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Light - Background
Light fills the universe. Even on the darkest night light still fills the universe. Light is a form of
energy. When we watch television every picture sends about one millionth of a joule of energy into
our eyes. Twelve hours of viewing would give about one joule which is roughly the energy needed
to put on a pair of jeans. Our main source of light energy is the sun. The nuclear fusion reactions
occurring in the core of the sun give off this energy. Hydrogen nuclei combine to produce helium
nuclei with the release of energy according to the equation, E = mc2, where m is the change in mass
and c is the speed of light in vacuum. The sun produces a vast amount of energy as about 6 × 10 8
tonnes of hydrogen are converted every second.
The earth, because of its size and distance from the sun, receives about 0.000 000 05% of this
energy as heat and light. The visible part of the sun, the intensely bright sphere of white light, is
called the photosphere. The temperature of the surface of the sun is about 6000 °C. Sun spots (dark
patches) are areas on the sun’s surface at a lower temperature due to variations in the sun’s magnetic
field. The sun, like most celestial bodies, spins on its axis. As the sun is not a solid body all parts of
it do not rotate in unison; the equator rotates once every 25 days while points near the poles take 34
days for one rotation.
Refraction
The earliest speculations about the nature of light are attributed to the Greek philosopher,
Pythagoras (c.582 BC–c.497 BC), who proposed that light consists of tiny particles which are sent
out by the object being viewed to the eyes of the viewer. An alternative theory, attributed to the
Greek mathematician Euclid (325 BC–?), is that the eyes send out rays which strike objects and give
the sensation of sight. Euclid also studied mirrors and discovered the laws of reflection.
The Greek astronomer, Ptolemy (c.75 AD–?) studied the refraction of light. He recognised the need
to make adjustments to the apparent positions of planets in the sky to arrive at their actual positions.
This change in location is due to the light bending on entering the earth’s atmosphere. He studied
air/water, air/glass and water/glass interfaces and concluded that the angle of incidence was
proportional to the angle of refraction which we now know is incorrect. The Arabian physicist,
Alhazen (c.965–1038), applied mathematics to his study of plane and curved glasses and mirrors. He
showed that spherical mirrors could not bring parallel rays to a sharp focus – spherical aberration –
and that light travels more slowly through a more dense medium, causing the bending at interfaces
between media of different density.
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The speed of light
The first measurement of the speed of light was made by the Danish astronomer, Ole Rømer (1644–
1710) about the year 1676. He observed the planet Jupiter and its satellites. Each of these satellites is
eclipsed when it moves behind the planet. The time between successive eclipses of a particular
satellite should be the same. Rømer found that when the earth was approaching Jupiter the eclipses
became progressively earlier and that when the earth was receding from Jupiter the eclipses became
progressively later.
He correctly attributed this variation to the variation in the time it took light to come from Jupiter to
Earth as the distance between the two planets changed, (The variation in the distance between Earth
and Jupiter is due to the fact that Jupiter, being farther from the sun, takes much longer –
approximately twelve times longer – to complete an orbit of the sun than the earth does.) Based on
the information available at that time Rømer calculated a value of 2.28 × 108 m s–1 for the speed of
light.
In 1849 the French physicist, Armand Fizeau (1819–1896) carried out the first terrestrial
measurement of the speed of light using mirrors to make light travel a round trip of some 17 km. He
used a rotating toothed wheel W (see diagram), to control the emerging and reflected light.
Light from the source S was focused by a converging lens through a half-silvered plate P onto the
edge of the toothed wheel. If the light was not blocked by a tooth then it passed through and
travelled a distance of 8.6 kilometres to the mirror M and back to the wheel’s edge. He adjusted the
speed of rotation of the wheel until he got no reflected light. He now knew that the light had
travelled the round trip in the time it took the gap in the wheel to be replaced by a neighbouring
tooth, blocking the light. This method was adapted by Michelson (1852–1931) who used a rotating
mirror to replace the toothed wheel. Today, as part of the 1983 redefinition of the metre, the speed
of light in vacuum, c, has been assigned an exact value. By definition the value of c is now 299 792
458 m s-1 exactly.
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Experiments which formerly measured the speed of light in vacuum are now used to calibrate length
standards. The daylight we use to see this print left the sun 8.5 minutes ago and travelled in a
straight line until it encountered the earth’s atmosphere, where it was continuously refracted due to
the density gradient of the atmosphere, until it reached the page. From the time man discovered fire
he has used light. He extended his day and defended his cave family using bundles of burning twigs.
Oil from seals and other animals was used in stone or shell lamps. Roman and Greek lamps had
spouts to hold the wick in the oil while the people of the Shetland Islands pushed wicks into the
bodies of the very oily stormy petrel birds and used them as lamps. As the quality of lamps
improved their uses increased, especially in the field of communications. The ancient Romans used
light to send messages from one army post to another. They used large wooden flags and codes to
convey the message but they were limited by the fact that these stations depended on direct line of
vision. The discovery of electric current gave us the electric telegraph and the telephone but, as
these required wire connections, armies still used light – flashing lamps and mirrors – to transfer
information. Marconi invented the wireless telegraph and this gave us radio, radar and television.
As a result the use of light for transferring information decreased. However, as the amount of
information being sent from place to place increased, the air waves became crowded and light is
now being used again. Light has a much higher frequency (c. 1015 Hz) than radio waves (c. 105–
107 Hz) or microwaves (108–1010 Hz). A system that operates at the frequency of light can transmit
much larger quantities of information than radio or microwaves. In this system the information is
encoded in pulses of light which are transmitted over large distances using optical fibres, at the end
of which the information is retrieved. Light wave telecommunication systems using optical fibres
already span the world and are being used to carry voice, video and computer data. Very fast
computers of the future will probably use light.
The microscope, which consists of two converging lenses, was invented by Zacharius Janssen about
1590. In the microscope the first lens is called the objective lens and the object to be viewed is
placed just outside its focus, to form a magnified inverted real image. This real image is the object
for the second lens, called the eyepiece lens, and is placed just inside the focus of the eyepiece lens.
The final image formed is magnified, virtual, still inverted and formed far away from the eyepiece
lens so that it can be viewed with a relaxed eye.
The telescope was invented by a Dutch optician, Hans Lippershey (1587–1619) in 1608.
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