# Visible Light

From the earliest times, people have wondered about the nature of light. The Greeks thought that light was a stream of particles. Isaac Newton thought this also. However, during the 18th Century, a scientist called Thomas Young showed that light was made up of waves. Young (1773 - 1829) was an interesting chap. He was an infant prodigy playing several musical instruments and learning several languages before leaving school. He trained as a doctor but was unsuccessful because he "lacked a suave bedside manner". From the study of the eye, he switched to light itself. Young conducted experiments that showed light was a wave because the waves interfered with each other, sometimes cancelling each other out. This was something that particles couldn't do.

It took a long time for scientists to accept light as a wave. At first they asked 'waves in what?' This was eventually answered by Albert Einstein. Young also worked out the wavelength of light. The wavelength of light is approximately 1mm (0.000001 m). This is small enough not to be visible to the naked eye, but not as small as atoms or molecules.

The wavelength of light is different depending on the colour. Newton had showed that white light can be split into the spectrum by passing it through a triangular piece of glass called a prism. This gave the familiar rainbow of colours. Violet light has the smallest wavelength, next is blue, then green, yellow, orange. Red has the longest wavelength of light.

For thousands of years people thought that light travelled at infinite speed. Galileo Galileii had attempted to measure the velocity of light. His crude methods failed because light was much faster than anybody imagined at the time. The best value for this velocity was eventually obtained by a character called Olaf Rømer (1644 - 1710). He was a Danish astronomer. While working in Paris he observed the moons of Jupiter and studied their mutual eclipses. He found that the times of these eclipses varied with the distance between Jupiter and the Earth. He deduced that light must take extra time to travel from Jupiter to Earth when the planet was further. From his knowledge of the difference in distance and the changes in the eclipse times he worked out the speed of light.

The velocity of light is one of the most important universal constants there is. The modern value for this speed is 300,000 km/s, or 300 million m/s (also written as 3 x 108 m/s). In miles it works out to 186,000 miles per second (seven times around the earth in one second).

For all waves, there is a relationship between the velocity, wavelength and a quantity called frequency. Frequency is how many waves pass a given point per second. The relationship is given by the simple equation below.

frequency = velocity of light / wavelength

For light, the frequency is 3 x 108 m/s divided by 0.000001 m which gives 3 x 1014 waves per second (or cycles per second). The unit per second is called a Hertz. So we can say light has a frequency of 300 million million Hertz (or 300 million MHz). This is indeed a high frequency!

# Invisible Light

William Herschel (1738 - 1822) was one of the most important astronomers that ever lived. He built his own telescopes which were better than any that existed at the time. In 1781 he discovered the first new planet since ancient times (Uranus). He made important discoveries about the stars: that not all stars were single; that they were not actually fixed in space but had tiny motions visible over hundreds of years; that they were not scattered randomly in the sky.

In 1800 he performed a famous experiment where he tried to measure the temperature of different colours of the spectrum by placing a thermometer on each colour. He found to his amazement that the hottest part of the spectrum was in a place where there was no colour at all. It was a spot beyond the red end of the spectrum. For the first time it was possible to talk about invisible light. This hot light became known as Infra Red (below the red) because it was shown to have longer wavelength than visible light.

Apart from its wavelength, Infra Red has all the other properties of light.

Johann Wilhelm Ritter (1776 - 1810) was famous as the discoverer of electroplating. In 1801 he was experimenting with a chemical called silver chloride (AgCl). This chemical is decomposed by light, liberating silver which makes the colourless substance turn black. This reaction is the basis of pre-digital photography.

In chemistry at that time there was a rumour that blue light was more efficient at initiating chemical change than red light. Ritter tried to measure the speed at which silver chloride broke down with different colours. He proved that blue light was indeed more efficient that red light. He was amazed, however, that the most vigorous reactions took place in the region beyond the violet where nothing could be seen.

This new radiation was originally called Chemical Rays but is now called Ultra Violet (beyond the violet). Ultra Violet differs from visible light only in its wavelength which is shorter.

# Electromagnetic Waves

James Clark Maxwell (1831 - 1879) was a Scottish physicist. Using his powerful mathematical brain he made theoretical contributions towards the invention of colour photography; he proved Saturn's rings could not be solid as the forces on them would cause them to break up; he proved that the temperature of a body was a reflection of the motions of its molecules thus killing off the idea that heat was something tangible like a gas.

Maxwell's crowning work came between 1864 and 1873. He was looking at the way electricity and magnetism worked. He found many similarities between them (negative / positive, north / south, etc). Mathematically, he devised a few simple equations that explained all the varied phenomena of electricity and magnetism and bound them together. One could not exist in isolation without the other. His work is usually called The Electromagnetic Theory. He showed that the oscillation of an electric charge produced an electromagnetic field moving outward from its source at a constant speed, c. The value of c could be worked out by a simple formula involving various electric and magnetic quantities.

When he did the calculation it turned out to be close to 299,792.5 km / s. This was too much of a coincidence for Maxwell.

He suggested that light itself was an electromagnetic wave. At the time it was not known what oscillating electric charge was involved. Whatever it was it had to be oscillating at 300 million MHz to produce the correct type of waves. The oscillations were later found when the properties of the atom were better understood.

Maxwell also suggested that since electric charges could oscillate with any frequency, there should be a whole family of electromagnetic radiation of which visible light was only a small part. Maxwell's work was so important that it survived the Physics revolution that occurred at the beginning of this century.

Maxwell's ideas were to be dramatically verified by a gentleman called Hertz. Heinrich Rudolf Hertz (1857 - 1894) was a German Jew before such a thing became dangerous. He set up electric circuits that produced oscillations and managed to produce electromagnetic radiation with a wavelength of 66cm (over a million times longer than light). This radiation could be picked up by other circuits set up quite a distance away. Later, the Italian M. G. Marconi devised a method for using these rays for communication.

The new radiation was first called Hertzian Waves; this became Radiotelegraphic Waves after Marconi. We now call them Radio Waves. Radio Waves have the longest wavelengths of any electromagnetic radiation being longer than Infra Red. Hertz died young from blood poisoning so he missed the communication explosion set off by his discovery.

Just before the turn of the century, four types of electromagnetic radiation were known: Radio Waves, Infra Red, Visible Light and Ultra Violet.

# More Waves

We now turn to Wilhelm Roentgen (1845 - 1923), another German physicist. He was a competent scientist working on Cathode Ray Tubes (later to be used for TV screens and computer monitors).

On the night of 5 November 1895, he noticed a glow coming from a chemical called barium platinocyanide. This chemical glowed whenever the tube was on, even if he put cardboard between it and the tube. Years later he was asked what he thought at this point. He replied I didn't think; I experimented.

Roentgen went on to show that the glow was caused by a highly penetrating but invisible radiation given off by the tube. It passed through paper, thin sheets of metal, flesh. It could ionise gases and had wave properties like light but only much shorter wavelengths. The new radiation was called X-Rays because of their mysterious properties. Roentgen refused to patent the discovery or make any financial gain out of it but he was awarded the first ever Nobel Prize for Physics.

When radioactivity was discovered, it was found to be caused by atoms breaking down and throwing out various particles. The radiation from radioactivity consisted of three different types of rays. These were called Alpha, Beta and Gamma Rays. Alpha Rays were shown to be nuclei of Helium thrown out by the disintegrating atom. Beta Rays were shown to be very fast electrons.

Gamma Rays were found to be very short wave radiation, more penetrating and of shorter wavelength than even X-Rays.

The table below describes the different radiations of the Electromagnetic Spectrum. I must emphasise that these radiations are all the same except for the difference in wavelength. They have different names because of historical reasons and the way they are generated. The boundaries between the different radiations are all artificial.

As you progress from Radio Waves through to Gamma Rays, the wavelength gets shorter (so they become more penetrating), the frequency gets higher (so the oscillation needed to produce them gets faster), and the energy gets higher (so it takes more energy to produce Gamma Rays than it does to produce Radio Waves).

Name
Wavelength (m)
Frequency (Hz)
Energy (J)
Radio Waves 104 - 10-3 103 - 1010 10-30 - 10-23
Infra Red 10-3 - 10-6 1010 - 1014 10-23 - 10-19
Visible 10-6 1014 10-19
Ultra Violet 10-6 - 10-8 1014 - 1016 10-19 - 10-17
X-Rays 10-8 - 10-10 1016 - 1019 10-17 - 10-14
Gamma Rays 10-10 - 10-14 1019 - 1024 10-14 - 10-10

Radio Waves are produced when free electrons are forced to move in a magnetic field, or when electrons change their spin in a molecule. They are used for communication and to study low energy motions in atoms. All electrical goods generate Radio Waves.

Radio Waves from space can be used to study cool interstellar gases. Radio Waves cannot be detected by humans.

Infra Red radiation is produced by the vibrations of molecules. Human skin feels this radiation as heat. Microwave ovens work by using Infra Red radiation of the correct frequency to make the water molecule vibrate faster. A faster vibrating molecule is a hotter molecule. Only the food which contains water is affected. The plate which is a dry mineral is unaffected.

Infra Red is used as an analytical tool for molecules in Chemistry. Cool, proto-stars are studied with Infra Red detectors.

Visible and Ultra Violet Light is produced by chemical reactions and ionisations of outer electrons in atoms and molecules.

There are many chemical reactions that are instigated by this radiation: the chemical retinal in animal eyes, chlorophyll in plants, silver chloride in pre-digital photography, the chemical melanin in human skin, silicon converts light to electricity. Light is the most familiar electromagnetic radiation because the Earth's atmosphere is transparent to it. Light (and a little of the Infra Red and Ultra Violet on either side of it) can pass through the atmosphere. Living organisms have evolved to use these waves.

Visible Light is simply the part of the electromagnetic spectrum that reacts with the chemicals in our eyes. Bees can see more Ultra Violet than we can. Snakes can detect Infra Red.

X-Rays are produced by fast electrons stopping suddenly, or by ionisation of the inner electrons of an atom. They are produced by high energy processes in space: gases being sucked in to a black hole and becoming compressed; exploding stars. They are used in medicine to look through flesh.

In Physics the waves are small enough to pass between atoms and molecules so they can be used to determine molecular structures.

Gamma Rays are produced by very high energy processes, usually involved with the nucleus of atoms. Radioactivity and exploding stars produce Gamma Rays. They are very dangerous because if they strike atoms and molecules they will do lots of damage. If the molecules are the long and complex molecules of life, death and mutation could occur.