Concepts for Teachers

In simple terms light is what enables us to see. For thousands of years people have been using various means of producing light to help them see things at night; they used fires, torches, rush-lights, oil lamps and, from the early 1800s, gas lights. People studied how light was reflected by mirrors, how a glass lens could be used to produce magnified images of things or to start a fire. They discovered that a beam of light generally bends or refracts when it goes from air into water or glass.

We know that people have been speculating about the nature of light for 2500 years. One of the earlier theories was that light was composed of streams of particles.

"The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." (Lucretius, On the nature of the Universe, 55 BC)

This was also Isaac Newton’s understanding and on this basis he tried to explain reflection and refraction (1642-1727). Many people thought that light travelled instantaneously while others thought that it had to have a finite speed. Galileo tried to measure it but did not succeed.

Over several years (1671-1677) Ole Römer’s observations led him to conclude that light travelled at a finite speed. He noticed that forty orbits of the moon Io around Jupiter were 22 minutes shorter when the Earth was approaching Jupiter than when it was moving away. (He didn’t actually calculate the speed of light but, using his data, it is found to be 276,000 km/s (8% less than the accepted value today – a remarkable achievement). Newton accepted this view:

"For it is now certain from the phenomena of Jupiter's satellites, confirmed by the observations of different astronomers, that light is propagated in succession and requires about seven or eight minutes to travel from the sun to the earth." ( Principia , Book I, section XIV; 1687).

Waves on water can interfere with one another. If two stones are thrown into a still pond they can produce a characteristic interference pattern. In 1800 Thomas Young demonstrated this property of waves on water and showed that sound behaved in a similar way. He went on to show in 1801 that when light was projected through two slits that were close together the same pattern was produced. He concluded that light was some kind of wave and from his results he was able to calculate the wavelength of light.  In this he was challenging Newton ’s particle theory (which had no theoretical basis). Gradually more people accepted that the wave theory of light was more reasonable.

This new theory raised a fundamental question: if light is a wave, then what is waving? People invented a theoretical medium for light waves; it was called ‘the aether’. It was not a material, it had no mass, it was invisible but it helped people to conceptualise light as a wave. (The idea was eventually abandoned in the early 1900s.)

In his later years Michael Faraday endeavoured to find some connection between light and electromagnetism. After many failures he eventually (in 1845) demonstrated that polarised light could be twisted by a strong magnetic field. Faraday had discovered many things but he regarded this as his greatest achievement.

This is a remarkable finding and it brings us back to reconsider the fundamental nature of light. Our everyday experience does not suggest that light might have some electric/magnetic properties – but yet it has. Building on Faraday’s experimental work in electromagnetism James Clerk Maxwell (1831-1879) constructed a theoretical description of electromagnetic waves and showed that these waves should travel at the speed of light. It became clear that light was in fact an electromagnetic wave and that other electromagnetic waves were also possible – including radio waves.

Light is a form of electromagnetic radiation. The colour of light depends on its wavelength which ranges from about 0.4 microns (for violet) to about 0.7 microns (for red). A micron is a millionth of a metre or a thousandth of a millimetre. The visible spectrum is just one small part of the electromagnetic spectrum. X-rays and ultraviolet light have shorter wavelength (and greater energy) than visible light. Infrared, microwaves and radio waves have longer wavelength (and lower energy) than visible light.

In the early 1900s a number of scientists tried to explain a discovery (first noted by Hertz in 1887) called the photoelectric effect. This is really another story but suffice it to say that the only workable explanation involved considering light to be composed of particles (now called photons). This was not a return to Newton ’s idea; the wave nature of light was well established at that stage. Light has wave-like characteristics and particle-like characteristics. It appears to have a dual nature.

Note: It is not intended that this summary be presented to primary school pupils but it should provide a basis for understanding the complexity of light and should help the teacher to guide pupils’ questioning and reflection.

Some useful applets:

Refraction: http://physics.uwstout.edu/physapplets/a-city/physengl/refraction.htm

Adding colours: http://physics.uwstout.edu/physapplets/wave/colors_g.htm