The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
Explain How does Light Travel in Straight Line
If we shine a torch on a dark night, we will see that the beam of light produced by torch travels straight into darkness. This observation shows that light travels in straight lines.
The beams of search lights at the airports (or from light houses in the sea) show that light travels in straight lines. And the beam of light coming from the projection room of a cinema hall and falling on the screen also shows that light travels in straight lines.
If we place an object in the path of light, a shadow is formed behind the object. The formation of shadow by a source of light also suggests that the light travels in straight lines. This is because if light could bend and travel in curved lines, then it would have reached behind the object and hence no shadow could have been formed.
The fact that we cannot see around a corner also shows that light travels in a straight line. It cannot bend to make us see around a corner. Our inability to see things behind a wall also shows that light travels in a straight line. The property of light travelling in straight lines is called rectilinear propagation of light. We will now describe two simple activities which will show us that light travels in straight lines.
Activity 1
Fix a lighted candle on a table. Take a rubber tube, stretch it straight and look through it at the flame of ’ the candle with one eye (keeping the other eye closed) [see Figure(a)], We will find that when the rubber tube is straight, we can see the light of candle flame through it.
Now, bend the rubber tube a little and look through it at the candle flame again [see Figure(b)], We cannot see the light of candle flame through a bent rubber tube. This means that the light of candle flame can travel through the hole of straight rubber tube but not through the bent rubber tube. From this observation we conclude that light travels in a straight line.
Activity 2
Take three cardboards of the same size. Place these three cardboards together one over the other and make a hole in their centre by hammering a nail through them. In this way, we will get three cardboards, each having a hole at the same place in the centre. We can now perform the activity as follows:
(i) Fix the three cardboards vertically on the table in such a way that their holes are in a straight line as shown in Figure.
(ii) Place a burning candle behind the farthest cardboard. The candle should have its flame at about the same height as the holes in the cardboards.
(iii) Look at the candle flame through the hole in the first cardboard (Figure 9). We can see the candle flame through the holes of the cardboards. Thus, when all the cardboard holes are in a straight line, light from the candle flame reaches our eye.
(iv) Now push the middle cardboard slightly out of its position so that the three holes do not remain in a straight line. If we look through the hole in the first cardboard, we cannot see the flame of candle now. This means that when all the holes of the cardboards are not in a straight line, the light of candle flame does not reach our eyes.
Thus, when all the holes of the cardboards are in a straight line, light can reach us, but when all the holes are not in a straight line, then light does not reach us. This shows that light travels in a straight line