SHADOWS AND ECLIPSES
A Well-Known Shadow
When you walk along a street at night, a shadow is produced in front of you as soon as you pass a lamp. You see it often and also notice that the length of the shadow increases as you walk farther away from the lamp.
Fig. 1 shows how these shadows are formed. When you are at AB, the length of the shadow is BX; when you walk to a position CD, the length of the shadow increases to DY. These shadows are found by drawing a straight line first from the lamp L to A and producing the line to meet the ground at X, and, in the second case, from L to С produced to meet the ground at Y. In doing this we assume that light travels in a straight Line.
Fig. 1. Shadows cast by a lamp
Light Travels in a Straight Line
Mount four rectangular pieces of cardboard, of the same size, on wooden blocks (Fig. 2). Make small holes in the centres А, В, С of three of the boards and leave D without a hole. We now wish to get the three holes in a straight line to look through the holes. By doing this we assume that light travels in a straight line - the very fact we wish to prove. Some other method must be used. It is better to define the shortest distance between two points as a straight line. Put a piece of thread through the three holes, pull it taut, and move the three cardboards until the string passes unhampered through the centre of each hole. The holes are now in a straight line. Remove the string and place the fourth cardboard behind the three. Darken the room and place a lamp in front of the first hole. A light spot should be seen on the last cardboard at D. The light must have passed in a straight line through А, В and С to D (Fig. 2). Now move card В sideways - no light is seen on the last cardboard. This shows that light travels only in a straight line.
Fig. 2. The room should be darkened for this experiment.
There are some remarkable natural phenomena which depend upon shadows. The Sun is the centre of our system and we depend upon the Sun for our light and heat. The Earth travels round the Sun once every 365 days on an almost circular course of radius 93,000,000 miles. While on this journey the Earth also rotates about its own axis once every 24 hours. The portion of the Earth facing the Sun is experiencing day while on the other side of the Earth is night. The Earth is just one of the planets circling round the Sun.
The Earth also has its own companion, the Moon, which travels round it once every lunar month. We are all familiar with the lovely glow of moonlight on a clear night. The Moon is not really glowing, but is reflecting light falling upon it from the Sun and this reflected light is our moonlight.
Two interesting events arising from this rotation of the Moon round the Earth may be observed from time to time. One occurs when the Moon comes between the Sun and the Earth, and the other when the Earth is between the Sun and the Moon.
Speed of Light
Light travels very quickly at a speed of about 186,000 miles per second. This means that light from the Sun takes just over eight minutes to travel the 93,000,000 miles to the Earth. Many of the distant stars are suns in other solar systems, and are so far away that it would be foolish to attempt to state their distances from us in miles. Instead a special unit called a light-year is used. One light-year is the distance that light can travel in one year at a speed of 186,000 miles per second. This distance may be estimated as follows:
In 1 second light travels 186,000 miles.
In 1 year light travels 186,000 × 60 × 60 × 24 × 365 =... miles.
In water, however, sound travels over four times as fast as in air, 4700 ft. per second.
Facts and Theories about Light
Light sources, other than those which shine by reflected light, are generally hot; and in the case of solids and liquids (such as filament lamp or the surface of molten iron) the nature of the light emitted depends largely on the t° of the sources - the lower the t° of the source, the greater the preponderance of red light in the emission. The following is an approximate guide to colour t° for surfaces which are good radiators:
Light of all types travels through space at the rate of 186,000 miles/sec, or 3 × 1010 cm./sec.
This was first discovered by a Danish astronomer, Roemer, who in 1674 found that light from a celestial source (he observed the eclipse of the light from one of Jupiter's moons) took 1000 seconds to cross the Earth's orbit of 186,000,000 miles. The speed is the same for light of all colours. It is also the same for all types of thermal radiation and for radio waves whether of long or short wave-length. The common speed of such widely different types of radiation is part of the evidence which suggests that they are different aspects of similar phenomena. The name electro-magnetic radiation has been given to all these radiations. Present day theory assumes that light is a disturbance, propagated in space, similar to that sent out from a radio aerial, but whereas the electrical oscillations in an aerial have a frequencv of the order of a million per second, the electrical oscillations set up by the electrons in the atoms of a light-source have a frequency of the order of a thousand million per second.
About the beginning of the 18th century there was much speculation as to whether light consists of minute weightless particles shot out by source (Corpuscular Theory) or a disturbance spreading out from the source as a wave motion (Wave Theory).
Huyghens (1629 - 1695) developed the Wave Theory. Newton (1642 - 1727) recognizing, that both theories could explain all that was then known about light, somewhat favoured the Corpuscular Theory. He was influenced in this by the fact that water ripples and sound waves can bend round obstacles, a phenomenon known as diffraction. Point sources of light cast sharp shadows with no diffraction. Though Newton had observed peculiarities at the edges of shadows he did not consider that they were examples of diffraction. It was not until the beginning of the 19th century that Young (1773 - 1829) and Fresnel (1788 - 1927) showed that diffraction of light does occur, and that the apparentlv straight-line travel of light is the result of its very short wave-length.
The refraction of light passing from air into water was explained by the Corpuscular Theory as due to an increase in speed of the waves.
In 1851 Foucault showed that the speed of light in water is less than in air.
Wave-Lengths and Frequencies of Electro-Magnetic Radiation
Measurement of wave-length of light in vacuo shows that it lies between 0.70 and 0.35 of a micron (millionth' of a metre); the larger value is for red light and the smaller for violet light. To produce such a small wave-length, the frequency of the oscillation which gives rise to it must be very great. It can be calculated from the general formula for wave motion.
The speed of light is the same for the various colours only in vacuum. In transparent substances the speed is Jess for violet light than for red.
The preceding table gives values of wave-length and frequency of various types of electro-magnetic radiation in vacuo, the frequency is given in megacycles/sec.
1 = 1 micron
Light and other short-wave-length radiation has an electrical effect. If it falls on a plate of iron coated with selenium, a voltage is produced.
The energy of a light wave has a corpuscular nature.
corpuscular theory корпускулярная теория
disturbance нарушение, помеха
emission распространение, изучение, эмиссия
emit испускать, излучать
filament lamp лампа накаливания
glow светиться, сверкать
light-year световой год
medium-wave волна средней длины
obstacle помеха, препятствие
oscillation вибрация, колебание
per second в секунду
phenomenon (phenomena - мн. ч.) явление
point source точечный источник
preponderance перевес, преобладание
propagate распространять(ся), передавать на расстояние через среду (звук, свет, тепло)
radar радар, радиолокация
radiation радиация, излучение
radiator излучатель, радиатор
at the rate со скоростью
refraction рефракция, преломление
ripple покрывать рябью
rotate вращать (ся)
shoot out вылетать, выбрасывать
speed скорость, ускорять
spread out разбрасывать
t° = temperature температура
1010 = ten to tenth power
vacuum (vacuo - мн. ч.) вакуум
wave-length длина волны
wave-motion движение волны
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