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

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

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.