Electromagnetic Radiation Discoveries

All matter produces radiation.

Radio Waves

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.

Heinrich Rudolf Hertz (1857 - 1894) set up electric circuits that produced oscillations and produced electromagnetic radiation with a wavelength over a million times longer than visible light. These waves 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. At first these waves were called Hertzian Waves and then Radiotelegraphic Waves. We now call them Radio Waves. Radio Waves have the longest wavelengths of any electromagnetic radiation.

Infra Red Radiation

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. Astronomers study cool proto-stars with Infra Red detectors.

William Herschel (1738 - 1822) was a very important astronomer who built the best telescopes of his time, which he used to discover Uranus, the first new planet since ancient times. In 1800 he performed famous experiments to measure the temperature of different colors of the spectrum by placing a thermometer on each color. He found to his amazement that the hottest part of the spectrum was in a place where there was no color 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.

Visible and Ultra Violet Light

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 photography, melanin in human skin, and the way 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.

Ancient Greeks thought that light was a stream of particles. Isaac Newton thought this also. However, an 18th Century a scientist called Thomas Young showed that light was made up of waves. Young’s experiments showed light was a wave because the waves interfered with each other, sometimes canceling each other out. This was something that particles couldn't do.

The colors we see depend on the different wavelengths of visible light. Newton 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 colors. Violet light has the smallest wavelength, next is blue, then green, yellow, orange. Red has the longest wavelength of visible light.

In 1801 Johann Wilhelm Ritter (1776 - 1810) was experimenting with silver chloride. This colorless chemical is broken down by light, liberating silver which makes it turn black. This reaction is the basis of pre-digital photography. In chemistry at that time there was a rumor that blue light was better at breaking down silver chloride than red light. Ritter measured the speed at which silver chloride was broken down by different colors. He proved that blue light was indeed better than 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.

X-Rays

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.

Wilhelm Roentgen (1845 - 1923) was a German physicist who worked 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 with 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.

Gamma Rays

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.

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.

Electromagnetic Radiation

Historic Discoveries

Across

4 This chemical can be used to convert light into electricity

6 He first used radio waves to transmit information

8 This type of radiation is found in the electromagnetic spectrum somewhere between short wave radio and infrared light. Humans use it for both transmitting information and quickly warming up left-overs.

9 A continuous range of differences, an orderly arrangement.

10 The highest energy rays known. (Higher energy means shorter wavelength)

12 These are arranged in energy levels surround the nuclei of atoms. If they are separated from an atom, that atom can be said to have been "ionized"

14 The coolest visible (to humans) light

16 These reptiles can use infra red radiation to detect their prey

17 When free electrons are forced to move in this type of field, radio waves are produced.

22 Having enough energy to liberate an electron from an atom or molecule.

24 Cathode ray tubes make these computer parts

25 What Roentgen did instead of thinking

26 The first scientist to demonstrate the wave properties of light

27 This frequency of light is much more efficient in causing chemical reactions in sliver chloride

28 Silver chloride was the most important chemical in the pre-digital (black and white) form of this craft. Silver chloride breaks down when exposed t light and turns black. (The more the exposure to light, the blacker it gets)

30 These rays are actually very fast (high energy) free electrons

32 He discovered radio waves in the 19th century. His name is also a term for "cycles per second."

33 An early name for radio waves

Down

1 This type of radiation is experienced by living things as heat

2 Scientific name for a living thing

3 Something waves can do that (normal) particles cannot

5 He discovered x-rays

7 Ancient Greeks and Isaac Newton thought that light was made up of these. Einstein later forced modern physicists to admit it might be true somehow.

8 This chemical is produced in human skin to protect against ultra violet rays. The more of this chemical, the darker the skin.

10 The color of chlorophyll - and therefore the color of most leaves in plants

11 This protects the earth from most radiation coming from outer space except visible light and radio waves

13 Caused by the breakdown of atoms - now known to consist of Alpha, Beta, and gamma rays

15 The hottest visible (to humans) light

17 Even thin sheets of this will not block gamma rays

18 This chemical captures energy from light. Plants use it to make their own food.

19 His experiments with photographic chemicals lead to his discovery of ultraviolet light

20 This astronomer used prisms and thermometers to discover infra red light

21 Formed when atoms share or exchange electrons

23 These insects can see more of the ultraviolet spectrum than we can

29 The part of the eye where chemical reactions caused by light occur. These reactions are detected by optic nerves and transmitted to the brain.

31 These rays are actually helium nuclei (consisting of two protons and two neutrons)