Security Inks Light Interference

Sentences given in each question, when properly sequenced, form a coherent paragraph. The first and last sentences are 1 and 6, and the four in between are labelled A,B,C and D. Choose the most logical order of these four sentences from among the four given choices to construct a coherent paragraph from sentences 1 to 6.

1.  Security inks exploit the same principle that causes the vivid and constantly changing colours of a  film of oil on water.

1. When two rays of light meet each other after being reflected from these different surfaces, they have each travelled slightly different distances.
2. The key is that the light is bouncing off two surfaces, that of the oil and that of the water layer below it.
3. The distance the two rays travel determines which wavelengths, and hence colours, interfere constructively and look bright.
4. Because light is an electromagnetic wave, the peaks and troughs of each ray then interfere either constructively, to appear bright, or destructively, to appear dim.

6.  Since the distance the rays travel changes with the angle as you look at the surface, different colours look bright from different viewing angles.