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THE CHEMICAL ACTION OF LIGHT
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Fig. 11.
the ray strikes the water, a perpendicular line d / be drawn, the rule is that when a ray passes from a less dense medium (for example, air) into a denser one, it approaches the perpendicular, for n b is evidently nearer to the d / than n a. On the other hand, when a ray passes from a denser to a less dense medium, — for instance, from glass into air, — then the ray n b departs from the perpendicular n d — that is, the angle which the ray makes with the perpendicular after refraction is greater than the angle which it makes with it before.
Now, it is a remarkable fact that light of different colours is refracted unequally.
If a bundle of rays of white sunlight is suffered to fall on a piece of glass, the violet rays are deflected more than the blue rays, the blue more than the green, yellow, and red ; and the result of this is that the white bundle is decomposed into a rainbow-coloured fan, violet, indigo, blue, green, yellow, orange, and red.
This phenomenon is the cause of the rainbow. If a ray a falls on a drop of water (fig. 12), it is refracted and at the same time divided into a coloured fan, which is reflected from the back of the drop, suffers again refraction and dispersion at b, and issues as a broad bundle of colour. In open daylight this decomposition of white light by means of a prism cannot be clearly seen, because our eyes are dazzled by the bright light. In order to observe the pure colours Fig* 12 of the spectrum, it is best to pro
duce it in a darkened room, in which the light is allowed to enter only through a small slit (b, fig. 13).
The Spectrum. Fraunhofer's Lines. — When a prism 8 is