Interesting results are obtained when light filters in the form of sheets of gelatine coloured with various dyes are placed, in turn, in front of the slit in Fig. 25.2 or 25.3. By this means, the light transmitted by the filter can be analysed into its component
colours. It is observed that certain colours, depending on the colour of the filter, are now absent from the spectrum. The missing colours are those of light which has been absorbed by the filter while the remaining colours have been transmitted. Now one would expect red gelatine to transmit only red light, green gelatine only green light and so on. Indeed, this generally proves to be so when tested by experiment. But an unusual result is obtained with yellow gelatine. The spectrum of the light passing through most types of yellow gelatine is found to consist of red and green as well as yellow. What is even more striking is that this particular yellow light looks just the same to the eye as that which comes from a filter passing only pure yellow. To distinguish between the two, the former kind of yellow is called compound yellow light. Experiment shows that the yellow petals of flowers and most yellow paints are examples of compound yellow.