We can bypass the whole argument about ether by considering those wave properties which are independent of the medium. Waves in general obey the principle of superposition, which states that the net effect of two or more overlapping waves is simply the sum of the effects caused by each wave individually. This applies to waves in a string, waves in

* George Berkeley (1685-1753) wrote a treatise entitled 'The Principles of Human Knowledge', which formed the basis of a philosophy of immaterialism, concerning the reality of physical objects and the existence of things which cannot be observed.

Figure 8.1 Nothing interferes with love!

water, and all kinds of waves both transverse and longitudinal. It means that waves pass through each other and then continue on independently, unaffected by their mutual interaction. An example of this was shown in the picture of overlapping waves created by drops of rain falling on the surface of water, in Chapter 6.

We do not need to set up a special experiment to illustrate that light behaves in similar fashion. A common domestic situation is shown in Figure 8.1. Father's watching of the television is not interrupted by the visual messages of affection that travel across his line of sight.

The world is full of light. Light beams of all wavelengths criss-cross every bit of space in the universe. Now, mankind has added radio and television waves, radar, infrared and other radiation, all independently superimposing on top of each other and on top of the natural background. While the phenomenon of superposition may not be conclusive proof that light is a wave, it is consistent with the wave hypothesis.

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