Prof. Muni Mahendra Kumar
We have already seen that according to Doppler Principle,[1] when there is a relative motion between the observer and the source of any wave motion, the wavelengths of the wave undergoes a change. This relativity of colour to motion is regarded as a fact contradicting the objectivity of colour. A. d'Abro, an exponent of scientific theories, contends: "We now come to the so-called secondary qualities-colour and sound. Here, for instance, is what would commonly be called a red light. We apprehend it as red wherever we may be situated, so that, in contrast to the case of apparent shape, it might appear as though we were justified in claiming that the light was really red, i.e., red in an objective world devoid of all observers. But, if now, instead of occupying a succession of various positions at rest with respect to the luminous source, we move forward or recede from the light with sufficient speed, it will change colour. Colour, when considered from an impersonal objective standpoint, is thus just as indefinite as apparent shape. Can we at least combine these various colours, as perceived by the various observers, into one common colour, of which our private perceptions would constitute but different perspectives?
"Needless to say, the task is quite impossible. In other words, there exists in the case of colour no parallel to the objective cone of classical science, no possibility of speaking of objective colour. Colour remains private. Here, then, is a first reason for differentiating colour from real objective three dimensional shape, or, again, secondary from primary qualities."[2]
It follows from our foregoing discussion that the change in the colour due to the Doppler Effect is actually the change in the perceived colour (Cp), and not in the objective colour (Co). The change in the velocity of the observer creates an apparent change in the frequency of the wave-length emitted from the object (source). It is obvious that S (the function representing sensory equipment) is not independent of the velocity of the observer, and hence, the change in the velocity would produce a corresponding change in the value of S, which, subsequently, would give rise to change in Cp. Thus, it would be wrong to conclude on the basis of Doppler Effect that there exists no objective colour. On the contrary, the Doppler effect corroborates the fact that the change in the colour due to the change in the relative velocity of the observer with respect to the object (source) is only a change in the perceived colour; the objective colour of the object remains unaffected by the velocity of the observer.
