## The visible spectrum

In 1666, Isaac Newton showed that white light is made up of a continuous spectrum of colours, from red to orange, yellow, green and finally to blue, indigo and violet. He passed a beam of sunlight through a prism, and saw it fan out into its constituent colours. By putting a piece of paper on the far side of

Visible part of electromagnetic spectum.

the prism, he was able to look at 'individual' colours. He was able to recreate white light by bringing the colours together again using a second prism.

Figure 1.1 is a schematic representation, in that one would not normally see the spectral colours by looking at the beam from the side. In addition, principally owing to the finite width of the incoming beam, it is not possible to recombine the colours completely. In practice the final image is white in the centre with a combination of colours on each side.

1.3 Measuring the speed of light 1.3.1 The astronomical method

In 1676, the Danish mathematician Olaus Römer (1644-1710) found that eclipses of Jupiter's moons do not occur at the times predicted by Newtonian mechanics. They are about 11 minutes too early when Jupiter is closest to the earth and about 11 minutes too late when it is furthest away. Römer concluded that the

Figure 1.2 Jupiter's moons. The light message takes longer when Jupiter is further away.

discrepancy occurs because light takes longer to travel the larger distance (as indicated in Figure 1.2), and on the basis of the measured time difference of about 22 minutes, he calculated the speed of light to be 2.14 x 108 ms-1. Although not a particularly good estimate in modern times, this value is certainly of the right order of magnitude and a remarkable achievement at the time.

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