely linked. Looking ahead, the refractive index is calculated using Snells' Law, which says that:

the ratio sin(i) : sin(r) is constant

where i = angle of incidence and r = angle of refraction

If these terms are difficult to understand, read on and return to the equation later

To look up the refractive index of a material, try this website: http://www.wolframalpha.com/widget/widgetPopup.jsp?p=v&id=5f7039b1bb628805481cb58560a1208b&title=Index%20of%20Refraction&theme=red&i0=glass&podSelect=&showAssumptions=1&showWarnings=1

Now that all of the terminology has been explained, we will explore the trends of refraction. Whenever light travels from one medium to another medium that has a different optical density, refraction will occur*. When light travels from a less optically dense medium to a more optically dense medium, the light is bent towards the normal. When it travels from a less optically dense material to a more optically dense material, it bends away from the normal. Look at the diagram: light is refracted twice, once from air to glass and once from glass to air. Glass is more optically dense, and since the dotted lines represent the normals, you can see the rules being applied. First, the light bends towards the normal, and then away from it.


Notice that the ray leaving the glass is travelling in the same direction as the ray entering the glass, but has just been moved down. We say that it has been subject to parallel displacement. This will only happen if the light leaves the 2nd medium back into the same medium it started in.

*There is an exception: if the light enters the second medium along the normal (perpendicular to the new material's surface) then no refraction will occur.

Now we can return to the rule: the ratio sin(i) : sin(r) is constant. We can derive from the equation
 n = sin(i)/sin(r) with n being the refractive index. We can use this to find i, r, or n by substituting know values into the equation and simplifying.


Dispersion occurs when white light is refracted through a prism to give a spectrum of light. White light is composed of 7 different colours of visible light, which all have different wavelengths. As a result of this, the prism has a different refractive index for each colour. This means that the direction of each colour is changed by a different amount, and a spectrum or band of colours spreads out from the other side of the prism. This is not an exception to refraction, only an interesting application of it.

To summarise: refraction is the change in the direction of light as a result of the change of the medium in which it is travelling. When light travels into a more optically dense material, it bends towards the normal, and vice versa. The sine of the angle of incidence divided by the sine of the angle of refraction is equal to the refractive index, n. The magnitude of the refration is defined by the refractive index of the new medium.


The next post in the series is 'Total Internal Reflection' http://exploringgcses.blogspot.com/2012/01/total-internal-reflection.html

Louis Pasteur (1822 - 1895)

Louis Pasteur was the scientist who made the link between germs and disease, and then continued his work by making vaccines for diseases like chicken cholera. This post explores Pasteur's journey to his discovery, and how other scientists followed on from it.
History - Medicine through time - Fighting infectious disease


Science at that time: science was propelled forward  by advances that came before and with the industrial revolution. These include the invention of the thermometer (by Fahrenheit in 1709 and Celsius in 1742) and the invention of the microscope as early as 1683 (by van Leeuwenhoek). Microbes were known of at this point, but they were not believed to be the cause of disese, only the result of it. This theory is called spontaneous generation. This theory was only applied to microscopic organisms by this time because of the experiments of Francesco Redi in 1668, who showed that maggots did not spontaneously generate but came from eggs laid by flies. Pasteur was to disprove the theory of microscopic spontaneous generation.

Pasteur was an industrial scientist, not a doctor or pathologist (someone who investigates the cause and effect of disease). This meant that he dealt with the sientific problems facing big businesses. He made his discovery while investigating the reason why sugar beet mysteriously went sour during fermentation in 1857.

He believed that microbes were growing in the sugar beet and causing it to go sour. He proved this with the famous swan-necked flask experiment. In a swan necked flask, microbes cannot get in because they have to travel against gravity to go up the swan neck. Using this knowledge, he set his experiment as follows:

He took two sets of swan necked flasks and filled them with broth, which microbes love to feed on. He then strongly heated the broth in all of the flasks so that no germs would start inside the flasks. Since they couldn't enter through the swan neck, they would have to spontaneously generate to get in. He broke the swan necks of half of the flasks, which would allow microbes to float in on dust particles. After a while, all of the flasks with broken necks had lots of bacterial growth, but those with te swan necks had none. Since no microbes started in any of the flasks, this proved that microbes didn't spontaneously generate. He confirmed his idea in 1865-7 when studying a silkworm disease called pébrine.

Pasteur's ideas were followed up by other scientists such as Robert Koch, who started using this information to single out which microbes cause which diseases and then make a vaccine against that microbe. Pasteur followed this trend that resulted from his own idea by investigating the diseases anthrax, tuberculosis, cholera and others. This is explored in another post.

Important note: Jenner created the first vaccine against smallpox, so it would be unfair to claim that Pasteur and Koch began the trend entirely. However, Jenner created his vaccine without knowledge of why it worked, and so Pasteur's discovery is important because it represents the knowledge behind why vaccines work.

In summary, Pasteur was an industrial scientist who did not discover microbes, but proved that they did not spontaneously generate and also that they caused disease. He began and continued a trend of identifying microbes for the diseases they caused and creating vaccines for them.





Further reading:
http://bcs.whfreeman.com/thelifewire/content/chp03/0302003.html

The Reactivity Series

The reactivity series is a way of ranking elements, usually metals, according to their reactivity. This is useful because it allows us to work out which elements will displace others in compounds, and has uses in other parts of chemistry such as resistance to corrosion through the use of sacrificial metals. This post details the different reaction types that can be analysed using the reactivity series.

Chemistry - reaction types - reactivity

    High reactivity
Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Hydrogen
Copper
Silver
Gold
Low reactivity  

The reactivity series is used to predict the outcome of displacement reactions. In a displacement reaction, there is more than one element of a certain 'type' available to be in the compound. To explain what I mean by 'type', consider this:

Magnesium + copper oxide ---> Magnesium oxide + Copper

The copper is paired with the oxygen, but the magnesium could also bond with the oxygen. There are two elements competing for a place, and so the more reactive element will take that place. It does this by displacing the less reactive element.

Oxidation and reduction
Oxidation and reduction are ways of describing what has happened to an element or ion during a reaction. It is, as the name suggests, relevant to oxygen, but not exclusively.

• A substance has been oxidized if it gains oxygen. Oxidation is gain of oxygen.
•  A substance has been reduced if it loses oxygen. Reduction is loss of oxygen.
• A reducing agent is something that reduces something else.
• An oxidizing agent is something that oxidizes something else.

It can refer to the gain or loss of electrons as well. This is important to know and understand when you study the industrial chemistry topic. 

• Loss of electrons is oxidation.
• Gain of electrons is reduction

You can use the memory aid 'OIL RIG' to help you remember. Oxidation Is Loss, Reduction Is Gain. This is not to be confused with the gain or loss of oxgyen, because the gain of oxygen is oxidation, but the gain of electrons is reduction.


A REDOX reaction is one in which one element or ion is reduced, and another, different element or ion is oxidised.

Reactions of metals with water or acids uses the same rule, but instead of a metal displacing a metal it may displace hydrogen or carbon in the water or acid. Hydrogen and carbon have their places in the reactivity series too.

Reactions of metals with water -  Metals above hydrogen in the reactivity series react with water or steam to produce hydrogen.  Metals below hydrogen in the reactivity series don’t react with water or steam - they cannot displace the hydrogen.

Reactions of metals with dilute acids -  The pattern is the same as for the reaction between metals and water, except In each case the reaction is more vigorous

    We can use the reactivity series to make predictions about the results of a reaction. Given the opportunity and the right conditions, a more reactive element will always displace a less reactive one in a compound, but a less reactive one will not naturally displace a more reactive one.

Cells

There’s no such thing as a typical cell. Cells are the building blocks of life, and all living things are made from them. Each cell has internal organelles they are all based in the cytoplasm of the cell which has the texture of sloppy jelly. It contains many dissolved substances such as proteins. This post explores the different cell types and the structures within cells.

Biology - Cells and organisms

An Organelle is a small structure within a cell, meaning that all cells are made of organelles. The nucleus is probably the most important organelle because it contains DNA coiled up as chromosomes. The DNA contains all of the information about which proteins to make inside the cell, which reactions should happen and (in a broader context) what the organism as a whole will look like. 

The cytoplasm is the structure in which most of the reactions take place - as a general rule, it is found around the outside of the vacuole in plant cells or everywhere within the cell membrane in animal cells. Most of the other structures can be found outside of the cytoplasm, for example the chloroplasts, which contain chlorophyll to allow them to generate energy from sunlight (photosynthesis, only found in plant cells), or the mitochondria which carry out the respiration reactions in the cell.

The vacuole is the structure that is usually found in the centre of a plant cell. They are filled with cell sap, a solution of sugars. Some specialised animal cells have temporary vacuoles which store food and water, but this is not common.


Around the outside of the cytoplasm will be a cell membrane. All cells have a cell membranes but it is very hard to see in plants because it is against the cell wall. A cell membrane is a very thin layer of protein and fat which controls what goes in and out of the cell. We describe it as a partially permeable; this means it lets some things through but prevents others from entry to the cell.

Cell walls can only be found in plants, and are made of a polymer called cellulose. A cell wall is a very strong covering for the cell. It protects and supports the cell. It can withstand the internal pressures of turgor (when the cell has become stiff because it has absorbed so much water through osmosis. Turgor keeps plants stiff so that they can grow tall and compete for light). Spaces between the cellulose fibres allow even large molecules to pass through; it is described as fully permeable.


There are a few interesting exceptions to the rules. Xylem vessels in plant stems do not have nuclei because they are technically dead cells. Red blood cells have no nuclei because they need to conserve room in the nucleus to carry as much haemoglobin as possible.


Remember - there are many exceptions. This is just an accurate generalisation that can be applied for most GCSE purposes.

Oxygen and Oxides

Oxygen is quite a common element - it occurs naturally in the atmosphere. It combines with many other elements to make compounds that have very different properties. This post details the properties of oxygen and its compounds, and how the compounds can be tested for, identified or made.

Chemistry - Oxygen and Oxides


Unpolluted, dry air is made up of approximately 78.1% Nitrogen, 21% Oxygen, 0.9% Argon and 0.04% Carbon Dioxide. Oxygen is very easy to test for as it relights a glowing splint.

Identifying and testing for oxide compounds
There are two types of oxide compounds: metal oxides and non-metal oxides.
Most Metals Oxides don’t either react with, or dissolve in, water – those that do tend to form alkaline solution. Non-metal oxides often react with water to form acidic solutions – common exceptions are water and carbon monoxide.


Carbon Dioxide
Oxygen and carbon can form the covalent compound carbon dioxide. It can be made using the reaction  between dilute hydrochloric acid and calcium carbonate (also known as marble chips).
Carbon dioxide can be tested for with lime water. When mixed, the lime water turns a milky colour. Alternatively, you can test for it using a glowing splint. While oxygen alone will fuel a fire, carbon dioxide makes a glowing splint go out. Carbon dioxide is is used in fizzy drinks because it dissolves in water under pressure, and to put out electrical fires where using water could cause problems.






This article is due to be added to shortly.

Jenner and the Smallpox vaccine

The discovery of a vaccine to guard against smallpox was the first major step in counteracting infectious disease. This post explores the events preceeding the vaccine, the discovery of the vaccine and the public's reaction to it.
History - Medicine through time - 18th/19th Centuries


Vaccination is often confused with inoculation, as they are similar. Before the introduction of vaccines, inoculation was the best way of developing immunity to smallpox. Inoculation involves deliberately infecting someone with the pus from a person who only had a mild form of the disease, and hoping that the body would be able to fight it. By surviving the disease, the person could become immune to future.

The problem with inoculation was that it was very dangerous. People risked contracting a fatal form of the disease. It was introduced to Britain in 1718 by Lady Mary Wortley Montagu, and by the time Edward Jenner (1749-1823) began experimenting with his theories in 1796 many people had become rich by setting up inoculation houses and charging for the service. These people didn't want to lose their business to the new practice of vaccination that was introduced by Jenner, which is one of the reasons he received so much resistance when he introduced it.

How Jenner discovered his vaccine
Jenner was the student of John Hunter, 'the father of modern surgery', who taught him the importance of scientific observation. Jenner lived in the countryside and investigated the rumour that milkmaids, who were exposed to the non-fatal disease cowpox, didn't get smallpox.


This led him to believe that getting cowpox and defeating it somehow gave a person resistance to smallpox as well. He decided to investigate by infecting someone with cowpox, and then testing them with smallpox. He chose a young boy, James Phipps, and tried the experiment. he found that catching and defeating cowpox did indeed give immunity to smallpox. Jenner wrote about his discovery in 1798.


Because cowpox was non-fatal, this was an excellent way to safely give any healthy person immunity to smallpox. He had discovered the vaccine.

Reactions to the vaccine
As prevously said, Jenner's vaccination method would put many people out of business. They attacked his techniques and spread rumours of strange and horrible side effects that could result from Jenner's discovery. The picture in this post shows the fears that some people had about the vaccine's 'side effects'.


However, the vaccine was so effective that the resistance was crushed. Members of the Royal Family were vaccinated, he was given £30,000 by the government (1802-6) and Emperor Napoleon even released a prisoner at Jenner's request!

Vaccines are used to this day since they became compulsory for all children in 1853. They have had an effect of great magnitude on modern preventative medicine.


Further reading:
http://www.jennermuseum.com/edwardjenner.html
http://en.wikipedia.org/wiki/Edward_Jenner
http://www.historylearningsite.co.uk/edward_jenner.htm

The three types of ionising radiation

Ionising radiation comes in three main forms: alpha, beta and gamma radiation. They have different masses and properties, which are explored in this post.

Physics - Atoms and Radioactivity

Alpha radiation has the strongest ionising power. This means that it will ionise lots of particles as they travel. Because of this, they get 'used up' quickly, so also have the shortest range. They can only penetrate a few cm of air.

Alpha radation is emitted as alpha particles. Each alpha particle is made of 2 neutrons and 2 protons, giving it the greatest mass out of the three types of radiation. Because it has 2 protons but no electrons, it has a relative charge of +2.

It is important to remember that an alpha particle (a particle that has two neutrons and two protons) is the same as a helium atom without the electrons, so expect to see it written as α or 4He2.

Beta radiation - Ionising radiation is the result of an unstable nucleus, and in beta radiation a neutron splits into a proton and an electron so that the electron can be ejected and leave only a proton left. This will increase the relative charge of the nucleus by +1, which will help restore balance in the nucleus.

The electron that is emitted is called the beta particle. It is many thousands of times lighter than an alpha particle, and has a relative charge of -1 (because of the electron).

Beta particles are not as strongly ionising as alpha particles, but are stronger than gamma particles. They lie in the middle. As a result, their range is also between that of alpha particles and gamma particles. They can be stopped by 1- 2mm of aluminium.

Gamma radiation is an electromagnetic wave, so has no mass at all. It is the weakest in terms of ionising power, so has the longest range. A thick lead sheet is needed to stop gamma radiation.

It emitted in packets of energy called photons.

Summary: the stronger the ionising power, the shorter the range. Alpha particles are the strongest, and have the greatest mass and charge. Beta particles lie in the middle. Gamma rays have no charge or mass, because they are waves. They are the weakest, so they have the longest range.