‘And the light shone in the darkness and the darkness comprehended it not..’ Due to its high reactivity, phosphorus is never found as a free element in nature on Earth. The first form of phosphorus to be discovered (white phosphorus, discovered in 1669) emits a faint glow upon exposure to oxygen — hence its name given from Greek mythology,Φωσφόρος meaning “light-bearer” (Latin Lucifer), referring to the “Morning Star“, or Venus. Unsurprising, really, the stuff burns with a brilliant, intensely hot flame and travels through flesh like butter. I have a friend with an old phosphorus burn on his arm which looks more like a bullet hole. It vaporised the skin, leaving small, neat puckers. It’s a surprise then, to discover that Israel has been roundly and universally accused of using white phosphorus against civilians. It is not considered a chemical weapon and is not banned per se, it ignites and burns on contact with oxygen and creates a smokescreen to conceal troop movement.

It also interferes with infra-red optics and weapon-tracking systems, thus protecting military forces from guided weapons such as anti-tank missiles. When WP comes into contact with people or objects, though, it creates an intense and persistent burn. It can also be used as a weapon against military targets.

The IDF has said “smoke shells are not an incendiary weapon” and defended its actions. Using any weapon for a creative purpose other than that for which it might have originally been designed seems to be at the heart of the matter. Alternatively, Hamas’ legendary disregard for the safety of its own people is reason enough to fire phosphorus shells themselves then blame Israel for doing it.  Additionally American troops used similar phosphorus ‘flares’ in Iraq, which illuminate an area, but if they land in houses, they ricochet around burning the house from the inside, rather like napalm.

I am so tired of this. Once again, the propaganda machine muddies the waters and Israel needs continually clear, unambiguous strategies to get her off the back foot. War is nasty, brutish but usually mercifully short and collateral damage, although most regrettable, in an urban theatre is almost certain and engagement of any kind will almost invariably generate it. To the bleaters about war crimes – the old adage ‘we didn’t start the fight’ seems an appropriate opening salvo.

Diffraction – wave spreading around an edge

waterdiffractionThe wavelength of water waves may be several metres. If the wavelength is of a similar size to a gap in a harbour wall, then the wave will diffract  or spread out into circular ripples. This is quite a good aerial image which shows the effect clearly.  If the wavelength  is much smaller than the size of the gap, then only a little bit of diffraction will occur at the edge of the wave where the wave falls over on itself to form circular ripples and most of the wave passes straight through without significant direction change. We can see clearly that the depth of the water is pretty constant, if it weren’t the wavelength of the waves would change, and notice there’s no change in wavelength, frequency or speed.

Here are two images, one showing a narrow gap with circular diffraction, the other a wide gap with only small circular bits at each side.

These diagrams are important, so learn them.



Here’s a slightly more complicated diffraction image, showing multiple diffractions around different edges in sea waves.

diffraction It’s worth noticing that the waves actually pass through each other, their amplitudes adding up in space and time.

I just got back from the mountains where TV signals sometimes are a bit wobbly, because the waves can’t diffract well enough over the mountains to be received clearly in the valley. Work out the wavelength for yourself. The TV signal is a radio wave travelling at 300 million m/s and might have a frequency of  600MHz. Got it? Wavelength = speed/frequency = half a metre. Mountains are a lot bigger than this so there isn’t much diffraction around them. We have to use much lower frequencies or bounce the wave off a satellite.

The idea of diffraction is important in loudspeaker design. Speakers that produce low frequency bass notes are called “woofers”. They need to move a lot of air, so need to be quite large. Diameters of 30 cm or more are common.

However, a typical high treble note has a frequency of 5000 Hz or so, which corresponds to a wavelength of  almost 7cm – sound travelling at 330 m/s in air. This is much less than the diameter we need for a woofer, so if we try to use the woofer to generate a high frequency note, the sound wave will beam straight ahead without significant diffraction, just like water waves passing through a wide gap. So, you won’t hear those notes unless you are right in front of the speaker.


It’s clear, then that the speakers which  produce high frequency notes, called  “tweeters”, must have a much smaller diameter than required for the woofer.

 In the picture, the woofer is above and the tweeter is below. A simple electric circuit inside the enclosure directs low frequencies to the woofer and high frequencies to the tweeter.


refracLatin “frango,” = I break, as in fracture. The straws look broken as we view them through the water. The light from them is being refracted, changing direction and travelling more slowly in the water than the air around it.

How it works

soldierImagine two columns of soldiers marching in step side by side on the pavement. The pavement then changes into a muddy path. The soldiers nearer the path have to take smaller steps to stay in time so travel more slowly, resulting in the whole column changing direction. Light travelling from a less dense to a more dense medium slows down. If the angle between the media isn’t 90 degrees, the beam suffers a change in direction as well.  Light waves are refracted towards the normal when going from less dense to more dense material (air into glass or water) and vice-versa. Water is refracted towards the normal when travelling more slowly in shallower water. If we look at a ripple tank pattern the waves travel more slowly in the shallower water. If they enter the shallow water at an angle the wavefront’s direction changes.

refraction_imagelargeWe can see the shorter wavelength and change in direction at the top of this image very clearly where the speed is slower. There’s no change in frequency.


Screen Shot 2015-04-03 at 11.34.52All waves bounce off surfaces. Water waves hit walls and bounce off them, light hits mirrors and bounces off and so on. We can imagine the direction of the wave travel to be a streak, drawn as a straight line with a pencil. It will strike a surface then rebound at the same angle as it struck.

We never use words like ‘bounce off’ or ‘rebound’. Instead, we say that

The incident ray is reflected at the surface at an angle θ to the NORMAL (an imaginary line perpendicular to the surface) so that the angle of incidence equals the angle of reflection.

Also the incident ray, the normal and the reflected ray can all be drawn on the same flat sheet.

Here’s a little animation to show you…


An image in a mirror isn’t real. Someone could walk behind the mirror and the image in it would still be there.  This kind of image is called a virtual image, because it hasn’t been formed by real rays of light which meet at it. If you stand 1m away from a mirror, how far behind the mirror is your image?  Look in a mirror and wink at yourself. What do you notice? Hold up some writing in front of a mirror. What do you notice?

Now for the physics bits…For light (an electromagnetic radiation) a flat shiny surface, like a plane (flat) mirror, is a good reflector. Unlike the ‘hall of mirrors – above- a plane mirror is one which is straight and not curved so the image appears to be the same distance behind the mirror as the real object is in front of it. This is because the brain thinks that light travels in straight lines without changing direction.


we can see that angle i= angle r

The wavelength, frequency and speed is unaffected by the reflection.

Finally, the image is the same size and colour as the object.

Thanks to Apple Computers for a screenshot of their iMac wallpaper

Screen Shot 2013-07-06 at 3.12.52 PM

Hydrogen as a Fuel


cg-fuel-cell-51One day, the oil will run out, so we’d better have a few ideas ready so we needn’t go back to camels and bicycles for transportation. Under the hood will shortly look something like this….
The idea of a fuel cell has been around for quite a while. Sir William Grove invented the first fuel cell in 1839.  He knew that water could be split into hydrogen and oxygen by sending an electric current through it.  [electrolysis of water] He  suggested that by reversing the procedure  by combining hydrogen and oxygen you could produce electricity and water. He created a primitive fuel cell and called it a gas voltaic battery. Fifty years later, scientists Ludwig Mond and Charles Langer coined the term fuel cell while attempting to build a practical model to produce electricity.
This site provides an excellent overview of fuel cells generally showing examples of the many different types. You only have to know that hydrogen can be used as a fuel, but a quick update on modern applications is worth a few minutes to scan through.Simply put, hydrogen gas is pumped into a catalytic chamber stripped of its electrons which are used to drive the electrical circuit, collecting them at the other side and combine with oxygen from the air to make water. No greenhouse gases, no pollution.

Waves Basics

water1There are two types of wave motion, transverse where the oscillation direction is at 90 degrees to the travel, and longitudinal where the oscillation and travel directions are along the same straight line. Light is transverse, sound is longitudinal. Earthquake waves can be both.
Have a look at this animation. Compare the two different types of oscillations and also their speeds.

This is a really great little site that you can play around with waves on. ..

For basic wave theory from Absorb Physics,  I think this one is brilliant. Click here and get involved.  Can you get the exercises right?

For a slightly higher level treatment, check out this  animation fromWikipedia

All waves obey two simple equations.

Speed of wave = frequency x wavelength

Make sure that the units match on both sides

where frequency is the number of complete waves made every second and wavelength is the crest-to crest distance

Speed = distance travelled / time taken

Earthquakes can be incredibly devastating. The 2005 ‘quake in northern Pakistan killed over 80,000 people injured another 100,000 and made over 3 million people homeless.

More detailed science to follow. The water ripples in the picture above  overlap each other. Watch this space.