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.

Eclipses

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.

On the other hand sound travels at about 1120 ft. per second in air, which is only about	1	mile per second.
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In water, however, sound travels over four times as fast as in air, 4700 ft. per second.