Light Travels in a Straight Line

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.

Дата добавления: 2015-09-06